spectroscopic investigations of
nanoemulsion surfaces
by
Emma (Ngo. c Bích) Tran
A DISSERTATION
Presented to the Department of Chemistry and Biochemistry
and the Divison of Graduate Studies of the University of Oregon
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
June 2022
DISSERATION APPROVAL PAGE
Student: Emma (Ngo.c Bích) Tran
Title: Spectroscopic Investigations of Nanoemulsion Surfaces
This dissertation has been accepted and approved in partial fulfillment of the re-
quirements for the Doctor of Philosophy degree in the Department of Chemistry and
Biochemistry by:
Dr. Larry Scatena Advisor
Dr. Cathy Wong Chair
Dr. Jim Prell Core Member
Dr. Stephanie Majewski Institutional Representative
and
Krista Chronister Vice Provost for Graduate Studies
Original approval signatures are on file with the University of Oregon Graduate
School.
Degree awarded June 2022.
ii
© 2022 Emma (Ngo.c Bích) Tran
This work is licensed under a Creative Commons
Attribution-NonCommercial-ShareAlike
iii
DISSERTATION ABSTRACT
Emma (Ngo.c Bích) Tran
Doctor of Philosophy
Department of Chemistry and Biochemistry
June 2022
Title: Spectroscopic Investigations of Nanoemulsion Surfaces
Interfaces are ubiquitous and diverse in nature, as are surface-active molecules
that self-assemble at interfaces. Surfactants and polymers are often found at vari-
ous interfaces due to their interfacial properties. Such properties can be tuned by
the chemical environment and make-up of the interface. For example, the behavior
and functionality of surfactants may vary across a solid/liquid, air/liquid, and liq-
uid/liquid interface. The works of this dissertation focuses on the oil/water interface,
specifically the surface of oil-in-water nanoemulsions. Nanoemulsions are kinetically
stable oil droplets dispersed in water that are 100s of nanometers in diameter. They
are stabilized by various types and combinations of surfactants and polymers. From
an application standpoint, nanoemulsions are prevalent in cosmetics, food science,
drug delivery, and environmental remediation. The droplet surface is a common fea-
ture amongst a wide variety of these applications. Thus, emphasis has been placed
on understanding how to control and tune droplet stability and interfacial properties.
The studies detailed herein employs a novel surface-specific spectroscopy called
vibrational sum frequency scattering spectroscopy to develop molecular-level insights
into the self-assembly of surfactants and polymers to nanoemulsion surfaces. Dy-
namic light scattering, interfacial tensiometry, and ζ-potential measurements are uti-
lized concurrently to complement molecular-level details with results reporting on
the macroscopic properties of the system. The adsorption behavior and conforma-
tional arrangement of surfactants and polymers to the droplet surface contributes
significantly to colloidal stability and interfacial properties. Specifically, the chemical
environment, constituents of the adsorbed species, polymer layering behavior, and sur-
face charge are important details that influence nanoemulsion properties. This kind of
knowledge on interfacial phenomena and colloidal chemistry aids in the advancement
of technological, commercial, and industrial applications involving emulsions.
This dissertation contains published and unpublished co-authored material.
iv
CURRICULUM VITAE
NAME OF AUTHOR: Emma (Ngo.c Bích) Tran
GRADUATE AND UNDERGRADUATE SCHOOLS ATTENDED:
University of Oregon, Eugene
University of Nevada, Las Vegas
DEGREES AWARDED:
Doctor of Philosophy, Chemistry, 2022, University of Oregon
Masters of Science, Chemistry, 2018, University of Oregon
Bachelor of Science, Chemistry, 2017, University of Nevada, Las Vegas
AREAS OF SPECIAL INTEREST:
Surface and colloid chemistry
Interfacial phenomena of surfactants and polymers
Nonlinear spectroscopy
PROFESSIONAL EXPERIENCE:
Graduate Research Assistant in the Richmond Laboratory, Department of Chem-
istry and Biochemistry, University of Oregon, Eugene (2018–Present)
General chemistry lecture sequence Head Teaching Assistant, Department of
Chemistry and Biochemistry, University of Oregon, Eugene (2020–2021)
Presidential Undergraduate Research Scholars (PURS) Co-instructor, Depart-
ment of Chemistry and Biochemistry, University of Oregon, Eugene (2020–2021)
General chemistry laboratory sequence Head Teaching Assistant, Department
of Chemistry and Biochemistry, University of Oregon, Eugene (2019–2020)
General chemistry laboratory sequence Teaching Assistant, Department of Chem-
istry and Biochemistry, University of Oregon, Eugene (2017–2018)
GRANTS, AWARDS, AND HONORS:
Keana Fellowship, University of Oregon, 2021–2022
Graduate Student Award for Excellence in the Teaching of Chemistry, Univer-
sity of Oregon, 2019–2020
v
PUBLICATIONS:
Tran, E., Richmond, G. L. “Interfacial steric and molecular bonding effects con-
tributing to the stability of neutrally charged nanoemulsions” Langmuir, 2021
Tran, E., Mapile, A. N., Richmond, G. L. “Peeling back the layers: Investigating
the effects of polyelectrolyte layering on surface structure and stability of oil-in-water
nanoemulsions” Journal of Colloid and Interface Science, 2021 599, 706-716.
Tran, E., Carpenter, A. P., Richmond, G. L. “Probing the Molecular Structure of
Coadsorbed Polyethylenimine and Charged Surfactants at the Nanoemulsion Droplet
Surface” Langmuir, 2020 36, 31, 9081-9089.
Carpenter, A. P., Tran, E., Altman, R. M., Richmond, G. L. “The Formation
and Surface Stabilizing Contributions to Bare Nanoemulsions Created with Negligible
Surface Charge” Proceedings of the National Academy of Sciences, 2019 116 (19)
9214-9219.
vi
ACKNOWLEDGEMENTS
The most valuable thing PhD school taught me is that nothing great is ever truly
accomplished alone. My deepest appreciation goes to each and every one of you:
To the Richmond Lab: Dr. Geri Richmond–Thank you for pushing me to be
a more confident scientist. You have shown me that strong and accomplished women
in science need not fit a single mold. Dr. Larry Scatena–I am amazed with the
amount of work you accomplish while prioritizing your work/life balance. I am happy
and proud to have you as an interim advisor. Thank you for stepping up and giving us
your all. Dr. Fred Moore–Despite everything, you have always brought so much
positivity and joy to the lab. We could all use a kind heart like yours. Priscilla
Lewis–You are the backbone of the lab. Without you, everything would be in sham-
bles. Thank you for taking incredible care of us. To all Richmond lab members past
and present–Thank you for all the support, mentorship, and friendly SFG banter. I
also thank the Department of Energy for the funding that made this work possible.
To my committee: Dr. Cathy Wong, Dr. Jim Prell, and Dr. Stephanie
Majewski–Thank you for believing in me and in my work, especially when I strug-
gled to believe in anything at all.
To the chemistry department: Dr. Deb Exton–I don’t think many people
truly understand and appreciate how challenging teaching general chemistry is. I will
always admire your dedication to your students. Thank you for supporting me in all
my teaching efforts. Dr. Jeff Cina–I have enjoyed each and every one of our con-
versations and how unapologetically honest you are with your thoughts and advice.
Christi Mabinuori–You have done so much for our department, have always been
in our corner, and have never expected anything in return. I (and likely so many
others) cannot thank you enough. Dr. Mark Lonergan–Despite having so much
on your plate as department head, you have always made time to talk to me and
worked hard to support all our DEI efforts. Your thoughtfulness and commitment to
the chemistry department is inspiring.
To my dearest friends: Jack Maurer–I’m not sure if you remember, but you
were the first person I met in our cohort. We were checking in at the rec center for
CPR training. You are one of the few people I know who can talk about science so
passionately without making me feel insecure about my own knowledge and abilities.
vii
Not many people have that skill. Thank you for being such a great friend and mak-
ing us bomb food in astronomical portions (i.e., the meatball the size of my head).
Becca Schaefer–I admire your creativity, talent, honesty, and the fun activities
(with prizes!) you prepare for us when we’re all together. Ashley Mapile–Between
working with you in the lab, serving on the DEI committee, conversations about
the good, bad, and ugly, and all of our outdoor excursions, you have become one
of my closest friends. Your unshakeable kindness motivates me to believe that (just
maybe) humans aren’t the absolute worst. Lola Schaefer Maurer–I’ve said it
before and I’ll say it again: I like dogs more than I like humans. Your zoomies
make my day. Lydia Dysart–I value your commitment to our friendship and keep-
ing in touch, especially when we’re 2,500 miles apart. Morgan Mann–Thank you
for being a source of never-ending love and support. To Cassidi Howard, Phil
Lotshaw,Marc Foster, Deepika Sundarraman, Claire Albrecht, Dylan
Heussman, and Bia Assunção–the past five years wasn’t always a smooth ride,
but having you all around made the bumps much more manageable.
To Tiemo Landes: Danke for providing me with your relentless patience, under-
standing, and so much more, even during my worst days. You are my best friend
and I appreciate all our adventures together. Also, I’m looking forward to getting
my honorary PhD in physics after the countless times I’ve listened to you talk about
“entangled two-photon absorption.”
To the friends I call family: Lynne Truong–No words will ever truly capture
how I appreciate you. You’ve been my rock since we were kids. For always showing
up and never being too busy for me, cảm ơn. Sarah Molina–No matter how busy
our lives are, you’ve always kept me in your thoughts and never stopped telling me
how proud you are of me. You are one of the strongest, most resilient people I know.
For all your encouragement and love since we were teenagers, maraming salamat.
To my family: Although the generational, cultural, and language barriers will al-
ways keep us from seeing eye-to-eye, everything I’ve done up to this point is for you.
To Dr. Kathleen Robins: Everything I’ve done up to this point is because of
you. Thank you for the unwavering support and guidance I never knew I needed. If
it weren’t for you, I would not be here writing this today, because I would not have
even gotten a Bachelor’s degree, let alone a doctorate. Truly.
viii
A final acknowledgement goes to my Vietnamese-American community. This is
long overdue, but it wasn’t until a couple years ago that I felt proud of my Vietnamese
culture and where I came from. My Vietnamese name is Ngo.c Bích and I was always
ashamed of it (it’s still a constant struggle for me), and I trust that people can imagine
why. The Model Minority Myth paints all Asian-Americans in the same light and sets
the expectation that we can all be successful if we just work hard enough. But the
truth is that success does not revolve around work alone. Success relies on resources,
guidance, support, and luck–all things many Vietnamese immigrants do not have.
I used to be consumed with anger. I was angry that my parents didn’t speak
English, didn’t help me with homework, didn’t teach me how to ride a bike, or
swim, or ski, didn’t expose me to the recreational activities that I now love, didn’t
understand nor appreciate what a PhD is, let alone the science. But I’ve learned that
those aren’t truly the reasons that made me upset. I shouldn’t have been upset at
my parents for the things they didn’t do, and I should have been more grateful for
the things they did do. To leave behind your home, comfort, and all of your family to
settle in foreign land with unfamiliar faces takes an enormous amount of courage–an
amount of courage I wish I had.
Instead, I was upset because people didn’t see and understand the hardships of
a Vietnamese immigrant, yet casually assumed that “Asians are good at math and
intelligent and successful” and expected the most from us. I encourage you all to
think about the number of times you’ve seen a Vietnamese name on a publication, as
the face of a recreational sport, the lead of a research group or project. The reason
why there is so few of us is because we are from a country that was devastated by
colonization and war, leaving most of my family living well below the poverty line.
When those are your circumstances, you can work as hard as you want, and will
never be where most are. So this acknowledgement goes to Vietnamese-Americans
who have defied all odds and paved the way for my journey and my story.
Whatever I do next, I will pay it forward.
ix
TABLE OF CONTENTS
Chapter Page
I. BACKGROUND AND THEORY . . . . . . . . . . . . . . . . . . . . . . 1
Colloid and Interfacial Chemistry . . . . . . . . . . . . . . . . . . . . 1
Vibrational Sum Frequency Spectroscopy at Planar Interfaces . . . . 2
Vibrational Sum Frequency Scattering Spectroscopy at Droplet Interfaces 5
II. EXPERIMENTAL DETAILS . . . . . . . . . . . . . . . . . . . . . . . 7
Nanoemulsion Preparation . . . . . . . . . . . . . . . . . . . . . . . . 7
Dynamic Light Scattering . . . . . . . . . . . . . . . . . . . . . . . . 9
ζ-potential Measurements . . . . . . . . . . . . . . . . . . . . . . . . 10
Pendant Drop Tensiometry . . . . . . . . . . . . . . . . . . . . . . . . 11
VSFSS Laser Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
VSFS Laser Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Sum Frequency Spectral Fitting and Analysis . . . . . . . . . . . . . 15
III. INTERFACIAL BEHAVIOR OF pH-TUNABLE POLYETHYLENIMINE 16
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
PEI pH-dependent Surface Activity . . . . . . . . . . . . . . . . . . . 18
Effect of Charged Surfactants on PEI Adsorption . . . . . . . . . . . 19
Effect of Charged Surfactants on PEI Surface Structure . . . . . . . . 22
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
IV. POLYELECTROLYTE LAYERING ON DROPLET SURFACES . . . 33
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
DTAB/PSS complexes on nanoemulsion stabiity . . . . . . . . . . . . 35
The effects of cationic PEI: Added electrosterics . . . . . . . . . . . . 40
The effects of nonionic PEI: Added hydrophobicity and sterics . . . . 44
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
V. NANOEMULSION STABILIZATION VIA STERIC EFFECTS . . . . 50
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Steric Stabilization of Nanoemulsions via Poly(N-Vinylacetamide) . . 51
DLVO and Extended DLVO Theory . . . . . . . . . . . . . . . . . . . 55
Interfacial Bonding Effects Contributing to Droplet Stability . . . . . 61
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
x
Chapter Page
VI. ZWITTERIONIC SURFACTANTS AND CO-STABILIZERS . . . . . 69
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Surface activity of DDAPS with various co-additives . . . . . . . . . . 71
Adsorption and net ordering of DDAPS at a planar oil/water interface 74
Zwitterionic DDAPS on nanoemulsion formation . . . . . . . . . . . . 78
DDAPS in the presence of salts at the droplet interface . . . . . . . . 80
Synergy between DDAPS and co-surfactants at the droplet interface . 82
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
VII. SUMMARY AND OUTLOOK . . . . . . . . . . . . . . . . . . . . . 86
APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
xi
LIST OF FIGURES
Figure Page
3.1 Molecular structures of PEI, SDS, and DTAB . . . . . . . . . . . . . 16
3.2 Surface pressure measurements of PEI, PEI/SDS, and PEI/DTAB as
a function of pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3 ζ-potential measurements of nanoemulsions stabilized by SDS alone
and d-SDS/PEI as a function of pH . . . . . . . . . . . . . . . . . . . 19
3.4 ζ-potential measurements of nanoemulsions stabilized by DTAB alone
and d-DTAB/PEI as a function of pH . . . . . . . . . . . . . . . . . . 21
3.5 VSFSS measurements of the CH stretching region of nanoemulsions
stabilized by various combinations of PEI and SDS under acidic con-
ditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.6 VSFSS measurements of the CH stretching region of nanoemulsions
stabilized by various combinations of PEI and SDS under basic conditions 25
3.7 VSFSS measurements of the CH stretching region of nanoemulsions
stabilized by various combinations of PEI and DTAB at pH 3 . . . . 27
3.8 VSFSS measurements of the CH stretching region of nanoemulsions
stabilized by various combinations of PEI and DTAB under basic con-
ditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.9 Cartoon illustration of PEI/SDS and PEI/DTAB coating nanoemul-
sions under acidic and basic conditions . . . . . . . . . . . . . . . . . 30
4.1 Molecular structures of DTAB, PSS, and PEI . . . . . . . . . . . . . 34
4.2 Size distribution measurements, average diameters, PDIs, and ζ-potentials
of nanoemulsions stabilized by DTAB and PSS . . . . . . . . . . . . . 35
4.3 Photograph images of nanoemulsion samples stabilized by various com-
binations of PSS, DTAB, and PEI . . . . . . . . . . . . . . . . . . . . 36
4.4 VSFSS spectra of the alkane CH and aromatic CH stretching regions
of nanoemulsions stabilized by DTAB and PSS . . . . . . . . . . . . . 38
4.5 Size distribution measurements, average diameters, PDIs, and ζ-potentials
of nanoemulsions stabilized by DTAB, PSS, and PEI at pH 3 . . . . . 41
4.6 VSFSS spectra of the alkane CH and aromatic CH stretching regions
of nanoemulsions stabilized by d-DTAB, PSS, and PEI at pH 3 . . . 43
4.7 Size distribution measurements, average diameters, PDIs, and ζ-potentials
of nanoemulsions stabilized by DTAB, PSS, and PEI at pH 10 . . . . 45
4.8 VSFSS spectra of the alkane CH and aromatic CH stretching regions
of nanoemulsions stabilized by d-DTAB, PSS, and PEI at pH 10 . . . 46
xii
Figure Page
4.9 Summary illustration of macroscopic and molecular-level studies of
polymer multilayering on nanoemulsion surfaces. . . . . . . . . . . . . 48
5.1 Molecular structures of SDS and PNVA, along with diameter and ζ-
potential measurements of nanoemulsions stabilized by SDS and PNVA 51
5.2 ζ-potential measurements of nanoemulsions stabilized by SDS and PNVA
prepared using thoroughly cleaned versus “insufficiently cleaned” glass-
ware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.3 Diameters, PDIs, and ζ-potentials of nanoemulsions stabilized by SDS
and PNVA as a function of time . . . . . . . . . . . . . . . . . . . . . 54
5.4 Theoretical interaction potential diagrams calculated using standard
and extended DLVO theory . . . . . . . . . . . . . . . . . . . . . . . 57
5.5 Experimental interaction potential diagrams calculated using standard
and extended DLVO theory for nanoemulsions stabilized by SDS alone
and SDS/PNVA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.6 VSFSS measurements of nanoemulsions stabilized by SDS and PNVA
in the CH, SO, and CO stretching regions . . . . . . . . . . . . . . . 61
6.1 IFT of DDAPS and salts (NaCl and MgCl2) as a function of time . . 71
6.2 IFT of DDAPS and co-surfactants (SDS and DTAB) as a function of
time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.3 VSFS measurements of DDAPS alone at the CCl4/water interface in
the CH stretching region . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.4 VSFS measurements of DDAPS and various co-additives at the CCl4/water
interface in the CH stretching region . . . . . . . . . . . . . . . . . . 76
6.5 d+/r+ ratios of DDAPS and various co-additives at both planar and
droplet interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.6 Diameters and ζ-potential measurements of nanoemulsions stabilized
by DDAPS and various co-additives . . . . . . . . . . . . . . . . . . . 79
6.7 VSFSS measurements of nanoemulsions stabilized by DDAPS and salts
at the hexadecane/water interface in the CH stretching region . . . . 81
6.8 VSFSS measurements of nanoemulsions stabilized by DDAPS and co-
surfactants at the hexadecane/water interface in the CH stretching
region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
xiii
LIST OF TABLES
Table Page
5.1 Fitting parameters for spectra of SDS/PNVA stabilized nanoemulsions
in the C=O stretching region, corresponding to Figure 5.6e. Note that
the parameters for the arbitrary background peaks have been omitted
for clarity. Complete fit parameters can be found in Table A.17. . . . 66
6.1 Characteristic CH resonances at an oil/water interface.[12, 185] . . . 74
A.2 Fitting parameters and d+/r+ ratios for h-SDS and PEI/h-SDS spectra
shown in Figure 3.5b. Nanoemulsions were either stabilized with 1 mM
h-SDS or 5.2 mM PEI and 1 mM h-SDS at constant pH 3. . . . . . . 87
A.3 Fitting parameters for PEI/d-SDS spectra shown in Figure 3.6a for pH
7.5, 8, 10, and 11. PEI and d-SDS concentrations were fixed at 5.2 mM
and 0.1 mM, respectively. . . . . . . . . . . . . . . . . . . . . . . . . 88
A.4 Fitting parameters for PEI/d-SDS and PEI/h-SDS spectra shown in
Figure 3.6b for pH 11. PEI and d-SDS/h-SDS concentrations were
fixed at 5.2 mM and 1 mM, respectively. . . . . . . . . . . . . . . . . 89
A.5 Fitting parameters for PEI/d-DTAB spectra shown in Figure 3.8a for
pH 8, 10, and 11. PEI and d-DTAB concentrations were fixed at 5.2
mM and 0.1 mM, respectively. . . . . . . . . . . . . . . . . . . . . . . 90
A.6 Fitting parameters for PEI/d-DTAB and PEI/h-DTAB spectra shown
in Figure 3.8b for pH 11. PEI and d-DTAB/h-DTAB concentrations
were fixed at 5.2 mM and 1 mM, respectively. . . . . . . . . . . . . . 91
A.7 Fit parameters for VSFSS spectra of nanoemulsions stabilized by either
0, 0.1, or 1 mM d-DTAB, in conjunction with 5 mM PSS, shown in
Figure 4.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
A.8 Fit parameters for VSFSS spectra of nanoemulsions stabilized by 0.1
mM d-DTAB, 5 mM PSS, and either 0, 1, or 10 mM cationic PEI at
pH 3, shown in Figure 4.6). . . . . . . . . . . . . . . . . . . . . . . . 93
A.9 Fit parameters for VSFSS spectra of nanoemulsions stabilized by 0.1
mM d-DTAB, 5 mM PSS, and either 0, 1, or 10 mM nonionic PEI at
pH 10, shown in Figure 4.8. . . . . . . . . . . . . . . . . . . . . . . . 94
A.10 Parameters used to calculate the Hamaker constant for hexadecane-in-
water nanoemulsions from Equation 27. . . . . . . . . . . . . . . . . . 95
A.11 Concentrations of SDS used and the corresponding Debye length cal-
culated from Equation 28. . . . . . . . . . . . . . . . . . . . . . . . . 95
xiv
A.12 Parameters used to calculate the interaction pair potentials in Fig-
ure 5.5d using extended DLVO theory for nanoemulsions stabilized by
SDS/PNVA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
A.13 Fitting parameters for spectra of h-SDS/PNVA stabilized nanoemul-
sions in the C–H stretching region, corresponding to Figure 5.6a. . . . 96
A.14 Fitting parameters for spectra of d-SDS/PNVA stabilized nanoemul-
sions in the C–H stretching region, corresponding to Figure 5.6b. . . . 97
A.15 Fitting parameters for spectra of SDS (alone) stabilized nanoemulsions
in the S=O stretching region, corresponding to Figure 5.6c. . . . . . . 97
A.16 Fitting parameters for spectra of SDS/PNVA stabilized nanoemulsions
in the S=O stretching region, corresponding to Figure 5.6d. . . . . . 98
A.17 Fitting parameters for spectra of SDS/PNVA stabilized nanoemulsions
in the C=O stretching region, corresponding to Figure 5.6e. . . . . . 98
A.18 Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS alone, corresponding to Figure 6.3. . . . . . . . . . 99
A.19 Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and NaCl, corresponding to Figure 6.4a. . . . . . . 100
A.20 Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and MgCl2, corresponding to Figure 6.4b. . . . . . 101
A.21 Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and SDS, corresponding to Figure 6.4c. . . . . . . . 102
A.22 Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and DTAB, corresponding to Figure 6.4d. . . . . . 103
A.23 Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and NaCl, corresponding to Figure 6.7a. . . . . . . . . . . . 104
A.24 Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and MgCl2, corresponding to Figure 6.7b. . . . . . . . . . . . 105
A.25 Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and d-SDS, corresponding to Figure 6.8a. . . . . . . . . . . . 106
A.26 Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and h-SDS, corresponding to Figure 6.8b. . . . . . . . . . . . 107
A.27 Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and d-DTAB, corresponding to Figure 6.8c. . . . . . . . . . . 108
A.28 Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and h-DTAB, corresponding to Figure 6.8d. . . . . . . . . . 109
xv
CHAPTER I
BACKGROUND AND THEORY
Colloid and Interfacial Chemistry
A colloid is a type of mixture in which particles with nanometer to micron diameters
are evenly distributed throughout an aqueous solution. In order to maintain colloidal
stability, surface-active agents (i.e., surfactants) and polymers are often used to mod-
ify the surfaces of colloids. Thus, the intersection of colloids and interfacial chemistry
is an area of research that provides necessary insights into controlling the properties,
functionality, and stability of colloidal systems.
Nanoemulsions are an example of a colloid. Oil-in-water nanoemulsions, specifi-
cally, are oil droplets measuring between 20-500 nm in diameter. The oil (dispersed
phase) is dispersed through an aqueous solution of emulsifiers (continuous phase) via
low- and high-energy processes.[1–3] Unlike two immiscible liquids—such as oil and
water—spontaneously forming a planar oil/water interface that is thermodynamically
stable, nanoemulsions are kinetically stable and will eventually phase separate over
time. However, based on the stabilizing agents, nanoemulsions can be stable for weeks,
months, and in some cases, years, making them advantageous for applications.[4–6]
Applications utilizing nanoemulsions include food science,[5, 7] cosmetics,[8] drug
delivery systems,[9–11] and material synthesis.[1] In each of these applications, na-
noemulsions play a variety of roles from improving the appearance of food to en-
capsulating and increasing the bioavailability of active pharmaceutical ingredients.
Their functionality and properties depend on environmental considerations such as
pH, salinity, and temperature, as well as the surfactants and polymers coating the
droplet surface. Thus, by developing a strong understanding of the interfacial chem-
istry occurring at the droplet surface, nanoemulsions can be designed for diverse
applications with specific properties and tunable behavior.
Liquid/liquid interfaces are notoriously challenging to study experimentally, es-
pecially in a way that provides molecular-level details on conformational structure.
Surface techniques such as X-ray photoelelectron spectroscopy require a vacuum and
is therefore incompatible for probing liquid/liquid interfaces. Other scattering tech-
niques such as neutron scattering and ellipsometery are able to provide information
regarding the thickness and composition of thin layers at an interface (including liq-
uid/liquid interfaces), but do not provide molecular-level information. Moreover,
1
none of these techniques are able to probe the surfaces of liquid droplets suspended
in another liquid medium. This dissertation sheds light on nanoemulsion surfaces
by providing molecular-level information using vibrational sum frequency scattering
spectroscopy, complemented by pendant drop tensiometry, dynamic light scattering,
and ζ-potential measurements.
In this dissertation, the contents of Chapters III, IV, and V have been published
with co-authors. Chapter VI contains work that is not yet published, but is currently
under review at the time of submission. Chapter VI also contains co-authors.
Vibrational Sum Frequency Spectroscopy at Planar Interfaces
Prior to discussing the details of probing a droplet liquid/liquid interface, it is nec-
essary to first review sum frequency generation at planar liquid/liquid interfaces.
Vibrational sum frequency spectroscopy (VSFS) is a powerful experimental tool for
probing interfaces because it is inherently surface-specific with chemical selectivity.[12,
13]
As light moves through a material, the electric field of that light exerts a force on
the valence electrons of the molecules making up the material. With low intensity,
non-coherent light (e.g., ambient light), the force exerted is small. In an isotropic
(uniform in all orientations and direction) medium, the induced electric dipole µ⃗ is
given as:
µ⃗ = µ⃗o + αE⃗, (1)
where µ⃗o is the permanent dipole of the material and α is the polarizability of the
electrons.
In condensed phase, the sum of all the molecular electric dipoles yields a dipole
moment per unit volume known as the bulk polarization P⃗ . Very few materials have
a permanent dipole, thus if µ⃗o is omitted, the bulk polarization can be described as
(1)
P⃗ = εoχ E⃗, (2)
where (1)χ is the macroscopic average of α, known as the first-order, linear suscep-
tibility. εo is vacuum permittivity. The induced dipole emits light that oscillates at
the same frequency as the incident electric field and describes linear processes such
as reflection and refraction.
For high intensity, coherent light (e.g., laser), the non-linearity of the material
response must be considered. This non-linearity is accounted for by additional terms
2
in the induced electric dipole given by
2 3
µ⃗ = µ⃗o + αE⃗ + βE⃗ + γE⃗ +⋯, (3)
where β and γ are the first- and second-order hyperpolarizabilities, respectively.
Again, if it is assumed that most materials do not have a permanent dipole moment
and omit µ⃗o, then the bulk polarization becomes
= (1) (2) (3)P⃗ P⃗ + P⃗ + P⃗ +⋯ (4)
= (1) (2) 2 (3) 3εo(χ E⃗ + χ E⃗ + χ E⃗ +⋯), (5)
where (2) and (3)χ χ are the second- and third-order non-linear susceptibilities, re-
spectivity. Like (1)χ and , (2) (3)α χ and χ are the macroscopic average of β and γ,
respectively.
The sum frequency component of the second-order non-linear polarization can be
expressed as
P⃗i(ωSF) = (2)χijk E⃗j(ωIR)E⃗k(ωvis), (6)
where E⃗j and E⃗k are the electric fields of the IR and visible pulse, respectively, and
(2)
χijk
is a third-rank tensor describing the material surface response. For sum frequency
generation, the induced second-order polarization will oscillate at the sum of the
incident IR and visible frequencies. Note that additional second-order non-linear
processes such as second harmonic generation and difference frequency generation
exist, but the focus here is placed on sum frequency generation.
The surface-specificity of the VSFS technique lies in the second-order non-linear
susceptibility, (2)χijk , which is a third-rank tensor containing 27 possible non-zero el-
ements. In centrosymmetric media, inversion symmetry requires the following rela-
tionship to be true:
(2) = (2) = (2)χijk χ−i−j−k −χijk (7)
This necessarily results in (2)χijk = 0. The outcome here is that sum frequency gener-
ation is strictly allowed only where inversion symmetry is broken. The interface of
two immiscible liquids is a primary example of where inversion symmetry is broken,
enabling VSFS to specifically probe the interface and ignore all bulk contributions.
Interfaces can be described with C∞v symmetry in the plane of the interface. By
applying the symmetry operators in the C∞v (or C4v for simplicity) point group to
each element of (2)χijk , the result is that out of the 27 tensor elements, only 7 is non-zero.
Further, of the 7 non-zero tensor elements, only 4 are unique because the interface is
3
isotropic within the plane of the interface (i.e., x = y). Ultimately, VSFS probes the
following tensor elements: (2) = (2) , (2)χxxz χyyz χxzx =
(2) (2) (2) (2)
χyzy, χzxx = χzyy, and χzzz.
Experimentally, the polarization of the IR, visible, and generated SF light can be
selected to probe specific elements of (2)χijk . The polarization denoted as S or P are
with respect to the plane of incidence (i.e., the plane in which the beams are travel-
ing) with S being perpendicular and P being parallel. The polarization schemes and
tensor elements probed with VSFS are summarized below:
Polarization scheme (2)χijk element probed
SSP (2)χxxz
SPS (2)χxzx
PSS (2)χzxx
PPP (2), (2) (2) (2)χzzz χyyz, χyzy, χzyy
(2)
χijk is a macroscopic quantity related to the molecular hyperpolarizability,
(2)
βabc, by
(2) (2)
χijk = N ⟨TiaTjbTkc⟩ βabc (8)
where N is the density of molecules, Tmn represents the coordinate transformation
from the molecular frame (a, b, c) to the laboratory frame (i, j, k), and the ⟨⟩ de-
notes averaging over all molecular orientations. The molecular hyperpolarizability is
expressed as
(2) 1 M= ab
Ac
βabc 2h̵ ( (9)ων − ωIR − iΓ)
where Mab and Ac are the Raman and IR transition moments, respectively, which ne-
cessitates that all sum frequency-active vibrational resonances must be both Raman-
and IR-active. ων is the frequency of the vibrational resonance, ωIR is the frequency
of the IR beam, and −1Γ is the relaxation time of the excited vibrational resonance. It
can be seen that as ωIR is tuned to be resonant with ων , the term (ων −ωIR)→ 0 and
the magnitude of (2)χijk increases. Thus, the sum frequency response is enhanced at the
resonant frequency. Measuring the sum frequency response as a function of IR fre-
quencies ultimately yields a vibrational spectrum of only surface-adsorbed molecules,
and all interference from bulk contributions are ignored.
4
Vibrational Sum Frequency Scattering Spectroscopy at Droplet Interfaces
While traditional planar sum frequency generation has been employed to study inter-
faces since 1987, sum frequency scattering was developed by Roke et. al. relatively
recently in 2003.[13, 14] Similar to the description above, centrosymmetry is broken
at the surfaces of nanoemulsions and thus, a scattered sum frequency response can be
generated to provide molecular-level information on suspended droplets. At the far
field, sum frequency photons interfere to generate a scattering pattern. The scattered
sum frequency response is given by
(2)
E0,i ∝ Γi,j,k(
(2)
χ ,R, θ) ∶ E1,jE2,k (10)
where (2)Γ is the effective particle susceptibility, which depends on the non-linear sec-
ond order susceptibility (2)χ , particle radius R, and scattering angle θ. The complete
derivation for the expressions describing (2)Γ can be found elsewhere.[14–18] Our focus
here is to briefly describe the relation between (2)Γ and (2)χ which is given by
⎛Γ1⎞ ⎛2F1 − 5F2 0 0 0 ⎞⎛χ1⎞
⎜⎜⎜ ⎟⎟ ⎜⎜ ⎟⎜⎜ ⎟⎜
⎟⎟
⎜⎜⎜
Γ2⎟⎟⎟ ⎜⎜⎜ F= 2 2F1 0 0 ⎟⎜χ2⎟⎟⎟ ⎜ ⎟⎟
⎟⎜⎜⎜ ⎟⎟⎟ . (11)
⎜⎜Γ3⎟⎟ ⎜⎜⎜ F2 0 2F 0 ⎟⎟⎟⎜⎜1 ⎜χ ⎟3⎟⎟
⎝Γ ⎠ ⎝4 F2 0 0 2F ⎠⎝ ⎠1 χ4
Under the Rayleigh-Gans-Debye approximation for second-order scattering, F1 and
F2 are form factor functions that are expressed as
( ) (sin(qR) cos(qR)F1 qR = 2πi ( ) − ) (12)qR 2 qR
( ) ( sin(qR) cos(qR) sin(qR)F2 qR = 4πi 3 ( − 3qR)4 ( −qR)3 (qR) ) . (13)2
Note that q = ∣∣q⃗∣∣ = 2∣k⃗o∣ sin( θ) and R is the particle radius.2
5
The resulting scattered electric field amplitudes are given by
θ θ θ (2)
Eppp ∝ cos ( ) cos ( − α) cos ( − α + β)Γ2 2 2 1
+ cos(θ − α + β)Essp
+ cos(θ − α)Esps
+ cos(β)Epss (14)
θ (2)
Essp ∝ cos ( − α)Γ2 (15)2
(θ (2)Esps ∝ cos − α + β)Γ3 (16)2
θ (2)
Epss ∝ cos ( )Γ4 . (17)2
The angle between the incident IR and visible beams is β and the angle between the
IR wavevector and the phase-matched wavevector is α. Ultimately, by careful design
of the experimental and detection geometry, the polarization combinations PPP, SSP,
SPS, and PSS for scattering experiments are analogous to planar experiments in that
they probe equivalent tensor elements.[14, 15, 17]
6
CHAPTER II
EXPERIMENTAL DETAILS
Nanoemulsion Preparation
All nanoemulsions prepared for this work were done via ultra-sonication with a Bran-
son Sonifier 250 sonicator. The materials and recipe for each nanoemulsion system
vary depending on each project, which are outlined below. In general, all nanoemul-
sion samples were sonicated with an output control of ∼3 for 3 minutes. For each
formulation, the final nanoemulsion concentration is 1% v/v. That is, 1% v/v hex-
adecane is sonicated into the aqueous phase. All glassware used was soaked for a
minimum of 12 hours in a solution of concentrated sulfuric acid and AlNOCHROMIX
oxidizer. After soaking, each piece of glassware was thoroughly rinsed with ultra-pure
water (18.2 MΩ-cm resistivity) and dried in a 140◦C oven. This cleaning routine is
important for removing surface-active impurities and contaminants that may interfere
with the spectroscopic measurements.[19] It is implied that each recipe below may use
either hydrogenated or deuterated versions of a chemical, whichever is necessary for
the experimental conditions. The specific chemical used is specified in each chapter.
Chapter III. Chemicals used: Hexadecane, sodium dodecyl sulfate (SDS), dode-
cyltrimethylammonium bromide (DTAB), polyethylenimine (PEI), HCl, and NaOH.
This chapter contains nanoemulsions stabilized by PEI/surfactants and surfactants
alone. For PEI/SDS-stabilized nanoemulsions, a stock solution of nanoemulsions was
made by ultra-sonicating 2% v/v hexadecane into an aqueous solution containing
twice the desired concentration of SDS. Separately, a PEI stock solution was also
prepared at twice its desired concentration. When equal volumes of the stock 2% v/v
SDS-stabilized nanoemulsions and stock PEI solution were mixed, a final nanoemul-
sion sample with 1% v/v hexadecane stabilized by the target concentration of SDS
and PEI is obtained. The same method was used for preparing DTAB/PEI-stabilized
nanoemulsions.
For SDS-stabilized nanoemulsions, a similar approach was taken. A stock solution
of nanoemulsions was made by ultra-sonicating 2% v/v hexadecane into an aqueous
solution containing twice the desired concentration of SDS. By mixing equal volumes
of the 2% v/v SDS-stabiized nanoemulsions and water, a final 1% v/v nanoemulsion
sample is made with the desired SDS concentration. The same procedure was used
7
to prepare DTAB-stabilized nanoemulsions.
For pH dependent studies, small amounts of NaOH or HCl were added to the PEI
stock solutions made in H2O to adjust the pH accordingly. MilliporeSigma MColor-
pHast pH strips were used to measure the pH of nanoemulsion samples. Analogously,
for stock solutions made in D2O, small amounts of NaOD or DCl were added to adjust
the pD of the deuterated solution. We note that although we do not measure pD,
but rather pH in a D2O solution, it has been reported that a constant value of 0.45 is
the difference between measured pH in D2O versus pD.[20] Thus, it is assumed that
this fixed value will not affect the trends in which conclusions were drawn from.
Chapter IV. Chemicals used: Hexadecane, dodecyltrimethylammonium bromide
(DTAB), polyethylenimine (PEI), poly(styrene sulfonate) (PSS), HCl, and NaOH.
This chapter contains nanoemulsions that are stabilized by DTAB alone, and DTAB
in conjunction with either one or two polymer layers: PEI (under different pH condi-
tions) and PSS. For nanoemulsions stabilized by only DTAB, a stock nanoemulsion
sample is prepared by ultra-sonicating 2% v/v hexadecane with an aqueous solution
containing twice the desired concentration of DTAB. By mixing equal volumes of the
2% v/v DTAB-stabilized nanoemulsions with water, a final 1% v/v DTAB-stabilized
nanoemulsion sample is obtained with the desired DTAB concentration.
For DTAB/PSS-stabilized nanoemulsions, a 2% v/v stock nanoemulsion sample is
prepared with twice the desired concentration of DTAB (as described above). Sep-
arately, a stock PSS solution is prepared at twice its desired concentration. Upon
mixing equal volumes of the nanoemulsion sample and the PSS solution, a final 1%
v/v nanoemulsion sample is obtained with the desired DTAB and PSS concentrations.
For DTAB/PSS/PEI-stabilized nanoemulsions, a similar approach is used with
slight modifications. Here, a stock nanoemulsion sample is prepared by ultra-sonicating
2% v/v hexadecane with an aqueous solution containing twice the desired concentra-
tions of DTAB and PSS. Separately, acidic (pH 3) and basic (pH 10) PEI solutions
at twice their desired concentrations are prepared and set aside. By mixing equal
volumes of the 2% v/v stock nanoemulsions stabilized by DTAB/PSS with either
the acidic or basic aqueous PEI solutions, a final 1% v/v nanoemulsion solution is
obtained and stabilized by the appropriate polymer/surfactant combination at the
desired pH and concentrations. Again, MilliporeSigma MColorpHast pH strips were
used to measure the pH of the PEI solutions, as well as the final nanoemulsion samples.
Chapter V. Chemicals used: Hexadecane, sodium dodecyl sulfate (SDS), and poly(N-
8
vinylacetamide) (PNVA).
This chapter contains nanoemulsions stabilized by a combination of SDS and PNVA,
as well as SDS alone. To prepare the SDS/PNVA-coated nanoemulsions, 2% v/v hex-
adecane was sonicated with an aqueous solution containing SDS at twice the desired
concentration. Separately, a PNVA solution was also prepared at twice the desired
concentration. Upon mixing equal volumes of the nanoemulsion sample and the
PNVA solution, a final 1% v/v nanoemulsion sample was obtained with the desired
surfactant and polymer concentrations.
For SDS-stabilized nanoemulsions, a stock solution of nanoemulsions was made
by ultra-sonicating 2% v/v hexadecane into an aqueous solution containing twice the
desired concentration of SDS. By mixing equal volumes of the 2% v/v SDS-stabilized
nanoemulsions and water, a final 1% v/v nanoemulsion sample is made with the de-
sired SDS concentration.
Chapter VI. Chemicals used: Hexadecane, sodium dodecyl sulfate (SDS), dode-
cyltrimethylammonium bromide (DTAB), dodecyl-N,N-dimethyl-3-ammonio-1-
propanesulfonate (DDAPS), NaCl, and MgCl2.
This chapter contains nanoemulsions prepared with DDAPS in conjunction with one
of the following: SDS, DTAB, NaCl, or MgCl2. Unlike previous projects, these emul-
sions were created by ultra-sonicating 1% v/v hexadecane into an aqueous solution
containing the chemical components of interest at the desired concentrations (as op-
posed to twice the desired concentration). Because DDAPS alone cannot form stable
nanoemulsions, it must be mixed with the other co-additives prior to sonication.
Dynamic Light Scattering
Sample size distribution, average (Z-average) diameter, and the polydispersity index
(PDI) were measured with a Malvern Zetasizer Nano via dynamic light scattering
(DLS) measurements.[21–23] Using DLS, the Brownian motion of the nanoemulsions
were measured by illuminating the sample with a 633 nm laser and detecting the
scattered light at a back-scatter detection angle of 173◦. This detection set-up is
advantageous for a few reasons:
1. By measuring the back-scattered light, the incident beam does not need to
travel through the entire sample volume, thus shortening the path length of the
sample. With a shorter path length, samples that are more concentrated may
still be measured before reaching the saturation threshold.
9
2. Reduces the probability for multiple scattering events to occur, which also allows
for higher concentrations to be measured.
3. Minimizes contributions from larger contaminants and particulates. Larger par-
ticles will scatter light primarily in the forward direction. Thus, by measuring
the back-scatter, the probability of detecting light scattering off contaminants
is reduced.
The fluctuations in intensity of the light scattered by the sample is analyzed with
a correlation function, G(τ), given by
G(τ) = ⟨I(t)T (t + τ)⟩ , (18)
where I(t) is the intensity at time zero and I(t+τ) is the intensity at some time later.
A cumulant analysis fits the log of the intensity correlation function to a third-order
polynomial to obtain the decay rate, Γ (i.e. the first cumulant):
Γ = 2q D, (19)
where q is the scattering vector and D is the diffusion coefficient. The Stokes-Einstein
formula is used to relate the diffusion coefficient to the hydrodynamic radius, R:
kBT
D = , (20)
6πηR
where kB is the Boltzmann constant, T is temperature, and η is the solvent viscosity.
This cumulant analysis provides a cumulant mean or a “Z-average” particle size.
This mean is considered to be the most stable size value obtained by DLS, and is
the size to report if a number is required for quality control. This Z-average size
is only comparable to other size measurement techniques if the sample is spherical,
monodisersed, and monomodal.
ζ-potential Measurements
ζ-potential (ZP) measurements were determined with a Malvern Zetasizer Nano to
characterize the potential at the slip plane boundary of nanoemulsions, which is used
as an indicator of surface charge and emulsion stability.[23, 24] In a colloidal system,
there exists a layer of tightly bound ions referred to as the Stern layer. Ions of opposite
charge are attracted to the Stern layer and form a layer of loosely bound ions, called
10
the diffuse layer. As a voltage is applied, causing charged particles to move, solvent
molecules and ions within the diffuse layer will move with the particles, while ions in
the Stern layer remain stagnant. The boundary separating the Stern layer and the
diffuse layer is called the slip plane.
Using Laser Doppler Velocimetry, the change in frequency of back-scattered light,
∆f , can be measured upon illuminating the sample with a 633 nm laser. The change
in frequency of the back-scattered light is related to the particle speed, ν, by:
2ν sin ( θ)
= 2∆f , (21)
λ
where θ is the scattering angle and λ is the laser wavelength.
For particles moving at a constant speed under a uniform electric field, E, the
particle electrophoretic mobility, µ, is given by:
ν
µ = (22)
E
By measuring∆f , one can determine the particle speed and then the electrophoretic
mobility. Finally, applying the Smoluchowski approximation, the electrophoretic mo-
bility and the electrokinetic potential at the slip plane (i.e., the ζ-potential), ϕζ , can
be determined:
ϕζεε= oµ η , (23)
with ε as the dielectric constant of the continuous phase, εo as the vacuum permit-
tivity, and η as the solvent viscosity.
Pendant Drop Tensiometry
Interfacial tension (IFT) measurements were conducted using a KSV Instruments
Attension Theta pendant drop tensiometer. The experimental set-up is dependent
on the oil phase, which is either carbon tetrachloride (CCl4) or hexadecane, for the
studies detailed in this dissertation. For IFT measurements using CCl4 as the oil
phase (which is more dense than water), a droplet of aqueous solution containing the
chemical system of interest (e.g., surfactants, polymers, salts, etc.) at the desired
concentrations was suspended in a quartz cuvette of CCl4. For IFT measurements
using hexadecane as the oil phase (which is less dense than water), a droplet of
hexadecane is suspended in a cuvette containing the aqueous chemical system. In both
cases, a Hamilton Gastight syringe with a hooked needle (∼0.771 mm in diameter)
11
was used to create the droplet.
The pendant drop tensiometer illuminates the cuvette such that a camera is able
to capture images of the droplet silhouette. For all IFT experiments, the first image
was taken immediately upon creating the droplet and then subsequently after at a
specified fixed rate for a specified number of frames. By fitting the droplet silhouette
of each frame to mathematical models derived from the Laplace-Young equation, the
IFT was calculated using the tensiometry software. The error bars included in all
IFT traces shown herein represent the standard deviation of at least three different
scans measured on different days.
Tensiometry data can be reported and visualized in different ways. In the work
discussed herein, there are reports of surface pressure (SP) values as well as IFT over
time, both in units of mN/m. In Chapter 3, SP is reported which is the IFT value of
a clean CCl4/water interface minus the equilibrated IFT value of the chemical system
of interest. Thus, the more surface active a system is, the greater the SP value. For
these experiments, the measurement continues until the IFT value reaches a plateau
(i.e., the equilibrium IFT). Alternatively, for highly surface active systems where a
plateau is not observed within a reasonable time frame, the IFT value at ∼8 hours
is used. In Chapter 5, IFT values over time is reported, which shows the evolution
of IFT. Here, after the first image of the droplet is taken, each subsequent image is
taken at a rate of 1 frame per minute for a total of 60 minutes. With this approach,
although some chemical systems will not reach their equilibrated IFT value by the
end of 60 minutes, one can compare the shape of the traces over time.
VSFSS Laser Set-Up
For the vibrational sum frequency scattering spectroscopy (VSFSS) experiments con-
ducted in the Richmond lab, a Ti:Sapphire Libra regenerative amplifier laser from
Coherent generates a fundamental 800 nm beam that becomes the IR and visible
beams used for sum frequency generation (SFG). Within the amplifier is a Vitesse
(Coherent) oscillator and an Evolution pump laser (Coherent). The Vitesse oscilla-
tor outputs the 800 nm seed beam at an 80 Mhz repetition rate. The seed beam is
stretched temporally via a diffraction grating and then enters the amplification cavity.
The Evolution pump laser outputs a ∼15 W 532 nm beam, which was used to amplify
the seed beam. The amplified beam exits the amplification cavity, travels through a
compressor, and then exits the Libra amplifier with femtosecond pulse widths at a 1
kHz repetition rate and a power of ∼3 W.
12
The now amplified seed beam travels into an OPERA-SOLO 2 (Light Conversion)
optical parametric amplifier (OPA), where a portion of the beam immediately exits
the OPA to the experimental line of the VSFSS set-up, while the remaining portion
is used to generate the IR beam at the appropriate wavelengths needed to probe
specific vibrational resonances. The 800 nm seed beam that is allowed to travel
through the OPA is split once more, where ∼5% of the beam is sent to a saphhire
crystal to generate white light. This is the pre-amplification step, where the white
light is spatially chirped and overlapped with the 800 nm seed beam inside a Beta
barium borate (BBO) non-linear crystal. This step generates a signal and idler beam
via optical parametric generation. The generated signal beam is overlapped with the
remaining 800 nm seed beam that was not used in the pre-amplification step in another
BBO crystal. During this step, an amplified signal and idler pair is generated with
∼100x the power of the original signal and idler from the pre-amplification step. The
amplified signal and idler beams recombine in a GaSe crystal to generate the IR beam
via difference frequency generation. This IR beam exits the OPA and continues to
the experimental line of the VSFSS set-up. By adjusting the white light timing in the
pre-amplification step and optimizing the crystal angles, IR beams with wavelengths
of 3–10 µm can be generated.
The experimental line of our VSFSS set-up follows the design of others in liter-
ature.[14, 15, 25, 26] The portion of the 800 nm beam that immediately exits the
OPA (described above) is used as the visible beam for SFG. The visible beam travels
through an etalon, half-wave plate, polarizer cube, half-wave plate configuration to
allow for the control of the visible power and polarization. The visible beam then
travels off a retroflector, which is used to control the beam’s path length and hence,
the temporal overlap of the visible and IR beams. The broadband IR beam that exits
the OPA travels through two BaF2 wire grid polarizers and is then focused to a spot
size of ∼80 µm with a gold parabolic mirror (focal length = 50 mm). The IR beam
is focused insight the sample cuvette, while the visible beam is focused behind the
sample cuvette with a spot size of ∼500 µm. The sample cuvette consists of a CaF2
front window (CeNing Optics) and a quartz back cuvette (Helma QS, path length =
200 µm) to allow for proper transmission of the incident beams and sum frequency
response, respectively. The visible and IR beams are incident on the sample cuvette
with an opening angle of ∼20◦ and the scattered sum frequency response is collected
at ∼60◦ from the phase-matched direction.
The scattered sum frequency response is collimated by a plano-convex lens (focal
length = 20 mm) and travels through a half-wave plate, polarizer cube configura-
13
tion, which allows for filtering and controlling the polarization of the sum frequency
response. Finally, the collimated sum frequency response travels through another
lens (focal length = 100 mm), which focuses the light into a spectrometer (IsoPlane
Princeton Instruments) that spectrally disperses the signal onto a CCD (PIMAX 4
Princeton Instruments).
VSFS Laser Set-Up
The majority of this dissertation contains spectroscopic measurements using VSFSS,
which is specific to nanoemulsion interfaces. Chapter VI, however, contains work that
is a collaboration and includes traditional vibrational sum frequency spectroscopy
(VSFS) measurements made off of a planar carbon tetrachloride (CCl4)/water inter-
face. For the VSFS experiments discussed in Chapter VI, a brief overview of the
laser set-up is provided here, but more detailed descriptions can be found in publica-
tions[27–29] and dissertations[30–32] from the Richmond lab.
The VSFS employed a commercially available Nd:YAG laser system manufactured
by Ekspla. A flash-lamp pumped Nd:YAG rod generates a 1064 nm seed beam,
with ∼30 ps pulse widths, at a repetition rate of 50 Hz. The seed beam is split
into three different beams, two of which are frequency doubled to 532 nm. One
of the two 532 nm beams is sent directly to the CCl4/water interface. The other
532 nm beam and the remaining seed 1064 nm beam are sent to an Ekspla optical
parametric oscillation/optical parametric generation/difference frequency generation
(i.e., OPO/OPG/DFG) process to generate the tunable IR beam for experiments.
The IR wavelength can be tuned between the range of 2-10 µm. The IR beam is
directed—via a bottom-up geometry—to the sample cell containing the CCl4 as the
oil phase, and the aqueous phase containing the chemical system of interest. The
sample cell for VSFS experiments was custom machined from Kel-F with a CaF2
window for the incident beams and a BK7 glass window for the outgoing beams.
As the IR beam travels through the sample cell, it impinges with the visible beam
at 68◦ and 76◦, such that total internal reflection is achieved. A half-wave plate and
a Glan-Taylor polarizer were used to control the polarization of the visible beam;
while a periscope was used to control the polarization of the IR beam. Upon spatial
and temporal overlap of the visible and IR beams at the sample, the generated sum
frequency signal is sent through a series of filters and another half-wave plate/Glan-
Taylor polarizer combination for polarization selection. Finally, the sum frequency
signal is detected by a monochromator (model MS2001) and photomultiplier tube
(Hamamatsu R7899), with a 3 cm−1 resolution.
14
Sum Frequency Spectral Fitting and Analysis
The sum frequency response from a planar or droplet surface is generated by the
spatial and temporal overlap of a visible and IR beam, where the visible beam is at
a fixed wavelength and the IR beam is chosen to be resonant with the vibrational
mode of interest. The intensity of the sum frequency response is proportional to the
square modulus of the second-order susceptibility, (2). The (2)χ χ tensor contains both
non-resonant and resonant contributions where
(2) N
χresonant ∝ ε ⟨βν⟩ , (24)o
where N is the number density of molecules at the droplet surface, εo is the vacuum
permittivity, and ⟨βν⟩ is the macroscopic average of the molecular hyperpolarizability.
Thus, the SF response is dependent on both the population and net conformational
ordering of surface adsorbed species.
A standard fitting routine described by Bain et. al.[33, 34] was used to deconvolve
the non-resonant signal and each individual resonant vibrational modes. This fitting
procedure fits all experimental spectra to the following equation:
»»» iϕ ( 2 »
2
ν ωL−ων) »
+∞ A e exp [− ] »
(2) » ν 2 »
χ = »»» (2) iϕNR Γν »»»χNRe +∑∫ dω
»
» ω − ω + iΓ
L»»» . (25)
»» ν −∞ L IR L »»»» »
The first term describes the non-resonant susceptibility with an amplitude (2)χNR and
phase ϕNR. The second term describes the resonant susceptibility as a sum of all
SF-active vibrational transitions, where Aν is the peak amplitude, ϕν is the phase,
ΓL is the Lorentzian linewidth describing the homogenous broadening, and Γν is the
Gaussian linewidth describing the inhomogenous broadening. The frequencies of the
Lorentzian, IR, and resonant vibrational modes are described by ωL, ωIR, and ων ,
respectively. The Lorentzian linewidths were chosen to reflect the vibrational lifetime
of the transition of interest. All spectra shown herein were fit and analyzed with Igor
Pro 6, using code developed by Moore et. al.[35]
15
CHAPTER III
INTERFACIAL BEHAVIOR OF pH-TUNABLE
POLYETHYLENIMINE
This work was published in Volume 36 of the journal Langmuir in July 2020.
Emma Tran designed the study, performed the experiments, analyzed the data, and
wrote the manuscript. Andrew Carpenter provided initial training on the laser system
and general feedback on the manuscript. Geraldine Richmond was the principal
investigator for this work and provided editorial assistance and general feedback.
Introduction
In pharmaceutical applications, many active ingredients are water-insoluble, mak-
ing it difficult to stabilize, protect, and increase the bioavailability of the active in-
gredient. Research in this area has been directed towards developing nanocarrier
drug delivery systems, which utilize emulsions to encapsulate a hydrophobic drug
to then be delivered to a target site within the body.[36] A layering of surfactants
and polymers around the oil core is often implemented in the design of nanocarrier
templates to enhance the stability and efficacy of the nanocarrier. The performance
and versatility of the nanocarrier depends heavily on the tunability of the interfacial
polymer–surfactant (PS) behavior that emerges from altering physiological conditions
such as pH, ionic strength, and temperature.[37, 38]
Figure 3.1. Molecular structures of (a) protonated PEI, (b) deprotonated PEI, (c)
sodium dodecyl sulfate (SDS), and (d) dodecyltrimethylammonium bromide (DTAB).
In this work, polyethylenimine (PEI) is studied in conjunction with either sodium
dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB) to investigate
the molecular factors contributing to the stabilization of nanoemulsions that occur
during PS coadsorption. Molecular structures of PEI, SDS, and DTAB are shown in
16
Figure 3.1. Central to this study is measuring changes to the molecular conforma-
tional arrangement of the polymer and co-surfactants upon variation of the polymer
electrostatics by altering the solution pH. PEI is a biocompatible, partially cationic
polymer whose charge density can be tuned by adjusting the solution pH, making it
an attractive option as the polymer shell in nanocarrier templates.[10] Approximately
70%, 30%, and 5% of the amine groups in PEI are protonated at pH 2.5, 5-6, and 10,
respectively.[39] While many previous studies of PEI employed interfacial techniques
such as surface tensiometry,[40–44] neutron reflectivity,[40–46] and ellipsometry,[47,
48] molecular-level details regarding PEI bonding and conformational ordering are
largely lacking, especially at the droplet interface. Due to the tunable cationic char-
acter of PEI, understanding its adsorption behavior and molecular conformational
ordering at a surfactant-stabilized nanoemulsion surface is essential for controlling its
performance as a nanocarrier layer under different environmental conditions.
To achieve this level of understanding, vibrational sum frequency scattering spec-
troscopy (VSFSS) has been employed herein to probe the vibrational dipoles of
PS complexes at the nanoemulsion interface, yielding insight into PEI/SDS and
PEI/DTAB molecular structure and behavior at the droplet surface. These spec-
troscopic studies are complemented with ζ-potential (ZP) and surface pressure mea-
surements. The findings reported in this chapter provide insights into the molecular
factors that contribute to the enhancement of PEI when coadsorbed with a similarly
or oppositely charged surfactant, and how the interfacial molecular properties vary as
the charge density on the polymer is altered by solution pH. Most notable is the strong
variation with pH in the conformational ordering of the polymer at the nanoemul-
sion surface when coadsorbed with either surfactant. These results have significant
implications for both nanoemulsion stability and functionality in the application of
drug delivery as different organs and membranes within the human body exist under
a wide range of pH values.
17
PEI pH-dependent Surface Activity
Figure 3.2. Surface pressure measurements as a function of pH of PEI alone (black
●), PEI/SDS (orange ●), and PEI/DTAB (pink ●). PEI concentration was kept
constant at 0.52 mM, while surfactant concentration was fixed at 0.01 mM. For com-
parison, the gray horizontal bar represents the surface pressures exerted by 0.01 mM
SDS and 0.01 mM DTAB individually.
The adsorptive behavior of PEI to a planar oil/water interface was determined
through pendant drop tensiometry, represented by the black ● in Figure 3.2. Al-
though surface tensiometry provides information about surface activity at a planar
(rather than a droplet) oil/water interface, such studies can still be informative re-
garding PS adsorption to the nanoemulsion oil/water surface. At low pH, PEI alone
at a concentration of 0.52 mM is not surface active. In acidic conditions, PEI is
highly soluble due to the high charge density. Thus, bulk solvation of the polymer is
more energetically favorable than adsorption. As the solution becomes more basic,
its surface pressure increases slightly, indicative of higher surface activity. This result
is consistent with previous studies from this laboratory showing pH-tunable polymers
desorb from a planar oil/water interface when the polymer is ∼20% charged.[28, 49,
50] PEI becomes more hydrophobic as pH increases into the basic regime, allowing
for polymer adsorption in part due to hydrophobic interactions between the polymer
backbone and oil phase, and because bulk solvation is less energetically favorable as
PEI becomes less charged. Additionally, weakly charged PEI has been shown to form
multilayers due to strong hydrophobic interactions coupled with ion-dipole interac-
tions between the SDS sulfate ion and NH dipole.[40] The enhanced surface pressure
18
observed may also be indicative of more PEI adsorption, potentially forming thicker
layers. It is noted later, regardless of the slight enhancement in PEI surface activity
as pH increases at the planar oil/water interface, we find that PEI alone does not
form and stabilize nanoemulsions at any pH. Emulsion samples created with 15 mM
PEI alone were on the order of microns and destabilized within ∼10 min by visual
observation.
Effect of Charged Surfactants on PEI Adsorption
PEI adsorption in the presence of SDS. Although PEI alone showed minimal
adsorption to the planar oil/water interface, its surface activity can be enhanced with
the addition of surfactants. The orange ● in Figure 3.2 depict the surface activity of
PEI/SDS in solutions of different pH, with PEI and SDS concentrations fixed at 0.52
mM and 0.01 mM, respectively. For comparison, the horizontal gray bar in Figure 3.2
represents the pH-independent surface activity of 0.01 mM SDS at a planar oil/water
interface under various pH conditions. PEI/SDS is surface active across the entire pH
range studied. The enhanced adsorption for this system increases with pH, indicating
that PEI/SDS surface activity is strongest when PEI becomes increasingly neutral as
pH becomes more basic.
Figure 3.3. ζ-potential measurements as a function of pH for nanoemulsions sta-
bilized by SDS (black ●) and PEI/d-SDS (orange ●). The polymer and surfactant
concentrations are 5.2 mM and 0.1 mM, respectively. Uncertainty bars are present,
but comparable to the size of the markers.
The addition of SDS to PEI also enhances nanoemulsion formation throughout the
pH regime of 311. As noted above, PEI alone does not form stable nanoemulsions
19
at any pH. ZP measurements of these SDS/PEI-stabilized nanoemulsions are shown
in Figure 3.3 (orange ●) along with ZP measurements of nanoemulsions stabilized
with SDS alone (black ●). For nanoemulsions stabilized by SDS only, the ZP remains
highly negative and largely invariant to changes in pH.
The ZP values of PEI/SDS-stabilized nanoemulsions start highly positive at pH 3
and become progressively less positive as pH increases. At low pH, PEI behaves as
a cationic polymer, making the surface of PEI/SDS-stabilized nanoemulsions highly
positive. Between pH 3 and 7 (Figure 3.3), ZP measurements for these samples are
constant, despite PEI going from ∼70% charged (pH 3) to ∼30% charged (pH 7).
Previous sum frequency studies at the planar solid/aqueous interface by Windsor et
al. and neutron reflectivity studies at the emulsion surface by Penfold et al. suggest
PEI encourages SDS adsorption.[44, 51] Enhanced SDS adsorption due to the presence
of PEI would result in increasingly negative ZP values between pH 3 and 7, which we
do not observe. There are two possibilities that may explain the constant ZP from
acidic to neutral pH. With the concentrations used, the number of SDS molecules (and
hence, negative charge) in solution is ∼2 orders of magnitude less than the number
of positively charged monomer units of PEI at pH 3 and ∼1 order of magnitude
less at pH 7. Thus, it is feasible that if additional SDS molecules adsorb to the
droplet surface, PEI counters the influence SDS has on the ZP because the amount of
positively charged monomer units is so much greater than the amount of negatively
charged SDS. It is also likely that cooperative interactions between PEI and SDS
enhance the adsorption of both species such that the net change in magnitude of
ZP is nominal. While it is unclear from ZP results alone whether PEI assists the
adsorption of SDS, our VSFSS results discussed below revisit this hypothesis in more
detail.
Beyond pH 7, ZP values of PEI/SDS become increasingly negative, indicative of
deprotonated PEI contributing less positive charge to the interface. At pH ⩾ 10, PEI
is considered to behave essentially as a neutral polymer.[39, 52] The highly negative
ZP originates from the negatively charged headgroup of coadsorbed SDS. However,
even in highly basic conditions, nanoemulsions stabilized by PEI and SDS exhibit ZP
values less negative than those of nanoemulsions stabilized by SDS alone. In addition
to the strong surface activity of PEI/SDS complexes in the basic regime, these results
confirm PEI is still interfacially present, even when it is only marginally positive.
As change in pH alters PEI charge density, knowledge of how the surfactant and
polymer surface structure are affected will aid in the informed design of nanocarrier
architecture. While both SDS and PEI are present at the interface stabilizing the
20
nanoemulsions, the molecular structure of these two species at the droplet interface
is still unknown. Past studies have reported PEI/SDS layer formation and thickness
at various interfaces,[40–46] but these studies do not provide molecular-level details
about PS conformational ordering around a nanoemulsion surface. Thus, in later
sections, we turn to VSFSS to determine the orientation of net dipoles of the SDS
alkyl tail and polymer backbone relative to the oil/water droplet surface, and how
that orientation changes under different pH conditions. We now investigate how
changing the surfactant headgroup from a negative to a postiive charge influences
PEI adsorptive behavior.
PEI adsorption in the presence of DTAB. Studies of PEI in conjunction with
DTAB provide an opportunity for further investigation of surface complexation be-
tween a positively charged surfactant and a charge-tunable polymer. PEI/DTAB
solutions at a planar oil/water interface (Figure 3.2) are surface active at pH ⩾ 9,
albeit less surface active than the PEI/SDS system.
Figure 3.4. ζ-potential measurements as a function of pH for nanoemulsions stabi-
lized by DTAB (black ●) and PEI/d-DTAB (pink ●). The polymer and surfactant
concentrations were fixed at 5.2 mM and 0.1 mM, respectively.
To confirm the presence of PEI at the droplet surface, mixtures of 5.2 mM PEI and
0.1 mM DTAB were used to form stable nanoemulsions. ZP measurements made on
these PEI/DTAB-stabilized nanoemulsions at varying pH are shown in Figure 3.4.
Though both species are cationic, changes in the magnitude of the ZP values aid in
the deduction of whether PEI is present at the surface. As can be seen in Figure 3.4,
21
there is minimal deviation between the ZP values of DTAB-stabilized nanoemulsions
(black ●) and PEI/DTAB-stabilized nanoemulsions (pink ●) at all pH values.
In acidic conditions, the nominal deviation in ZP values between the DTAB and
PEI/DTAB system, along with the significantly low surface activity discussed above
(Figure 3.2), suggests that a PS complex is not present at the nanoemulsion surface.
In basic conditions, while the surface activity of the PEI/DTAB system increases at
the planar oil/water interface (Figure 3.2), the changes in ZP values of nanoemul-
sions stabilized by DTAB alone versus PEI/DTAB are not substantial enough to be
conclusive. Therefore, to investigate this system in more detail, we turn to VSFSS
to corroborate our ZP results as well as determine the structural ordering of the
PEI/DTAB complexes.
Effect of Charged Surfactants on PEI Surface Structure
PEI surface structure in the presence of SDS. Surface spectroscopic mea-
surements of nanoemulsions confirm not only the presence of interfacial species but
also their molecular structure relative to the interface and thus, provide insight into
their interfacial bonding behavior. Recall that all nanoemulsions were prepared with
deuterated hexadecane as the dispersed phase and D2O as the continuous phase.
Thus, in all spectra hereafter, there are no CH contributions from hexadecane, and
IR absorption by the continuous phase has been minimized.
22
Figure 3.5. (a) VSFSS measurements of the CH stretching region of PEI/d-SDS
nanoemulsions at pH 3 (pink), 5 (yellow), and 7 (green). PEI and d-SDS concentra-
tions were fixed at 5.2 mM and 0.1 mM, respectively. (b) VSFSS spectra of the CH
stretching region of nanoemulsions stabilized by PEI/d-SDS (gray), h-SDS alone (or-
ange), and PEI/h-SDS (red). Here, PEI concentration remained the same at 5.2 mM,
while the surfactant concentration was increased to 1 mM. All spectra were collected
in SSP and where appropriate, lines are fits to data following the Richmond fitting
routine.
As shown in Figure 3.5, VSFSS measurements in the CH stretching region of na-
noemulsions stabilized by 5.2 mM PEI and 0.1 mM deuterated SDS (d-SDS) at pH 3,
5, and 7 all result in a lack of signal from the CH stretching vibrational modes of PEI.
Since ZP results show PEI is adsorbed to the nanoemulsion surface under these pH
conditions and surface pressure measurements confirm PEI adsorption at the planar
interface, the lack of SF signal under acidic conditions is attributed to a lack of net
dipole orientation of the polymer CH modes relative to the interface. Such scenario
is indicative of a disordered polymer molecular structure around the droplet surface
(e.g., tangled polymer strands). Highly charged PEI is reported to adopt an extended
conformation due to a high degree of intrachain chargecharge repulsion.[53] While this
would allow PEI to adsorb flatly to a planar surface, especially one with a tightly
packed surfactant monolayer, PEI adsorption to the nanoemulsion surface geometry
likely adopts more gauche defects, especially when the surfactants are isotropically
distributed in a diffuse monolayer, making the polymer chains more disordered.
To monitor the structural ordering of the surface adsorbed surfactant in the pres-
ence of interfacial PEI, SDS concentration was increased from 0.1 mM to 1 mM to
surpass the limit of detection of SDS with our laser system. VSFSS spectra in the CH
stretching region collected for nanoemulsions stabilized by PEI/d-SDS, PEI/h-SDS,
and h-SDS alone are shown in Figure 3.5b. Because the CH modes of PEI are SF
inactive at pH 3 and contribute no signal, as can be seen by the featureless gray trace
in Figure 3.5b, any spectroscopic signal from the CH stretching region is attributed
to the SDS alkyl tails. VSFSS measurements probing the SDS tails in the absence
(orange trace) and presence (red trace) of PEI show that with the addition of PEI,
the SF signal from the SDS CH vibrational modes increases. These vibrational modes
have been attributed to the CH2 symmetric stretch (d
+) at ∼2850 cm−1, CH3 sym-
metric stretch (r+) at ∼2873 cm−1, and a broad peak corresponding to a mixture of
modes that include the CH3 asymmetric stretch and a Fermi resonance arising from
a CH3 bend overtone with a CH3 stretch at ∼2935 cm
−1.[54, 55] Fits to all SF spectra
herein follow the Richmond laboratory fitting routine, which was discussed above in
23
Chapter 2.[34] The fitting parameters along with the uncertainties corresponding to
the fits in Figure 3.5 can be found in Table A.2 in the Appendix.
The enhanced intensities from each vibrational mode observed in samples stabilized
by PEI/h-SDS compared to samples stabilized by only h-SDS indicate that more SDS
has adsorbed to the droplet surface with PEI present. Furthermore, analysis of the
intensity ratios between the CH2 symmetric stretch and CH3 symmetric stretch (i.e.,
the d+/r+ ratio) yields a lower d+/r+ ratio for samples stabilized by both PEI/h-
SDS relative to h-SDS alone, which corresponds to a higher degree of conformational
ordering in the SDS alkyl tails and less gauche defects.[54, 56] We note that despite the
change in surfactant concentration, and thus the polymer/surfactant concentration
ratio, the PEI/SDS system with 1 mM d-SDS behaves the same as with 0.1 mM d-SDS
(i.e., both polymer/surfactant concentration ratios result in a completely disordered
PEI surface structure). While it has been shown that PS complexation and interfacial
aggregation behavior depend greatly on polymer/surfactant concentration ratios,[57]
it is evident that in this work, whether the polymer/surfactant concentration ratio is
∼50:1 or ∼5:1, the disorder in PEI interfacial structure is comparable between the two.
Together, these results illustrate that at the droplet oil/water interface, PEI is present
and encourages more SDS to adsorb, leading to more conformational ordering of the
surfactant tails. Our conclusions at the nanoemulsion surface are consistent with the
previous studies mentioned above, confirming that PEI promotes the adsorption of
SDS.[44, 51] Moreover, these results show that coadsorption also promotes SDS chain
ordering.
24
Figure 3.6. (a) VSFSS measurements of the CH stretching region of PEI/d-SDS
stabilized nanoemulsions at pH 7.5 (green), 8 (cyan), 10 (blue), and 11 (purple).
PEI and d-SDS concentrations were fixed at 5.2 mM and 0.1 mM, respectively. (b)
VSFSS spectra of the CH stretching region of nanoemulsions stabilized by PEI/h-SDS
(purple) and PEI/d-SDS (gray) at pH 11. Here, PEI concentration remained the same
at 5.2 mM, while the surfactant concentration was increased to 1 mM. Spectra were
taken under the SSP polarization scheme and lines are fits to data.
As pH increases, the charge density on PEI decreases, making it increasingly neutral.
To investigate the effects that deprotonation has on PEI surface behavior, VSFSS
spectra of the CH stretching region of nanoemulsions stabilized by 5.2 mM PEI and
0.1 mM d-SDS at pH 7.5, 8, 10, and 11 were measured (Figure 3.6a). The observed
broad peak at 2851 cm−1 corresponds to the CH2 symmetric stretch of PEI for all
traces. This SF intensity grows with increasing pH, consistent with its surface activity
at the planar oil/water interface (Figure 3.2). Note from Figure 3.5a, that this mode
was not observed for PEI/SDS nanoemulsions prepared in acidic pH. Minor contribu-
tions near 2870 cm−1 from the CH3 symmetric stretch and 2935 cm
−1 from the CH3
asymmetric stretch of the polymer end-caps are also acknowledged (see Table A.3
and A.4 for fitting parameters). The spectral presence of the terminal CH3 indicate
that they too have a preferential surface orientation with their CH modes having a
perpendicular component relative to the nanoemulsion surface. These spectroscopic
studies provide strong evidence not only that PEI is interfacially present but that the
25
polymer backbone and terminal methyl groups adopt an ordered and net orientation
to the droplet surface.[14, 15, 19, 58] The VSFSS results herein reveal the unique
conformational reorientation PEI undergoes between pH 7 to 7.5 from being largely
disordered in the acidic regime to progressively more ordered in the basic regime.
To determine the influence that nonionic PEI has on SDS structural ordering, VS-
FSS measurements of the SDS alkyl tails were obtained for 1 mM SDS in the presence
of 5.2 mM PEI at pH 11 (Figure 3.6b). Recall that the surfactant concentration was
increased in order for it to be spectroscopically detectable with our laser system. The
gray and purple traces in Figure 3.6b are the VSFSS spectra of nanoemulsions sta-
bilized by PEI/d-SDS and PEI/h-SDS, respectively. Similar to the discussion above
surrounding polymer/surfactant concentration ratios, PEI orders itself similarly with
0.1 mM d-SDS as it does with 1 mM d-SDS. With a comparison of the spectrum of
PEI with those of 0.1 mM d-SDS at pH 11 (Figure 3.6a, purple trace) and 1 mM
d-SDS at pH 11 (Figure 3.6b, gray trace), it is clear that the similar traces are both
indicative of ordered interfacial PEI, regardless of d-SDS concentration. While the
spectrum of PEI/d-SDS contains polymer contributions, specifically from the CH2
symmetric stretch mode near 2851 cm−1 (gray trace), the spectrum of PEI/h-SDS
contains contributions from both the polymer and surfactant CH2 and CH3 modes
(purple trace). Because it is not possible to decouple CH modes between PEI and
h-SDS, a quantitative d+/r+ analysis is not applicable here. Nevertheless, the overall
enhanced signal intensity and additional CH3 spectral features near ∼2935 cm−1 ob-
served for nanoemulsions stabilized by PEI/h-SDS are indicative of conformationally
ordered interfacial SDS molecules. Therefore, in basic conditions at the nanoemulsion
surface, PEI and SDS form ordered complexes.
PEI surface structure in the presence of DTAB. We now investigate the ef-
fects of surfactant charge on PEI interfacial ordering by switching anionic SDS to
cationic DTAB. Spectroscopic measurements taken in the SSP polarization scheme
of nanoemulsions stabilized by PEI/d-DTAB, PEI/h-DTAB, and h-DTAB only are
shown in Figure 3.7. These experiments were performed at pH 3 with 5.2 mM PEI
and 1 mM DTAB. Though the polymer/surfactant concentration ratio has changed
here, we are confident that PEI interfacial conformational structure is similar even
with higher DTAB concentrations, as it resulted in a lack of signal, very much like it
did with 0.1 mM d-DTAB (not shown).
26
Figure 3.7. VSFSS spectra of the CH stretching region of nanoemulsions stabilized
with 5.2 mM PEI and 1 mM h-DTAB (red) and 1 mM h-DTAB alone (orange). All
spectra were collected in SSP and all samples were prepared at pH 3.
We note that although there are differences in signal-to-noise between PEI/SDS
and PEI/DTAB, the VSFSS results obtained for the PEI/DTAB system are similar
to the results obtained for the PEI/SDS studies above, in that, no CH stretching
vibrational modes of PEI were detected (gray trace). However, unlike the PEI/SDS
system, VSFSS measurements of DTAB in the absence (orange trace) and presence
(red trace) of PEI resulted in nearly identical spectra. The orange and red spectra in
Figure 3.7 are characteristic of the DTAB alkyl tails stretching modes, consisting of
the CH2 symmetric stretch, CH3 symmetric stretch, and a combination of the CH3
asymmetric stretch with a Fermi resonance.[59, 60] This result combined with the
ZP measurements indicates that at pH 3, PEI and DTAB do not appear to form a
surface complex at the nanoemulsion surface. PEI likely remains solvated in the bulk
aqueous phase under these conditions, leaving the nanoemulsion surface polymer-free.
In basic conditions, PEI is no longer in its highly charged form. With less cationic
character on the polymer, there is less chargecharge repulsion between DTAB and
PEI, allowing for PEI to adsorb more readily to DTAB-stabilized nanoemulsions. The
surface pressure measurements (Figure 3.2) show PEI/DTAB mixtures exhibit higher
surface activity in basic conditions than acidic conditions. This enhanced surface
activity is also evident in the spectroscopic measurements of the nanoemulsions.
27
Figure 3.8. (a) VSFSS spectra of the CH stretching region of nanoemulsions stabi-
lized by 5.2 mM PEI and 0.1 mM d-DTAB at pH 8 (cyan), 10 (blue), and 11 (purple).
(b) VSFSS measurements of the CH stretching region of nanoemulsions stabilized by
PEI/h-DTAB (purple) and PEI/d-DTAB (gray) at pH 11. PEI concentration was
fixed at 5.2 mM, while surfactant concentration was increased to 1 mM. Spectra were
collected in SSP and solid lines are fits to data.
VSFSS spectra of the CH stretching region of nanoemulsions stabilized by PEI and
d-DTAB at pH 8, 10, and 11 are shown in Figure 3.8. Similar to the results obtained
from the PEI/d-SDS nanoemulsions above (Figure 3.6), SF intensity increases with
increasing pH (Figure 3.8a), showing conclusively that PEI, in the presence of DTAB,
is adsorbed to the nanoemulsion surface under basic conditions. Moreover, the strong
CH response from the CH2 backbone modes indicates a net dipole orientation along
the polymer chains, which increases with pH. The most pronounced peak corresponds
to the CH2 symmetric stretch at 2851 cm
−1 with additional contributions arising
from the CH3 modes from the polymer end-caps. It is noted that for nanoemulsions
stabilized with PEI/d-SDS in basic conditions (Figure 3.6a), there is elevated SF
intensity at lower frequencies of the CH stretching region and an apparent frequency
shift in the PEI CH2 symmetric stretch. This is not observed for samples stabilized
with PEI/d-DTAB (Figure 3.8a). This observation of seemingly higher background
SF signal around 2800 cm−1 for SDS versus DTAB has also been acknowledged by
other groups and corresponds to OD modes of D2O that are enhanced by the presence
28
of SDS and constructively interfere with the CH2 modes.[61–65] These OD modes are
also present in chemical systems involving DTAB, but the cationic charge likely results
in a destructive interference with the CH2 modes and hence, lower intensity.[61]
With a comparison of the spectra of nanoemulsions stabilized by PEI/d-DTAB
(gray) to PEI/h-DTAB (purple) at pH 11 (Figure 3.8b), information regarding DTAB
tail ordering can be inferred, despite contributions from the polymer CH modes.
In these experiments, the polymer and surfactant concentrations were fixed at 5.2
mM and 1 mM, respectively. Again, we note that despite the change to the poly-
mer/surfactant concentration ratio, the spectrum of PEI with 0.1 mM d-DTAB (Fig-
ure 3.8a, purple trace) versus 1 mM d-DTAB (Figure 3.8b, gray trace) are very
comparable, indicating that coadsorbed PEI/DTAB complexes structure themselves
similarly whether the surfactant concentration is 0.1 mM or 1 mM. In the presence
of h-DTAB, the SF intensity is increased due to the additional CH2, CH3, and Fermi
resonances from the surfactant alkyl tails that are no longer deuterated. For fitting
paramters corresponding to Figure 3.8, see Table A.5 and A.6 in the Appendix. To-
gether, these results demonstrate that in basic conditions, in conjunction with ordered
PEI, DTAB also possesses a net ordering at the nanoemulsion surface.
29
Conclusions
Figure 3.9. A pictorial representation of PEI/SDS in (a) acidic and (b) basic con-
ditions and PEI/DTAB in (c) acidic and (d) basic conditions. Molecules and na-
noemulsions are not illustrated to scale.
This study examines how PEI adsorption and structural ordering at a nanoemul-
sion surface can be tuned with surfactant charge and solution pH. Illustrated by Figure
3.9a, in the presence of anionic SDS at low pH, PEI is disordered and polymer ad-
sorption to the droplet surface is predominantly influenced by attractive electrostatic
interactions between the positively charged amine groups on PEI and the negatively
charged sulfate group on SDS. When PEI is highly charged, adsorption is limited due
to competitive bulk solvation of the polymer. At high pH when PEI is less charged,
bulk solvation is no longer energetically favorable and thus, the degree of PEI adsorp-
tion in basic conditions is stronger than in acidic conditions. Hydrophobic interac-
tions between PEI, SDS, and the oil phase encourage PEI adsorption to the droplet
surface. In conjunction with hydrophobic interactions, attractive chargecharge and
ion-dipole interactions between the sulfate anion and the polymer NH dipole assist
in PEI adopting a net ordering at the interface, depicted by Figure 3.9b, similar to
30
what previous studies have concluded with tensiometry[40–44, 66, 67] and neutron
reflectivity[40–46, 66, 67] experiments at the air/water and oil/water interfaces.
For the PEI/DTAB system in acidic conditions, the lack of attractive electrostatic
interactions between the polymer and surfactant leaves PEI fully solvated in the bulk
and DTAB as the only interfacially active component stabilizing the nanoemulsion,
as seen in Figure 3.9c. In basic conditions, PEI/DTAB exhibit similar adsorption
behavior and polymer structuring as PEI/SDS. For the PEI/DTAB system at pH>8,
hydrophobic interactions between PEI, DTAB alkyl tails, and the oil phase accompa-
nied by attractive ion-dipole interactions between the DTAB cation and NH dipole
govern polymer adsorption and ordering. These combined intermolecular interactions
drive PEI adsorption and impart a net ordering to the polymer chains. However, il-
lustrated by Figure 3.9d, the lack of attractive chargecharge interactions between
PEI and DTAB limits the overall surface activity of PEI/DTAB when compared to
PEI/SDS, where repulsive forces do not exist.
Being partially cationic, the surface behavior of PEI is dependent on its charge
density. Due to the pH-dependent behavior PEI exhibits, the amount of adsorption
and degree of ordering can be tuned to create nanoemulsions with contrasting inter-
facial structures, demonstrating the versatility of PEI for industrial and commercial
applications. The direct comparison between PEI/SDS versus PEI/DTAB confirms
that chargecharge interactions, ion-dipole interactions, and hydrophobic interactions
are all contributing to the strong adsorption and ordering of PEI in basic condi-
tions, which corroborates assumptions in literature regarding the strong interactions
between PEI and SDS at high pH.[40–42, 44, 46, 51, 68] Moreover, the results in
this work demonstrate PEI structural ordering at a droplet surface and its sensi-
tivity to pH perturbations. Using VSFSS, molecular-level detail about the polymer
at a buried oil/water interface has been revealed, which is information that has not
accessible until now.
These findings provide valuable insight into the tunable nature of PEI and how it
behaves at the nanoemulsion surface, which has direct implications for the informed
design of nanocarrier templates containing PEI as the polymer layer. Within half a pH
unit, PEI reorients itself from being completely disordered to adopting a net ordering
that persists with increasing pH. This suggests that the net structural ordering of PEI
is insensitive to pH values within the acidic regime but responds to changes in pH
values within the basic regime. Future work investigating the mechanism that drives
this reorientation would be necessary to take full advantage of PEI surface behavior for
both fundamental knowledge and layer-by-layer applications. These results provide
31
molecular-level details about a PS system at a nanoemulsion surface, which aids in
the optimization of nanocarrier technology and sets the foundation for future work
investigating polymerpolymer ordering at a curved interface.
32
CHAPTER IV
POLYELECTROLYTE LAYERING ON DROPLET SURFACES
This work was published in Volume 599 of the Journal of Colloid and Interface
Science in April 2021. Emma Tran designed the study, performed some of the experi-
ments, analyzed the data, and wrote the manuscript. Ashley Mapile performed some
of the experiments and provided general feedback on the manuscript. Geraldine Rich-
mond was the principal investigator for this work and provided editorial assistance
and general feedback.
Introduction
In the previous chapter, we studied nanoemulsions coated by a single polymer
layer at a molecular-level. In this chapter, we add a secondary polymer layer and
investigate the complexities of such a system from both a macroscopic and molecular
perspective.
Since the early 1990s, layer-by-layer (LbL) deposition has become a popular tech-
nique employed to prepare multiple layer thin films of oppositely charged polyelec-
trolytes through electrostatic-driven adsorption.[69, 70] Today, this technique is still
widely prevalent in applications involving the design of devices,[71] coatings,[70, 72,
73] sensors,[74, 75] quantum dots,[76] and drug delivery systems.[9, 11, 77, 78] While
most research conducted on LbL assembly has been primarily done to understand
thin film formation on a flat solid substrate, knowledge regarding LbL assembly at
curved surfaces or liquid/liquid interfaces is much more limited. As more drug de-
livery applications are incorporating LbL deposition and emulsions into their design,
having knowledge about multi-polymer layering behavior at the curved surface is
essential for intuiting structure-function relationships. Structural characterization
of thin films prepared via LbL deposition has been done using various reflectiv-
ity[79–82] and microscopy techniques,[74, 82, 83] which provides information such
as layer thickness and composition. Additionally, surface-specific techniques such as
reflection-absorption infrared spectroscopy[84] and sum frequency generation[84, 85]
has been utilized to determine the conformational ordering of polyelectrolyte layers
on planar surfaces. Yet, the molecular-level structural ordering of polymer layers
coating a curved surface has yet to be investigated.
In this work, we show on a molecular level the role that charged surfactant do-
decyltrimethylammonium bromide (DTAB) (Figure 4.1a) plays in the stabilization
33
of nanoemulsions with either a single- or two-polymer system. The polymers used
in this study are poly(styrene sulfonate) (PSS) (Figure 4.1b) and polyethylenimine
(PEI) (Figure 4.1c). While PSS is an anionic polymer, PEI is a cationic polymer in
acidic environments and a nonionic polymer in basic environments (Figure 4.1c).[39,
52] Thus, this polymer combination provides a unique opportunity to control the in-
terfacial electrostatics of the system through the addition of a secondary polymer and
solution pH. Ultimately, the effects of PEI as a secondary polymer on both PSS con-
formational ordering and overall nanoemulsion stability have been carefully described
in this work.
Figure 4.1. Molecular structures of (a) surfactant DTAB, (b) anionic polymer PSS,
and (c) partially cationic polymer PEI.
We characterize nanoemulsion quality and stability by monitoring the sample size
distribution, Z-average diameter, polydispersity index (PDI), and ζ-potential (ZP)
over 30 days with dynamic light scattering (DLS), as these metrics are important for
industrial and commercial applications. To complement our macroscopic character-
ization, we also employ vibrational sum frequency scattering spectroscopy (VSFSS)
to unveil molecular-level structural details at the nanoemulsion surface. Specifically,
we are able to determine the interfacial assembly and conformational ordering of the
anionic PSS layer, with and without the secondary polymer PEI, allowing us to un-
derstand the contributions from added electrostatics and steric effects. Together, our
34
DLS and VSFSS results provide a fundamental understanding of how LbL processes
at curved oil/water interfaces affect both macroscopic and molecular-level surface
properties of nanoemulsions. Moreover, the molecular-level insights gained in this
work are important for the rational design of applications involving foods,[86, 87]
cosmetics,[88, 89] and drug delivery systems,[9, 11, 77, 78] as these applications often
employ emulsions coated by alternating layers of oppositely charged polyelectrolytes.
DTAB/PSS complexes on nanoemulsion stabiity
Since the formation of nanoemulsions is a nonspontaneous event, understanding
and controlling nanoemulsion stability (i.e. sample size distributions, polydisper-
sity, ζ-potential) is important for many applications involving a shelf life such as
creams and cosmetics, or time-sensitive functions such as targeted drug delivery. The
time-dependent stability of nanoemulsions stabilized by d-DTAB/PSS at the natural
solution pH of 7 is summarized in Figure 4.2. The top panel (Figure 4.2a–c) shows
size distributions of nanoemulsion samples stabilized by a fixed PSS concentration
of 5 mM with 0, 0.1, and 1 mM d-DTAB, respectively. The bottom panel (Figure
4.2d–f) shows the diameters, PDIs, and ZP, respectively, for 5 mM PSS in conjunction
with 0, 0.1, and 1 mM d-DTAB.
35
Figure 4.2. Top panel: Size distribution measurements of nanoemulsions stabilized
by (a) 0 mM DTAB, (b) 0.1 mM DTAB, and (c) 1 mM DTAB, all in conjunction
with 5 mM PSS. Bottom panel: (d) Nanoemulsion Z-average diameter, (e) PDI, and
(f) ZP for samples stabilized by 5 mM PSS with 0 mM DTAB (gray ●), 0.1 mM
DTAB (blue ∎), and 1 mM DTAB (red ▲).
Fresh samples (Day 0) of nanoemulsions created with 5 mM PSS alone (i.e., no
added surfactants) had a broad size distribution with two distinct populations (Figure
4.2a, solid black). A broad size distribution has been reported to be due to a variety of
structures in solution.[90, 91] The broadness we observe is indicative of a combination
of nanoemulsions, micron-sized emulsions, and bulk PSS aggregates in solution. Over
time, depletion of larger sized emulsions is observed accompanied by an increase in
smaller sizes until eventually, the distribution becomes uni-modal (Figure 4.2a). We
attribute the depletion of larger size populations to the destabilization of micron-sized
emulsions. Over time, the sample becomes less opaque, a visual indicator of phase
separation and emulsion destabilization (Figure 4.3, left panel).[87, 92–94]
Figure 4.3. Photograph images of nanoemulsion samples stabilized by 5 mM PSS
alone (left), 0.1 mM d-DTAB, 5 mM PSS, and 10 mM cationic PEI (pH 3) (center),
0.1 mM d-DTAB, 5 mM PSS, and 10 mM nonionic PEI (pH 10) (right) taken on
Days 0, 7, and 29 of the 30-day period.
With no surfactant present, diameters of nanoemulsions stabilized with only PSS
fluctuate between ∼350–500 nm (Figure 4.2d, gray ●) with PDI values (Figure 4.2e,
gray ●) indicating non-uniformity in the sample. A nanoemulsion sample is consid-
ered to be monodispersed and suitable for application when the PDI < 0.3,[2, 77, 95]
which is not the case for these samples. The measured ZP starts highly negative and
becomes progressively more negative as the sample ages (Figure 4.2f, gray ●). From
the size distribution plots, the reduction in larger size population frees PSS from sta-
bilizing larger emulsions. Thus, with free PSS in solution, the formation of bulk PSS
36
complexes can contribute to both the more negative ZP values and the increase in
size populations with time, as observed from the distribution plots.
Nanoemulsions created with PSS in conjunction with 0.1 mM d-DTAB and 1 mM
d-DTAB resulted in uni-modal size distributions that remained as such for 30 days,
which can be seen in Figure 4.2b and 4.2c, respectively. Additionally, nanoemulsion
diameters decrease with fewer fluctuations. Smaller nanoemulsions are obtained with
1 mM d-DTAB (Figure 4.2d, red ▲) as compared to 0.1 mM DTAB (Figure 4.2d,
blue ∎); and while both samples show steady growth over time, they remain monodis-
persed based on their steady PDI values < 0.3 (Figure 4.2e, blue ∎ and red ▲). ZP
measurements are summarized in Figure 4.2f, showing that with the addition of 0.1
mM (Figure 4.2f, blue ∎) and 1 mM d-DTAB (Figure 4.2f, red ▲) to 5 mM PSS, ZP
becomes less negative compared to samples containing only PSS (Figure 4.2f, gray
●). This confirms d-DTAB is adsorbed to the droplet surface, resulting in a less net
negative surface charge. Samples containing 1 mM d-DTAB have more negative ZP
than 0.1 mM d-DTAB, despite having a greater bulk concentration of cationic surfac-
tant. We conclude that a higher bulk d-DTAB concentration increases the amount
of d-DTAB adsorbed to the nanoemulsions, which recruits more PSS to the surface,
making the net interfacial charge more negative. We note that it is possible for the
negatively charged sulfonate groups of PSS to re-orient in such a way that can result
in more negative ZP measurements without necessarily an increase in PSS adsorption.
However, previous surface tensiometry experiments have shown that the interfacial
population of PSS increases with increasing amounts of d-DTAB, which is consistent
and further supports our interpretation here.[60, 96, 97]
Altogether, we conclude that nanoemulsions stabilized by d-DTAB/PSS are stable
over a month’s time based on the robustness of the size distribution plots and mini-
mal variation in diameters, PDIs, and ZPs. We note that for d-DTAB concentrations
2–14 mM, visible aggregation was observed, prior to the creation of nanoemulsions.
This is commonly attributed to low-charged (near the isoelectric point) PS complexes
resulting in precipitation.[98, 99] Upon sonication, aggregation and precipitation pre-
vented the formation of stable nanoemulsions for this work, as nanoemulsions near
the isoelectric point are highly prone to flocculation.[100, 101]
37
Figure 4.4. VSFSS spectra of the (a) alkane CH stretching region and the (b)
aromatic CH stretching region of nanoemulsions stabilized by 0 mM d-DTAB (gray
●), 0.1 mM d-DTAB (blue ∎), and 1 mM d-DTAB (red ▲), all in conjunction with
5 mM PSS. All samples are at the solution natural pH of 7 and all spectra were
collected in the SSP polarization scheme. Spectra in the aromatic CH stretching
region of samples made with 0 mM d-DTAB/5 mM PSS and 1 mM d-DTAB/5 mM
PSS resulted in no signal and thus, are not shown.
From above, it is clear that d-DTAB concentration influences the long-term emul-
sion stability and sample quality based on the sample size distributions, diameters,
PDIs, and ZPs (Figure 4.2). To better understand the surface properties of nanoemul-
sions coated with d-DTAB and PSS at the molecular-level, we have employed VSFSS
to investigate the conformational ordering of PSS induced by adsorbed d-DTAB.
VSFSS spectra of 5 mM PSS sonicated with 0, 0.1, and 1 mM d-DTAB is shown
in Figure 4.4. The use of deuterated DTAB removes its spectral contributions in
the CH stretching region, allowing us to investigate the conformational structure of
strictly PSS. Figure 4.4a and 4.4b are VSFSS spectra of the aromatic CH and alkane
CH stretching modes of PSS, respectively. As these spectra were taken with the SSP
polarization combination, any resultant signal is indicative of vibrational modes with
a component of their dipoles perpendicular relative to the droplet surface. Without
the addition of d-DTAB, PSS yields no spectral response (Figure 4.4b, gray ●) even
though the highly negative ZP of these samples (Figure 4.2f, gray ●) indicates that
PSS is adsorbed to the droplet surface.
As noted above from ZP measurements, the addition of d-DTAB to PSS results
in increased co-adsorption to the nanoemulsion surface (Figure 4.2f). The impact
of co-adsorbed d-DTAB on the conformational arrangement and orientation of the
38
PSS pendant group and backbone is shown in Figure 4.4. All spectra shown herein
have been fit according to previous Richmond publications. While the details of the
fitting routine can be found elsewhere,[33, 34] the fit parameters for this work can
be found in Table A.7 in the Appendix. Spectral contributions from the aromatic
sp2 CH stretching mode of PSS near 3060 cm−1 are observed with 1 mM d-DTAB
(red ▲) and shown in Figure 4.4a.[60, 85] The alkane CH signatures corresponding
to the CH symmetric stretch (ss) near 2850 cm−12 , CH3 ss near 2880 cm
−1, CH2
asymmetric stretch (as) near 2910 cm−1, and CH3 as near 2975 cm
−1 are observed
with 0.1 mM (blue ∎) and 1 mM d-DTAB (red ▲) and depicted in Figure 4.4b.[60,
85] As cationic d-DTAB populates the droplet surface, it creates an interfacial electric
field that interacts with the anionic groups on PSS. We attribute the net ordering of
the PSS backbone to interfacial electrostatics, in contrast to the surface behavior of
adsorbed PSS alone, which does not show any net orientation. While a net orientation
of the aromatic CH modes of PSS appears only at a bulk d-DTAB concentration of
1 mM (Figure 4.4a), the backbone CH modes are aligned starting at a much lower
bulk d-DTAB concentration of 0.1 mM. We note that we do not achieve the same
spectral sensitivity to the aromatic CH modes as we do with the alkane CH modes
until the d-DTAB concentration is at 1 mM; however, the spectral appearance of
both types of CH modes are indicative of significant conformational ordering of PSS
perpendicular to the surface normal, in the presence of co-adsorbed d-DTAB. It is
interesting to note that the appearance of the aromatic CH modes (Figure 4.4a)
occurs at d-DTAB concentrations that increase PSS adsorption, as shown in the ZP
measurements (Figure 4.2f, red ▲). As mentioned previously, bulk concentrations of
d-DTAB between 2 and 14 mM resulted in complex aggregation, making it difficult to
create monodispersed nanoemulsions for this study and therefore, we refrained from
using higher bulk d-DTAB concentrations.
Ultimately, we attribute these orientation effects to electrostatic interactions be-
tween the cationic d-DTAB head group and the anionic sulfonate pendant group of
PSS, which then influences the net orientation of the polymer backbone. This is
in contrast to the surface behavior of adsorbed PSS alone, which is facilitated pre-
dominantly by hydrophobic interactions. In the absence of co-adsorbed d-DTAB, the
system lacks electrostatically driven interactions and thus, PSS does not show any
net orientation.
39
The effects of cationic PEI: Added electrosterics
With the adsorption of a secondary polymer, the effects of additional steric re-
pulsion and intermolecular interactions on emulsion stability were investigated. In
this section, we focus on the addition of cationic PEI at pH 3 as the second polymer
whereas later, we focus on this system at pH 10 for which PEI is then nonionic.[39,
52] For these studies, d-DTAB and PSS were sonicated together as described above
in Chapter 2 in Nanoemulsion Preparation, followed by PEI being added to the na-
noemulsion sample and allowed to adsorb to the droplet surface via electrostatic
interactions, mimicking common LbL adsorption techniques.[36, 102] The size distri-
bution of nanoemulsions stabilized by 0.1 mM d-DTAB, 5 mM PSS, and either 0, 1,
or 10 mM PEI is shown in Figure 4.5a–c, respectively. Figure 4.5d–f summarizes the
nanoemulsion diameters, PDIs, and ZPs, respectively, for samples containing 0.1 mM
d-DTAB, 5 mM PSS, and varying cationic PEI concentrations.
Samples stabilized with 0 mM (Figure 4.5a) and 1 mM PEI (Figure 4.5b) show a
constant, narrow, and largely uni-modal size distribution throughout the duration of
the study, demonstrating that the stability of the sample is preserved with the addi-
tion of 1 mM PEI. For 0 and 1 mM PEI, the nanoemulsions display similar diameters
(Figure 4.5d, orange ● and green ∎), and exhibit little change over the 30-day span.
PDI values for these samples (Figure 4.5e, orange ● and green ∎) remain largely in-
variant over time and are all < 0.3, indicating a monodispersed sample. Comparison
of the ZP measurements of samples with 0 mM PEI (Figure 4.5f, orange ●) and 1 mM
PEI (Figure 4.5f, green ∎) shows that the addition of 1 mM cationic PEI results in
a more negative ZP reading than the DTAB/PSS system. We attribute these obser-
vations to an increase in PSS adsorption through enhanced electrostatic interactions
with the presence of surface adsorbed cationic PEI, similar to the enhanced PSS ad-
sorption observed with a greater concentration of cationic d-DTAB described above.
Consequently, the overall net surface charge is more negative with more adsorbed
PSS. This enhanced adsorption of PSS will be confirmed below with our spectro-
scopic results. Overall, the unchanging size distributions, diameters, PDIs, and ZP
values with the addition of 1 mM cationic PEI confirm that the nanoemulsions are
stable with a second polymer layer.
40
Figure 4.5. Top panel: Size distribution measurements of nanoemulsions stabilized
by 0.1 mM DTAB, 5 mM PSS, and (a) 0 mM PEI, (b) 1 mM PEI, and (c) 10 mM
PEI. Bottom panel: (d) Nanoemulsion Z-average diameter, (e) PDI, and (f) ZP for
samples stabilized by 0.1 mM DTAB, 5 mM PSS, and 0 mM PEI (orange ●), 1 mM
PEI (green ∎), and 10 mM PEI (purple ▲). All samples were prepared at constant
pH 3 with the DTAB/PSS + PEI preparation scheme, in which DTAB and PSS were
first sonicated together to create nanoemulsions, then PEI was added and allowed to
adsorb to the droplet surface.
Contrary to the observed robustness and stability of the system with 1 mM PEI,
the addition of 10 mM PEI at pH 3 shows more dynamic behavior over the course of
30 days. The initial size distribution is comparable to that of samples containing 0
and 1 mM PEI (Figure 4.5c, solid black). Over time, the distribution shifts to slightly
larger sizes, accompanied by an increase in size populations exceeding 1000 nm. The
shift in size distribution to larger diameter is representative of the coalescence process,
as the nanoemulsions are collectively becoming larger. After Day 15, the broad uni-
modal distribution becomes bi-modal with changes to the relative intensities between
the smaller and larger size populations. Specifically by Day 30, the relative amount
of emulsions > 5000 nm in diameter surpasses that of emulsions < 500 nm, which is
evident of Ostwald ripening.[78, 91] Destabilization via Ostwald ripening occurs when
smaller droplets join with larger droplets, adding to the volume of larger droplets while
41
decreasing the population of smaller droplets.[78, 90, 91] Complementing the dynamic
behavior of the size distribution over time for the 10 mM PEI sample are diameters,
PDIs, and ZPs results (Figure 4.5d–f, purple ▲). Both the emulsion diameter and
PDI begins to significantly increase after Day 15, when the size distribution becomes
bi-modal. The ZP for samples containing 10 mM cationic PEI begins highly positive
around +60 mV, but continually decreases to a near-zero surface charge by Day 30.
Long-term droplet stability relies not only on the adsorption of chemical species,
but also the energetics that keep them there. While instantaneous PEI adsorption
to the droplet surface provides initial stability, as represented by the size distribu-
tion and PDI results of fresh samples (Day 0), the sample quality degrades over time
as size increases and the sample becomes more polydispersed. ZP values also sup-
port sample destabilization. As the surface charge goes to zero, there is decreased
inter-droplet electrostatic repulsion, which increases the probability of flocculation
and coalescence. Under the current solution conditions, PEI is highly charged and
soluble which presents an issue of competitive bulk solvation versus adsorption.[103]
Altogether, the increased variability in size distribution over time, the increase in
diameter and PDI, and the decrease in ZP magnitude indicate that PEI is desorbing
from the droplet surface and catalyzing the destabilization process. While we do
not quantify the amount of polymer desorption, the near-zero surface charge by Day
30 illustrates the depletion of charged PSS/PEI complexes at the droplet surface.
Thus, without any surface-active species at the droplet surface, there is no stabilizing
mechanism for the nanoemulsions. We note that while the size distribution of sam-
ples containing 10 mM PEI (Figure 4.5c, dotted black) show a population of sizes
measuring ∼500 nm, we are confident that these are bulk polymer structures in solu-
tion, rather than nanoemulsions. By Day 30, the sample is visually no longer opaque
and instead, largely transparent (see Figure 4.3, middle panel), confirming that an
emulsion is no longer present.[87, 92–94]
To understand how the amount of adsorbed charged PEI influences the backbone
and pendant group ordering of PSS, VSFSS measurements are used to probe the
droplet surface coated by d-DTAB, PSS, and PEI at constant pH 3. We only per-
form spectroscopic studies on fresh samples, prepared the day of our spectroscopic
experiments, to ensure the most uniform size distributions and PDIs < 0.3, making
them most suitable for VSFSS. In this preparation scheme, d-DTAB and PSS were
sonicated together. Upon dilution with a PEI solution, the final concentrations of
d-DTAB and PSS were fixed at 0.1 mM and 5 mM, respectively, while the final con-
centration of PEI varied between 0, 1, and 10 mM. Though many applications use
42
much higher bulk surfactant concentrations to stabilize emulsions, we have carefully
chosen to work with 0.1 mM d-DTAB for our spectrosopic studies to stay below max-
imum surfactant surface coverage.[25, 104, 105] This allows us to focus primarily on
how steric hindrance contributes to droplet stability. Additionally, this ensures that
any changes observed in the aromatic CH modes of PSS can then be attributed to
the added cationic PEI, since 0.1 mM d-DTAB has been shown to not impart any
detectable net alignment in the PSS pendant groups (Figure 4.4a).
Recall from above that samples containing d-DTAB/PSS leads to the polymer
displaying measurable conformational ordering of the PSS backbone and pendant
groups (Figure 4.4). Figure 4.6a shows the VSFSS spectra of the alkane CH stretching
region of nanoemulsions stabilized with 0.1 mM d-DTAB, 5 mM PSS, and either 0
mM PEI (orange ●), 1 mM (green ∎), or 10 mM PEI (purple ▲). These spectra
contain peaks characteristic of CH modes from an alkane chain, such as the backbone
of PEI and PSS.[51, 60, 85] The polymers both contain alkane CH stretching modes,
making it impossible to decouple the alkane CH stretches of PEI from those of PSS.
The SF response observed in this region is evidence that the alkane CH modes of
either PSS, PEI, or a combination of the two possess a net ordering perpendicular to
the droplet surface.
Figure 4.6. VSFSS spectra of the (a) alkane CH stretching region and the (b)
aromatic CH stretching region of nanoemulsions stabilized by 0.1 mM d-DTAB, 5
mM PSS, and 0 mM PEI (orange ●), 1 mM PEI (green ∎), and 10 mM (purple ▲),
all at constant pH 3. Spectra were collected in the SSP polarization scheme and lines
are fits to normalized data.
Additional information can be gained from the rise in intensity at and below 2800
cm−1, which corresponds to the tail of the OD stretching modes of highly coordinated
43
interfacial (heavy) water molecules. As shown in recent studies from this laboratory,
surface D2O has a broad stretch and libration combination mode in this region that is
highly sensitive to surface charge.[106] This mode interferes with the CH modes, with
its phase flipping by 180◦ as a surface goes from positive to negative. The increase in
this water signal with decreasing PEI concentration (shown in Figure 4.6a) mirrors
the change from a positively charged surface to one that is negatively charged, which
is consistent with ZP results in Figure 4.5f. By accounting for this mode in our
fits (see Appendix, Table A.8) the overall intensities of the CH modes are found
to increase with the addition of PEI. This is consistent with our ZP results above
showing that in the presence of cationic PEI as a secondary polymer layer, more PSS
is recruited to the surface (Figure 4.5f, orange ● and green ∎). In the presence of 10
mM cationic PEI, the ZP becomes highly positive and alkane CH intensities are the
highest (Table A.8). Since 1 mM PEI enhances PSS adsorption, we intuit 10 mM PEI
would further enhance PSS adsorption, leading to the increased intensities in the fits
for the alkane CH stretches. While PEI itself can contribute to the increased signal
observed between 2800 cm−1 and 3000 cm−1, we believe the signal increase is mostly
due to a higher interfacial PSS population as PEI has been reported to possess no
net orientation at the droplet surface in acidic conditions.[103]
Although the backbone CH modes of PEI and PSS are indistinguishable, only
PSS contains the aromatic CH modes.[60, 85] Figure 4.6b shows the VSFSS response
from the aromatic CH modes of the PSS pendant groups in the presence of 0.1 mM
d-DTAB and varying concentrations of PEI at pH 3. As shown, this mode near 3060
cm−1 increases with increasing PEI concentration. We attribute this signal increase to
enhanced surface PSS adsorption (and consequently, orientation) when more PEI is
adsorbed to the droplet surface. Recall ZP results demonstrate that a greater amount
of PSS is interfacially present with the addition of cationic PEI (Figure 4.5f). This
is further confirmation that PEI is interfacially present, as without it (Figure 4.6b,
orange ●), the aromatic CH modes are not detectable.
The effects of nonionic PEI: Added hydrophobicity and sterics
As pH increases from 3 to 10, PEI loses its cationic character and behaves as a
nearly neutral polymer.[39, 52] In this section, we address how nanoemulsion stability
and interfacial PSS ordering is altered upon the addition of a secondary nonionic
polymer. The size distributions of d-DTAB/PSS and either 0, 1, or 10 mM PEI
nanoemulsions at pH 10 were measured over 30 days and are shown in Figure 4.7a–c,
44
respectively (top panel). In Figure 4.7d–f (bottom panel), we summarize the results
from diameter, PDI, and ZP measurements, respectively, of nanoemulsions stabilized
by d-DTAB/PSS and 0, 1, or 10 mM nonionic PEI.
Figure 4.7. Top panel: Size distribution measurements of nanoemulsions stabilized
by 0.1 mM DTAB, 5 mM PSS, and (a) 0 mM PEI, (b) 1 mM PEI, and (c) 10 mM
PEI. Bottom panel: (d) Nanoemulsion Z- average diameter, (e) PDI, and (f) ZP for
samples stabilized by 0.1 mM DTAB, 5 mM PSS, and 0 mM PEI (orange ), 1 mM
PEI (green ), and 10 mM PEI (purple ). All samples were prepared at constant pH
10 with the DTAB/PSS + PEI preparation scheme, in which DTAB and PSS were
first sonicated together to create nanoemulsions, then PEI was added and allowed to
adsorb to the droplet surface.
As can be seen by the consistent, robust, and invariant size distributions over 30
days for all samples containing 0 mM PEI (Figure 4.7a), 1 mM PEI (Figure 4.7b),
and 10 mM PEI (Figure 4.7c), hydrophobic PEI preserves the nanoemulsion stability
with time at both low and high concentrations. Unlike what was observed for samples
containing 10 mM cationic PEI above at pH 3 (Figure 4.5c), with nonionic PEI,
competing factors such as bulk solvation no longer poses an issue to nanoemulsion
stability. For each PEI concentration, steady growth in diameter (Figure 4.7d) is
observed, while the PDI (Figure 4.7e) and ZP (Figure 4.7f) remains comparable and
45
largely constant. This demonstrates that under these preparation conditions, PEI
desorption is not occurring as it did with higher concentrations of cationic PEI. These
results present another avenue for droplet surface tunability because now, a higher
concentration of PEI can be used to coat the droplet surface without jeopardizing the
sample quality.
Figure 4.8. VSFSS spectra of the (a) alkane CH stretching reigon and the (b)
aromatic CH stretching region of nanoemulsions stabilized by 0.1 mM d-DTAB, 5
mM PSS, and 0 mM PEI (orange ○), 1 mM PEI (green □), and 10 mM (purple △),
all at constant pH 10. Spectra were collected in the SSP polarization scheme and
lines are fits to normalized data. Spectra collected in the aromatic CH stretching
regions of samples containing 0 and 1 mM PEI resulted in no signal and thus, has
been omitted for clarity.
Figure 4.8 shows the VSFSS measurements, collected in the SSP polarization
scheme, of the two polymers alkane CH modes (a) and the PSS aromatic CH modes
(b) for nanoemulsions stabilized with 0.1 mM d-DTAB, 5 mM PSS, and varying con-
centrations of PEI at pH 10. In Figure 4.8a, the alkane CH stretching modes increase
(see Table A.9 in the Appendix for fit parameters) with increasing PEI concentration
from 0 mM (orange ○) to 10 mM (purple △). Although it is difficult to interpret the
alkane CH region for this two-polymer system where both polymers contain alkane
CH signatures,[51, 60, 85] we conclude that both PSS and PEI are interfacially present
and arranged with their net dipoles perpendicular to the droplet surface. We note the
striking absence of the CH2 asymmetric stretch near ∼2910 cm
−1 in Figure 4.8a. The
ways in which the two polymers can complex with each other at the droplet surface
may cause interferences between the CH2 asymmetric stretches from the two different
46
polymers, ultimately resulting in a diminished net signal near ∼2910 cm−1. It is dif-
ficult to determine the exact orientation of the polymers to describe the interference
between a CH2 asymmetric stretch of PSS and a CH2 asymmetric stretch of PEI.
However, it is possible that the two together interacts in such a way that suppresses
this vibrational mode.
ZP measurements for these nanoemulsions (Figure 4.7f) are negative for all con-
centrations of PEI, evidence that PSS is coating the droplet surface, as PEI possesses
minimal charge at pH 10. Note that with increasing amounts of nonionic PEI, the ZP
values do not show significant changes that would suggest more PSS is recruited to
the droplet surface. With PEI being neutral in basic environments, PSS adsorption
is not enhanced as it was with increasing d-DTAB (Figure 4.2f) and cationic PEI
(Figure 4.5f) concentrations, due to the absence of additional electrostatics. Thus,
changes to the VSFSS spectra depicted in Figure 4.8a confirm that PEI is also present
at the droplet surface and contributing to the increase in spectral intensity.
The reduction in positive charges along PEI greatly reduces the net ordering of
PSS pendant groups, as shown in Figure 4.8b. At pH 3 (Figure 4.6b, green ∎), 1 mM
cationic PEI imparts an appreciable SF response in the aromatic stretching region.
Conversely, at pH 10, 1 mM nonionic PEI results in no discernible signal (not shown
for clarity). When the amount of charge on PEI is significantly reduced in basic
conditions, 10 mM PEI is needed to impart a very low-lying, yet measurable level of
net ordering amongst the PSS pendant groups (Figure 4.8b, purple △).[60, 85] We
conclude that with nonionic PEI, the electric field it induces at the droplet surface
is not strong enough to achieve the level of net ordering in PSS as observed with its
highly cationic form.
Previously with highly cationic PEI, electrostatic interactions are the dominant
intermolecular forces influencing PSS adsorption and net orientation. This resulted
in nanoemulsion samples that are coated with highly ordered PSS pendant groups,
but are not stable for the 30-day period. Conversely, at pH 10, although the lowered
solubility of PEI caused the reduction in net alignment of the PSS pendant groups,
the nanoemulsions did remain stable during the study, even with higher PEI concen-
trations. At pH 10, electrostatic interactions between d-DTAB and PSS still exist,
but without competitive bulk solvation of PEI causing its desorption (as occurred
above at pH 3), nanoemulsions containing hydrophobic PEI remain stable for much
longer. Thus, it is the balance of electrostatics between d-DTAB and PSS, as well
as the hydrophobic interactions between d-DTAB/PSS complexes and nonionic PEI
that contributes to the long-term stability of the sample.
47
Conclusions
Figure 4.9. Summary illustration of macroscopic and molecular-level studies of
polymer multilayering on nanoemulsion surfaces.
LbL deposition on planar surfaces has been employed for a broad range of applica-
tions, as it is a convenient method for tuning surface properties.[71, 73–76] Specifically
with the multi-layering of polymers around cargo-loaded emulsions for biopharmaceu-
tical advancements, both the macroscopic sample properties as well as molecular-level
interfacial properties of the system are of interest.[9, 11, 77, 78] In this study, we have
demonstrated that nanoemulsions < 500 nm with PDI < 0.3 can be created with a
stabilizing surfactant layer and coated with two different polymers: anionic PSS and
charge-tunable PEI, but the long-term stability of this system is highly dependent on
factors such as surfactant concentration, PEI concentration, and solution pH. More-
over, these multipolymer coated nanoemulsions are highly versatile, in which many
routes can be taken to modify their macroscopic and molecular surface properties.
While many applications rely on high bulk concentrations of small molecule surfac-
tants to stabilize emulsions, with the combined effects of two polymers, nanoemulsions
can be prepared with much lower bulk surfactant concentrations.
By adjusting the balance of electrostatic, hydrophobic, and steric interactions,
nanoemulsions with long- and short-term stability can be made with varying degrees
of conformationally ordered polymer layers. DTAB/PSS-stabilized nanoemulsions
remained stable throughout the 30-day span, and while varying DTAB concentration
influences PSS adsorption and backbone alignment, the effect it has on the ordering of
the PSS pendant groups is minimal. Alternatively, the charge density of PEI plays an
important role in altering both the amount of PSS adsorption and its net interfacial
ordering, as well as the duration in which the nanoemulsion sample remains stable.
In the DTAB/PSS/PEI combination, cationic PEI encourages more PSS to adsorb to
48
the droplet surface with highly oriented pendant groups, in comparison to its nonionic
counterpart. This is caused by the increased electrostatic interactions at the interface
when PEI is charged. However, over time, highly charged PEI is shown to desorb from
the droplet surface, catalyzing sample destabilization. Conversely, within the 30-day
period, destabilization does not occur for nanoemulsion samples containing nonionic
PEI due to its enhanced hydrophobicity, making desorption from the interface to the
bulk less energetically favorable. The added steric effects from PEI as a secondary
polymer layer also preserves the stability of the sample.
Together, our time-dependent nanoemulsion sample characterization and spectro-
scopic investigations of PSS/PEI polymer layering behavior at the curved oil/water
interface demonstrate how nanoemulsions can be made to have both tunable stability
and interfacial properties. The versatility of this surfactant/two-polymer system is
highly advantageous for applications such as drug delivery systems, which depend
on timely destabilization of the nanoemulsion to release the encapsulated cargo, or
applications that require a long shelf life such as cosmetics and emulsified foods.
49
CHAPTER V
NANOEMULSION STABILIZATION VIA STERIC EFFECTS
This work was published in Volume 37 of the journal Langmuir in October 2021.
Emma Tran designed the study, performed the experiments, analyzed the data, and
wrote the manuscript. Geraldine Richmond was the principal investigator for this
work and provided editorial assistance and general feedback.
Introduction
In the work discussed thus far, the focus has been placed on how polyelectrolytes
influence not only the surface functionality, but also the overall quality (i.e., droplet
size distribution, PDI, growth rate) of the nanoemulsion samples. From a more funda-
mental perspective, colloidal stability depends on a balance of different intermolecular
forces. As mentioned previously, nanoemulsions are kinectically stable and thus, over
time, will destabilize and phase separate into immiscible oil and water layers (i.e.,
the thermodynamically stable system). To enhance the stability of nanoemulsions for
applications, ionic surfactants and polyelectrolytes are frequently used as emulsifying
agents, effectively adding a layer of electrostatic charge around the droplet surface.
Common polyelectrolytes used in applications include PEI and PSS, as discussed in
Chapter 3 and Chapter 4 above. As a result, the charge–charge repulsion between
droplets acts as a stabilizing factor. However, the incorporation of nonionic surfac-
tants and polymers in application design has gained interest due to their advantageous
properties such as low cost, lowered toxicity, and increased biodegradability.[107] A
deeper understanding of the stabilizing mechanisms that nonionic emulsifiers provide
is important for guiding the informed design of these products.
This chapter is aimed towards better understanding the stabilizing effects due to
steric hindrance from nonionic polymer layers. For these studies, we have created
polymer/surfactant-stabilized nanoemulsions that have roughly no surface charge,
yet are stable for upwards of a month. The stabilizing agents used are a combi-
nation of surfactant sodium dodecyl sulfate (SDS) and nonionic polymer poly(N-
vinylacetamide) (PNVA). Although SDS is anionic, under specific PNVA concentra-
tions, the stable nanoemulsions possess nearly zero ζ-potentials. Using a combination
of experimental techniques such as dynamic light scattering, ζ-potential, and surface
spectroscopic measurements, we characterize the properties of these “uncharged” na-
noemulsions to learn about the intermolecular factors contributing to their stability.
50
Additionally, we calculated interaction pair potentials using the framework of ex-
tended DLVO (named after Derjaguin, Landau, Verwey, and Overbeek) theory to
better understand how the thickness of the polymer layer contributes to the stability
of these droplets.
Our calculations show that extended DLVO theory can be applied to these “un-
charged” nanoemulsions to explain the significant forces that contribute to droplet
stability. But beyond simply steric effects, we learn from our spectroscopic results
that the interfacial-bonding network surrounding the droplet surface is critical for
maintaining the stability of nanoemulsions with a negligible ζ-potential (ZP). The
strongly bonded, rigid polymer networks that are formed act as shells, protecting the
oil droplets from colliding and ultimately being destabilized. The insights from this
investigation provide a better understanding of the factors governing the interfacial
behavior of nonionic polymers. This opens up avenues for applications to integrate
more nonionic emulsifiers into their designs for a variety of purposes.
Steric Stabilization of Nanoemulsions via Poly(N-Vinylacetamide)
a)
b) c)
Figure 5.1. (a) Molecular structures of surfactant SDS and polymer PNVA. (b)
Droplet diameters and (c) ZP measurements as a function of SDS concentration for
nanoemulsions stabilized by SDS alone (black ●), SDS with 2.5 mM PNVA (orange
∎), and SDS with 5 mM PNVA (pink ▲). Error bars represent error-propagated
standard deviations from triplicate measurements of 5-7 different samples prepared
on different days.
The diameter and ζ-potential (ZP) of nanoemulsions are informative of initial
51
droplet stability. Nanoemulsions are in the size range of 100s of nanometers in di-
ameter. A highly charged surface typically reflects a stable colloid system.[2, 108]
Figure 5.1a shows the molecular structures of surfactant SDS and polymer PNVA.
Figure 5.1b,c depicts the measured diameters and ZP, respectively, for nanoemulsions
stabilized by SDS and PNVA, as a function of SDS concentration. The black, orange,
and pink traces represent samples containing SDS in conjunction with 0, 2.5, and 5
mM PNVA, respectively. It can be seen that nanoemulsions stabilized by only SDS
(black ●) measure between 300 and 500 nm and possess a highly negative ZP, which
is characteristic of SDS-stabilized nanoemulsions.[59, 103] As PNVA is added to the
sample and adsorbs to the droplet surface, two notable phenomena are observed: the
nanoemulsion size increases significantly (Figure 5.1b) and the magnitude of the ZP
decreases to ∼0 mV (Figure 5.1c). It is noteworthy that these droplets have roughly
zero ZP values when coated with the polymer. In previous literature reports, na-
noemulsions stabilized by nonionic surfactants with minimal ZP were stable for only
5-10 days.[108, 109] In other reports, nanoemulsions prepared with nonionic surfac-
tants have been shown to still possess an appreciable ZP (−20 to −50 mV).[110, 111]
It has been noted that the observed charge is attributed to trace impurities within
the surfactants and/or surface-active contaminants.[19, 112] Thus, we are careful to
ensure that the nanoemulsions we have prepared are as clean as possible.[19]
52
Figure 5.2. ZP measurements of nanoemulsions stabilized by SDS alone (black ●
or ○) and SDS/PNVA (pink ▲ or △). The SDS concentration was varied from
0.1, 0.5, 1, and 2.5 mM, while the PNVA concentration was fixed at 5 mM. The
solid traces depict samples prepared with thoroughly cleaned glassware (see Chapter
2, Nanoemulsion Preparation), and the dashed traces depict samples prepared with
loosely cleaned glassware (which we refer to as “insufficiently cleaned”).
In Figure 5.2, we demonstrate that insufficiently cleaned glassware can influence
the overall magnitude of the ZP of the prepared nanoemulsions. While we cannot
control any impurities that may be present in the purchased SDS or PNVA, we can
control the cleanliness of our system. Figure 5.2 shows the ZP measurements for
nanoemulsions stabilized by SDS alone and SDS/PNVA prepared in “cleaned” (▲ or
●) and “insufficiently cleaned” (△ or ○) glassware. The PNVA concentration was
fixed at 5 mM, while the SDS concentration ranged from 0.1, 0.5, 1, and 2.5 mM.
It can be seen that between “cleaned” and “insufficiently cleaned” glassware, samples
prepared with clean glassware show ZP values that are lower in magnitude. The
additional charge from contaminants may alter the interpretation of droplet stability,
especially in discussions involving extended DLVO Theory where the emphasis is on
uncharged surfaces. Therefore, we ensure that the samples we have prepared are as
clean as possible to minimize any trace contaminants that may influence the surface
charge and thus, the stability of the droplets.
Both the increase in droplet size and reduction in ZP magnitude are strongly in-
dicative of emulsion destabilization. We define our “uncharged” nanoemulsions as
those with ZP values between −10 and +10 mV, as this range is considered “neu-
tral” for nanoparticles.[113] Without an appreciable ZP, the nanoemulsions are more
susceptible to flocculation, coalescence, and Ostwald ripening, all processes that will
generally increase the droplet diameters in the sample. However, in stark contrast to
what initial size measurements would suggest, despite the bulk nanoemulsion sample
being comprised of larger sized, “uncharged” droplets in the presence of PNVA, we
measure that these samples remain stable upward of 35 days, and the dynamics of
the system are not trivial. Figure 5.3 shows the measured diameters, PDIs, and ZP
of these “uncharged” nanoemulsions stabilized by SDS and 5 mM PNVA, over the
course of 35 days.
Initially, nanoemulsions stabilized with SDS/PNVA are much larger than those
stabilized by SDS alone. We ascribe the relative difference of ∼100-300 nm in droplet
diameters to the PNVA coating on the droplet surface and the formation of stratified
polymer layers. The decrease in ZP magnitude due to nonionic polymer adsorption
53
has been observed previously for nanoparticles.[114] It is important to recall that
ZP measurements are not a direct measurement of the surface charge, but rather, a
measurement of the potential difference across the slipping plane of a colloid.[113, 115]
The adsorption of nonionic polymers to a colloidal surface, specifically in thick layers,
shifts the slipping plane further into the bulk phase, and consequently reduces the
measured ZP value.[114] Ultimately, this phenomenon accounts for both the formation
of larger droplets (due to polymer adsorption) and the near-zero ZP values since
PNVA is nonionic.
Figure 5.3. (a) Measured diameters, (b) PDIs, and (c) ZP of nanoemulsions stabi-
lized with SDS concentrations from 0.1 (orange ●), 0.5 (green ∎), 1 (blue ▲), and
2.5 (purple ▼) mM, in conjunction with 5 mM PNVA, as a function of time. Error
bars represent error-propagated standard deviations from triplicate measurements of
3 different samples.
Over time, rather than growing in size, samples containing 0.1 (orange ●), 1
(blue ▲), and 2.5 (purple ▼) mM PNVA are observed to shrink in droplet diameter
with time (Figure 5.3a). Accompanying the reduction in sizes is an increase in ZP
magnitude (Figure 5.3c). This observation can be explained by the desorption of
PNVA. As PNVA desorbs from the droplet surface, the nanoemulsion diameter shrinks
and the interfacial electric field induced by SDS becomes more significant, resulting
in more negative ZP values. Despite the complex dynamics of this system, we believe
the nanoemulsion samples are still stable as reflected by the monodispersity of the
system with PDI values < 0.3 (Figure 5.3b).[2] If the droplets were destabilizing
via flocculation, coalescence, or Ostwald ripening, the sample would become more
polydispersed, which would be reflected by an increase in the PDIs.
The nanoemulsions we have prepared with SDS/PNVA show interesting proper-
ties and dynamics. While deeper investigations will be necessary to fully describe
54
the dynamics of this system, we conclude that these “uncharged” nanoemulsions are
stable, even during periods where the ZP values were ∼0 mV. We believe the factors
contributing to the stability of the nanoemulsions when they are “uncharged” are
steric effects and the molecular structure of the adsorbed complexes, topics that will
be discussed in greater detail in the sections to come.
DLVO and Extended DLVO Theory
Calculations. DLVO theory describes colloid stability through interaction pair po-
tentials derived from attractive van der Waals and repulsive charge-charge (also de-
scribed as “electric” double-layer) interactions between two spheres. The interaction
potential W between two spheres (i.e., droplets) of radii R1 and R2 as a function of
interdroplet distance D is expressed as[116, 117]
W (D) = Wvdw +Welec
A R1R2 R R= 1 2 −κD− ( ) + Ze (26)6D R1 +R2 R1 +R2
where A is the Hamaker constant equal to −200.55 × 10 J for hexadecane-in-water
nanoemulsions, −1κ is the Debye screening length, and Z is an interaction constant.
The Hamaker constant for hexadecane-in-water nanoemulsions is[117, 118]
− 2 ( 2 − 2 )23 ε
= ( hexadecane εwater) 3hν n nA kBT ε + ε + √
e hexadecane water . (27)
4 3hexadecane water 16 2 (n2 2 2hexadecane + nwater)
Parameters used to calculate the Hamaker constant for this work can be found in
Table A.10 in the Appendix.
The Debye screening length is given by[119]
−1 0.304
κ = √ (28)
I(M)
where I is the ionic strength in molar (M). For this work, the ionic strength and
corresponding Debye length can be found in Table A.11 in the Appendix.
The interaction constant is defined as[116, 117]
(kT
2 zeψ
Z = 2 o −164πεoε e ) tanh ( ) J m (29)4kT
where ψo is the droplet surface potential in mV. In our work, we use experimentally
55
measured ζ-potentials to calculate the interaction potentials. This has been done as
an approximation in previous work following the rationale that the ζ-potential values
closely mimic the electrostatic potentials.[117, 118]
First, we note that for two droplets of roughly the same size, R1 ≈ R2 ≡ R. Then
Eq. 26 above becomes
( AR 1 −κDW D) = − + RZe (30)
12D 2
Second, we note that at room temperature ( = ◦T 25 C), the interaction constant
becomes[116]
= −11 2
ψo −1
Z 9.22 × 10 tanh ( ) J m (31)
103
The equation describing the van der Waals attractive forces is chosen for the case
where the interdroplet distance is less than the droplet radius.[116] The interdroplet
distance is defined as the distance from one edge of a nanoemulsion to the next.
For our nanoemulsions stabilized by a steric polymer layer with ζ-potential mea-
suring ∼0 mV, extended DLVO theory was used to calculate the interaction potentials.
Under the framework of extended DLVO theory, two additional repulsive terms are
introduced[120–122]
W (D) = Wvdw +Welec +Wosmotic +Wentropic (32)
with
4πR 2 1 2 D 1 D
Wosmotic = ν ϕp ( − χ)L ( − − ln ) (33)2 2L 4 L
where R is the droplet radius, ν is the volume of one solvent molecule, ϕp is the
volume fraction of polymer, ϕ is the volume fraction of droplets, χ is the Flory-
Huggins constant for polymers, L is the steric layer thickness, Mw is the polymer
molecular weight, and ρ is the density of the oil core. The equations chosen for both
Wosmotic and Wentropic are for the case where the interdroplet distance is less than the
steric layer thickness.[120–122]
⎢⎡ ⎛ 3 − D 2 D ⎤2πR ⎞ 3 − ⎥
Wentropic =
2 ⎢
ϕL ρ ⎢⎢⎢⎢
D
ln⎜D ( L ) ⎟ D ⎥− 6 ln( L ) + 3 (1 + )⎥⎥⎥⎥ (34)Mw ⎢⎣L ⎝L 2 ⎠ 2 L ⎦⎥
In the following sections, DLVO and extended DLVO theory will be used to calculate
interaction pair potentials for SDS/PNVA-stabilized nanoemulsions.
56
General Application of DLVO Theory. To better understand the steric effects
stabilizing our “uncharged” nanoemulsions (Figure 5.3), theoretical frameworks de-
tailing colloid stability were applied. While DLVO theory has been predominantly
employed within the context of solid particle stability, we have applied the framework
to soft nanodroplets. Figure 5.4 summarizes the results from calculating interaction
pair potentials using the framework described by standard DLVO theory and extended
DLVO theory, as described above.[116–122]
Figure 5.4. Theoretical interaction potential diagrams calculated using (a) standard
and (b) extended DLVO theory. Calculations using standard DLVO theory were
done for droplets with varying ZP values and no steric layers, while calculations
using extended DLVO theory were done for droplets with ZP = 0 mV and varying
steric layer thickness. The interaction potentials are in units of kT . Equations and
parameters used to generate plots can be found in the Calculations section above and
the Appendix.
Since the 1940s, the fundamental concepts describing colloid stability have been
largely captured by DLVO theory. It explains colloid stability through the interaction
potential W , at some distance D, between two droplets, which is a balance between
attractive van der Waals and repulsive electrostatic forces (Equation 26). Theoretical
interaction potentials were calculated using DLVO theory where all parameters were
fixed, except for the ZPs, which were varied from 0 to -50 mV to progressively increase
the degree of electrostatic repulsion, and can be seen in Figure 5.4a. At infinitesimally
small interdroplet distances, the droplets fall into a “primary minimum” where they
are not stable. A “secondary minimum” is established with a rise in an energy barrier
due to charge-charge repulsion, which is a stable region for colloidal systems. In
Figure 5.4a, for a colloid surface where the ZP is 0 mV (black), only a primary
minimum exists. As the ZP magnitude increases, a progressive increase in the energy
57
barrier is observed. The larger the energy barrier, the more time the droplets spend
in the secondary minimum, which demonstrates that an appreciable ZP is crucial for
stability.
An important aspect of stability that is not captured by standard DLVO theory
is steric hindrance, which arises from adsorbates that are larger and contain bulkier
groups in their chemical structure. Especially in nanoemulsion applications where
macromolecules such as polymers are adsorbed in multiple layers, the thickness of the
steric layer becomes important. Extended DLVO theory accounts for these additional
repulsive forces through osmotic and entropic repulsion. Osmotic repulsion occurs
when the steric region of two droplets overlaps and the osmotic pressure within the
overlap region increases. To offset the increased osmotic pressure, the droplets will
repel one another. From an entropic perspective, entropy is decreased in the overlap
region where the steric layer is compressed. To re-establish a state with higher entropy,
the droplets will again repel.
Extended DLVO theory was applied to calculate the interaction potentials for
droplets with ZP values fixed at 0 mV. The only parameter varied was the thickness
of the steric layer, as shown in Figure 5.4b. The black trace in Figure 5.4b is the
same black trace from Figure 5.4a, calculated with standard DLVO theory, to serve
as a visual comparison. Immediately, we see that the energy barrier and secondary
minimum is recovered, and the height of the energy barrier increases as the steric
layer becomes thicker. This demonstrates that in order to accurately describe the
stability of the “uncharged” nanoemulsions we have created, extended DLVO theory
is certainly a more fitting model.
Applying Extended DLVO Theory to “Uncharged” Nanoemulsions. It has
been demonstrated above that steric effects can provided meaningful elements to
the description of colloid stability, especially in systems with marginal electrostatic
repulsion such as the “uncharged” nanoemulsions prepared in this work. The top
panel of Figure summarizes the interaction potentials calculated for nanoemulsions
stabilized with varying concentrations of SDS (Figure a) and with SDS/PNVA com-
plexes (Figure b). The bottom panel of Figure represents a zoomed in perspective of
the interaction potentials calculated for the uncharged nanoemulsions using standard
(Figure c) and extended (Figure d) DLVO theory. The concentration of PNVA was
fixed at 5 mM, while the concentration of SDS varied from 0.1 (orange), 0.5 (green),
1 (blue), and 2.5 (purple) mM.
58
Figure 5.5. Top panel: Experimental interaction potentials calculated using stan-
dard DLVO theory for nanoemulsions stabilized by (a) SDS alone and (b) SDS/PNVA.
Bottom panel: Zoomed in plot of interaction potentials calculated with (c) standard
and (d) extended DLVO theory for “uncharged” nanoemulsions. The PNVA con-
centration was fixed at 5 mM, while the SDS concentration was varied between 0.1
(orange), 0.5 (green), 1 (blue), and 2.5 (purple) mM. The interaction potentials are
in units of kT.
The stability of surfactant-stabilized nanoemulsions has been well-documented in
literature.[1, 2, 4] Thus, it is expected that nanoemulsions stabilized by anionic SDS
would be stable due to the high charge density at the droplet surface, as predicted
by DLVO theory in Figure 5.5a, which summarizes the interaction potentials for na-
noemulsions stabilized with increasing SDS concentrations. Upon the addition of 5
mM PNVA to samples with various SDS concentrations, the magnitude in ZP de-
creases to a constant level near zero with increased SDS concentrations (Figure 5.1c).
The calculated interaction potentials using standard DLVO theory for these samples
are shown in Figure 5.5b. At a low SDS concentration of 0.1 mM (orange), the DLVO
interaction potential indicates that the remaining ZP is still sufficient to stabilize the
59
nanoemulsions, while predicting that samples with near-zero ZP are not stable at all.
From the zoomed in perspective shown in Figure 5.5c using standard DLVO theory,
nanoemulsions prepared with 5 mM PNVA and 0.5 (green), 1 (blue), and 2.5 mM
(purple) SDS will immediately fall into the primary minimum and destabilize, accord-
ing to standard DLVO theory. Without the charge-charge repulsive forces balancing
the van der Waals attractive forces, there would indeed be no stabilizing mechanism
for the droplets. However, the thickness of the polymer layers has not been accounted
for in standard DLVO theory.
Figure 5.5d shows the interaction potentials calculated using extended DLVO the-
ory for nanoemulsions prepared with 5 mM PNVA and 0.5 (green), 1 (blue), and 2.5
mM (purple) SDS. The parameters used to calculate the interaction potentials can be
found in Table A.12 of the Appendix. The main distinguishing features between Fig-
ures 5.5c and 5.5d are the appearance of the energy barrier and secondary minimum
using the extended theory. Note that the energy barrier increases with increasing
polymer thickness (green to purple). This demonstrates that as more PNVA adsorbs
to the nanoemulsion surface, the stability of the droplets is enhanced. We also note
that in order to establish steric stabilization, the thermal energy of the system should
exceed the van der Waals attraction.[118] Work done by Morozova et al. has demon-
strated that the minimum thickness of the steric layer must adhere to the following
criterion:
AR
L > (35)
24kBT
where L is the steric layer thickness, A is the Hamaker constant for hexadecane-in-
water nanoemulsions, R is the droplet radius, kb is the Boltzmann constant, and T is
temperature. Our estimated PNVA (i.e., steric) layer thickness satisfies this criterion,
further confirming the stabilizing effects PNVA has on droplets with near-zero ZP.
Counter to standard DLVO predictions, we have demonstrated above that
SDS/PNVA-stabilized nanoemulsions exhibiting nearly zero ZP can still be highly
stable (Figure 5.3). We predominantly attribute the stability of these droplets to the
thick steric layers formed by the adsorption of nonionic PNVA, which is supported
by our interaction potential calculations. To account for the steric effects in these
nanoemulsions, we applied extended DLVO theory (which has previously been exclu-
sive to hard colloid systems such as nanoparticles)[120, 122] to calculate a different
set of interaction potentials shown in Figure 5.5d. Steric hindrance introduces ad-
ditional repulsive forces that prevent droplet flocculation and coalescence. However,
intermolecular interactions between the oil droplet, surface adsorbates, and aqueous
medium may also reveal insights that size measurements and DLVO theory cannot
60
predict. Thus, determining the conformational arrangement of the SDS/PNVA com-
plexes at the droplet surface is highly informative and will be the topic of discussion
for the next section.
Interfacial Bonding Effects Contributing to Droplet Stability
a) b)
c) d)
e) f)
61
Figure 5.6. Top panel: VSFSS measurements of nanoemulsions stabilized by (a)
h-SDS/PNVA and (b) d-SDS/PNVA in the C–H stretching region. The spectra were
collected in the SSP polarization combination, probing dipole components perpendic-
ular to the droplet surface. Note that the spectra in (b) were scaled by a factor of 4,
in order to be plotted on the same intensity scale as the spectra in (a). Middle panel:
VSFSS measurements of nanoemulsions stabilized by (c) SDS alone and (d) SDS with
PNVA in the symmetric S=O stretching region. The spectra were collected in the
PPP polarization combination, probing dipole components perpendicular and paral-
lel to the droplet surface. Bottom panel: (e) VSFSS measurements of nanoemulsions
stabilized by SDS/PNVA in the carbonyl C=O stretching region. The spectra were
collected in the SSP polarization combination, probing dipole components perpen-
dicular to the droplet surface. (f) Illustration denoting intra- (red and green dotted
lines) and intermolecular (blue dotted lines) interactions between neighboring PNVA
strands. For spectra with or without PNVA, the SDS concentrations were 0.1 (or-
ange), 0.5 (green), 1 (blue), and 2.5 (purple) mM. For samples stabilized with both
SDS and PNVA, the PNVA concentration was fixed at 5 mM.
Hydrophobic Interactions between PNVA Backbone and SDS Alkyl Chain.
Polymer adsorption and self-assembly is often driven by electrostatic interactions
between the polymer and the substrate, especially in the cases where the polymer
is a polyelectrolyte.[36, 123, 124] However, because PNVA is a nonionic polymer,
hydrophobic interactions become more essential in the adsorption process. To inves-
tigate the hydrophobic behavior of SDS/PNVA complexes at the oil/water droplet
interface, VSFSS measurements of nanoemulsions stabilized by 5 mM PNVA and
0.1 (orange), 0.5 (green), 1 (blue), and 2.5 (purple) mM SDS in the CH stretch-
ing region were obtained and are depicted in Figure 5.6 (top panel). All spectra
shown here were collected in the SSP polarization scheme, which probes dipole com-
ponents perpendicular to the droplet surface. Figure 5.6a are spectra collected with
standard hydrogenated SDS (h-SDS) and PNVA, which provide insight into the net
conformational ordering of the SDS/PNVA complexes. In order to decouple the CH
resonances of the SDS alkyl chain from the PNVA backbone, deuterated SDS (d-SDS)
was used in place of h-SDS. This redshifts the CH resonances of SDS and thus, all
sum frequency responses shown in Figure 5.6b are attributed solely to PNVA. We
note that the spectra shown in Figure 5.6b were scaled by a factor of 4 in order to
be clearly plotted on the same axis scale as the spectra in Figure 5.6a. The CH
resonances observed near ∼2850, 2890, and 2930 cm−1 correspond to the methylene
CH2 symmetric stretch, the methyl CH3 symmetric stretch, and the methylene CH2
asymmetric stretch, respectively, and are typical of CH containing surfactants and
polymers.[59, 125]
62
Figure 5.6a shows the SF intensity increasing with increasing bulk h-SDS concen-
tration. As the nanoemulsions are coated with both h-SDS and PNVA, the observed
increase in intensity may be attributed to a few factors. A higher bulk h-SDS con-
centration can result in a greater population of h-SDS at the interface, which can
consequently recruit more PNVA to adsorb. Additionally, a higher interfacial concen-
tration of h-SDS/PNVA complexes can induce a stronger degree of net orientation
within the surfactant tails and PNVA backbones via hydrophobic interactions. While
these are all possible explanations for the progressively increasing SF intensity, it is
not possible to decouple the contributions from h-SDS to those from PNVA, which is
where utilizing deuterated SDS becomes important. The fit parameters can be found
in Table A.13 of the Appendix.
Figure 5.6b shows the spectra of nanoemulsions stabilized by d-SDS and PNVA.
Since d-SDS no longer contains CH resonances, all SF intensity observed here de-
scribes PNVA interfacial behavior and conformational ordering. As the bulk PNVA
concentration is fixed at 5 mM, enhancement in the SF response is attributed to
enhanced PNVA adsorption with increasing surfactant concentration, thus forming a
thicker polymer layer. The increased SF intensity with increasing d-SDS concentra-
tion in Figure 5.6b can also be explained by PNVA adopting a stronger net orientation
around the droplet surface. However, we believe a higher PNVA population adsorbing
to the surface is a more likely interpretation as it is consistent with our nanoemulsion
size measurements above (Figure 5.1b), depicting that droplets coated with SDS and
PNVA are substantially larger in diameter than those coated with SDS alone. This is
also in agreement with extended DLVO calculations above (Figure 5.5d), predicting
that a thick steric layer is a considerable stabilizing factor for droplets with negli-
gible ZP values. Figure 5.6b also illustrates that the PNVA backbone maintains a
net orientation upon adsorption, likely forming strongly bonded polymer networks.
Previous work in literature has reported polymer self-assembled structures to result
in a stratified, interpenetrated network of polymer strands.[126–129] This network is
formed through complex interactions between the polymer and neighboring species.
Thus, it is likely that the net orientation of the PNVA backbone allows the polymer
strands to partake in such interactions, leading to the formation of a rigid network.
Table A.14 in the Appendix shows the peak assignments and fitting parameters used
to fit spectra shown in Figure 5.6b.
We conclude that the hydrophobic portions of the SDS/PNVA complexes formed
at the nanoemulsion surfaces have conformationally ordered surfactant alkyl tails
and polymer backbones. The enhancement in adsorbed PNVA due to increasing
63
amounts of interfacial SDS suggests that the thick polymer layers are preventing the
“uncharged” droplets from coalescence and immediate destabilization.
Bonding Environment around SDS Headgroup. Further insights into the
molecular nature of the SDS/PNVA co-adsorption can be obtained from spectro-
scopic measurements of the polar regions of the surfactant and polymer. One would
expect that the SDS head groups rest directly at the nanoemulsion surface, inter-
acting with interfacial water and the co-adsorbed PNVA. The bonding interactions
between the sulfate head group and other interfacial species can highlight additional
sources of droplet stability. Figure 5.6 (middle panel) shows VSFSS spectra in the
symmetric S=O stretching region of nanoemulsions stabilized by SDS alone (Figure
5.6c) and SDS/PNVA (Figure 5.6d).[58, 130–132] The PNVA concentration was fixed
at 5 mM, while the SDS concentration varied between 0.1 (orange), 0.5 (green), 1
(blue), and 2.5 (purple) mM. These spectra were collected in the PPP polarization
combination, probing perpendicular and parallel dipole components relative to the
interface.
In both Figures 5.6c and 5.6d, the SF intensity of the symmetric S=O stretch
near ∼1040 cm−1 increases with increasing SDS concentration. For nanoemulsions
stabilized with 0.1 and 0.5 mM SDS alone (Figure 5.6a) or in conjunction with PNVA
(Figure 5.6b), the SF intensities were of comparable magnitude (see Table A.15 and
A.16 in the Appendix for fitting parameters). However, samples containing 1 and
2.5 mM SDS without PNVA are appreciably lower in intensity. Our nanoemulsions
are initially prepared with only SDS; PNVA is then added afterward and allowed
to adsorb. Thus, we attribute the relative increase in the SF response between SDS
alone (Figure 5.6c, blue and purple) versus SDS/PNVA (Figure 5.6d, blue and purple)
to more highly oriented SDS head groups, induced by a thicker steric PNVA layer.
Note that the largest nanoemulsions with the lowest magnitude ZP (Figure 5.1) were
also prepared with 1 and 2.5 mM SDS in conjunction with PNVA. Alternatively,
an increase in SDS adsorption (and thus, population) driven by PNVA adsorption
would also increase the intensity of the S=O resonance. However, this interpretation
is unlikely, as it is not supported by our ZP measurements above (Figure 5.1c). If
PNVA encouraged more SDS to adsorb, the ZP would consequently become more
negative, rather than remain nearly neutral.
As discussed above, strongly bonded networks of polymer strands would induce
more rigidity to the SDS/PNVA complex at the droplet interface. This rigidity
would reduce the possible orientations the SDS headgroups could adopt, resulting
64
in a stronger net alignment, as observed in Figure 5.6d. The enhancement in the
SDS net ordering due to the presence of PNVA provides insight into the interfacial
chemistry occurring at the boundary of the oil droplet and the PNVA layers. To
investigate how the steric PNVA layer bonds with its environment at the droplet
surface, we need to probe other functional groups on PNVA.
Bonding Environment around PNVA Monomer Units. The polar regions
of PNVA are composed of secondary amide functional groups, with a carbonyl C=O
resonance. This vibrational mode is sensitive to changes in bonding environment
due to intermolecular forces that are attracted to the lone pairs of electrons on the
oxygen. Figure 5.6e shows VSFSS spectra, collected in the SSP polarization scheme,
of nanoemulsions stabilized by SDS/PNVA in the amide C=O stretching region.[125,
133–135] Again, the PNVA concentration was fixed at 5 mM, while the SDS con-
centration varied from 0.1 (orange), 0.5 (green), 1 (blue), to 2.5 (purple) mM. The
carbonyl spectral region has been notoriously difficult to probe due to a highly in-
tense nonresonant response that obscures any resonant responses from the species
of interest.[136, 137] In order to measure the resonant C=O responses from PNVA,
the visible pulse was carefully de-timed from the IR pulse by ∼500-600 fs, to re-
main within the vibrational lifetime of C=O stretching resonance.[138] This detiming
approach allows for the resonant response to be measured, while minimizing any un-
wanted overlapping contributions.[139–141] Its presence indicates that as the polymer
adsorbs to the interface, it does so with the carbonyl mode adopting a net orientation
perpendicular to the interface as opposed to solely random in orientation. Though
this method has allowed us to capture a discernible peak from the C=O resonances,
we note the presence of a residual elevated background in all spectra is still present.
This background is likely manifested from the initial broad nonresonant response
that was minimized, but not fully eliminated. Additionally, an elevated background
can also be attributed to a variety of effects including: (3)χ contributions, dispersive
D2O background, and a nonresonant contribution from the calcium fluoride window
containing our sample.[54, 142–144]
In order to fit the C=O stretching region and obtain meaningful information on the
C=O resonance, two non-physical background peaks were identified and incorporated
into the fits. We emphasize that these background peaks do not present physical
meaning, but rather allow us to properly fit the C=O mode of interest. Using the
spectrum with the lowest C=O response (0.1 mM SDS/5 mM PNVA), two background
peaks were placed at 1593 and 1777 cm−1. The peak parameters were determined and
fixed for all spectra in Figure 5.6e, except the peak amplitude. The background peak
65
amplitudes were allowed to vary because the elevated background intensity visibly
varies for different samples. In doing so, we were able to fit the C=O stretching region
for nanoemulsions stabilized by SDS/PNVA and extract the fit parameters shown in
Table A.17 of the Appendix. This process has allowed us to fit our spectra to a
peak near ∼1630 cm−1, corresponding to the “associated” C=O stretch for secondary
amides.[125, 133–135]
Table 5.1. Fitting parameters for spectra of SDS/PNVA stabilized nanoemulsions
in the C=O stretching region, corresponding to Figure 5.6e. Note that the parame-
ters for the arbitrary background peaks have been omitted for clarity. Complete fit
parameters can be found in Table A.17.
Fixed [PNVA] = 5 mM SDS concentration:
Peak [SDS] = [SDS] = [SDS] = [SDS] =
assignment Parameter 0.1 mM 0.5 mM 1 mM 2.5 mM
Amplitude 0.01 ± 0.001 0.01 ± 0.001 0.03 ± 0.001 0.03 ± 0.002
Phase 0 0 0 0
C=O stretch Lorentzian 5 5 5 5
Peak position 1629 ± 1 1628 ± 1 1629 ± 1 1629 ± 1
Gaussian 14 ± 1 15 ± 2 20 ± 1 21 ± 2
The fit parameters for specifically the C=O peak are provided in Table 5.1 for easy
reference. It can be seen that while the changes in peak amplitude are negligible, the
Gaussian width broadens with increasing SDS concentration as well as with increasing
thickness of the PNVA layer. This indicates that as more PNVA adsorbs to the
droplet surface, there is an increase in molecular bonding between the interfacial
species. A strongly bonded PNVA network would provide avenues for both intra–
and intermolecular forces to exist, as illustrated in Figure 5.6f. Among the monomer
units of PNVA, hydrogen bonding can occur intramolecularly between the N–H and
the C=O moieties (illustrated by the red dotted lines). Additional hydrogen bonding
can occur intermolecularly between the polymer N–H and C=O groups on neighboring
strands (illustrated by the blue dotted lines), as well as the C=O group and interfacial
water molecules. Carbonyl “head-to-head” interactions (i.e., C=O⋯C=O) are also
common and will impart more stiffness to the adsorbed polymer layers (illustrated by
the green dotted lines). These types of molecular interactions are consistent with what
has been observed for other carbonyl-containing species.[125, 145–148] Ultimately, the
broadening of the Gaussian width supports the interfacial picture that a stratified,
rigid polymer network is contributing greatly to droplet stability.
66
Conclusions
Interfaces are ubiquitous in nature and present in various applications. Thus, the
knowledge around chemical processes that occur at the junction of two immiscible
liquids is highly valuable. Specifically, the interest surrounding oil and water has
been predominantly motivated by efforts to develop safer dispersants for oil remedia-
tion, design stimuli-responsive nanocarriers for drug delivery, and understand complex
mechanisms that contribute to oil-in-water emulsion stability. While nanoemulsions
are widely prevalent in several applications, their stability is often a limiting factor
in their versatility. Colloid stability has been primarily described by DLVO theory,
which relates the stability to a balance of attractive van der Waals and repulsive elec-
trostatic forces. The problem that arises with an “uncharged” nanoemulsion surface is
the lack of repulsive charge-charge repulsion between neighboring droplets. As DLVO
theory would predict, such a colloid system would have very minimal stability.
In the work presented in this chapter, we have created nanoemulsions stabilized
by anionic surfactant SDS and nonionic PNVA, with ZP measuring ∼0 mV. Contrary
to what standard DLVO theory would predict, these “uncharged” nanoemulsions are
stable upward to 35 days with robust size distributions and constant PDI values in-
dicative of sample monodispersity. In order to more accurately model the stability of
these nanodroplets, we applied extended DLVO theory to calculate interaction poten-
tials that would imply a stable colloid system. We learn that the added steric effects
from the PNVA layer result in additional repulsive forces that prevent immediate
coalescence and destabilization.
By employing surface-specific VSFSS, we were able to confirm the presence of both
the surfactant and polymer at the nanodroplet surface and develop a molecular-level
understanding of how the conformational arrangement and adsorption behavior of
SDS and PNVA complexes contribute to the emulsion stability. As illustrated in
Figure 5.6f, the probing of interfacial C–H, S=O, and C=O vibrational modes re-
veals that the SDS/PNVA complexes formed at the droplet surface are in a stratified,
strongly bonded network of polymer strands. The adsorbed PNVA strands interact
intramolecularly with neighboring monomer units and intermolecularly with neigh-
boring polymer strands and interfacial water molecules. This ultimately forms a rigid
network that acts as a stabilizing mechanism for these “uncharged” droplets.
All in all, we have illustrated that nanoemulsions with negligible ZP can be created
without them immediately coalescing. By explicitly accounting for steric effects, the
extended DLVO theory underlines the importance of accounting for factors such as
67
steric hindrance in colloid stability. Additionally, the conformational ordering of the
SDS/PNVA complexes and especially the arrangement of the PNVA layers upon ad-
sorption are crucial stabilizing factors that DLVO theory, size, and ZP measurements
alone cannot capture. Our findings provide insight into the basic science behind the
interfacial phenomena aiding the stabilization of oil droplets via nonionic species,
expanding the applicability of “uncharged” nanoemulsions.
68
CHAPTER VI
ZWITTERIONIC SURFACTANTS AND CO-STABILIZERS
This work was not yet published, but under review at the time this dissertation
was submitted. Emma Tran designed the study, performed some of the experiments,
analyzed the data, and wrote the manuscript. Konnor Jones and Gabrielle Cano
performed some of the experiments and provided general feedback on the manuscript.
Frederick Moore provided editorial assistance and general feedback on the manuscript.
Lawrence Scatena was the principal investigator for this work and provided editorial
assistance and general feedback.
Introduction
Amphiphilic molecules such as surfactants play a significant role in diverse pro-
cesses including emulsification,[149] foaming,[150–152] interfacial property modifica-
tion,[153–155] excipient formulation,[156, 157] drug delivery,[158, 159] environmental
remediation[155, 160] and oil recovery.[161, 162] While surfactants have hydrophobic
and hydrophilic regions, their structural details vary. Surfactant head groups gen-
erally fall into two classes: charged (i.e., anionic or cationic) and uncharged (i.e.,
nonionic or zwitterionic).[163] While there exists an exhaustive body of literature on
the former, studies investigating the latter are an active area of research. The shift
to understanding uncharged surfactants was motivated by the desire and need to
incorporate more environmentally compatible chemicals into applications involving
pharmaceuticals, environmental remediation, and food science. Nonionic and zwitte-
rionic surfactants have been shown to be less ecotoxic and more biodegradable, while
still exhibiting sufficient emulsifying and foaming capabilities, but details are still
largely lacking.[164–166] Thus, investigating the interfacial properties and behavior
of such surfactants at a molecular-level will yield additional insight and knowledge
that can promote their usage in various formulations.
In this chapter, we focus on the zwitterionic surfactant dodecyl-N,N-dimethyl-3-
ammonio-1-propanesulfonate (DDAPS) and investigate its interfacial behavior in the
presence of different co-additives at both a planar and droplet (e.g., nanoemulsion)
oil/water interface. DDAPS adsorbs spontaneously to a planar interface, but for na-
noemulsions, the interface is formed through ultra-sonication and DDAPS’s presence
at the interface stabilizes the droplet surface. The self-assembly of ionic surfactants
69
at planar and colloidal surfaces has been studied to a great extent. Relatively, little
work has been done to understand how zwitterionic surfactants behave at a planar
versus droplet (i.e. curved) interface–and this is especially pertinent to oil spill reme-
diation via chemical dispersant systems. In this application, a large oil slick is doused
with a mixture of surfactants that are intended to disperse the oil slick into smaller oil
droplets. Once the oil droplets are small enough, microbes can then naturally degrade
the oil.[167, 168] In this example, the large oil slick is the thermodynamically stable,
planar oil/water interface; while the dispersed oil droplets are the kinetically stable,
curved oil/water interface. Thus, by comparing the two interfacial geometries, we are
able to learn how DDAPS’s behavior in the presence of different co-additives varies be-
tween systems governed by different energetics. The co-additives in this study include
salts and charged surfactants to mimic a more application-relevant environment.[169,
170]
By utilizing interfacial tensiometry coupled with surface-specific vibrational spec-
troscopic techniques, we were able to elucidate the molecular-level details of DDAPS
adsorption and molecular conformational ordering with and without co-additives.
Traditional sum frequency (SF) spectroscopy was employed to investigate DDAPS at
the planar oil/water interface, while sum frequency scattering spectroscopy was used
to probe the nanoemulsion surface. The results confirm that the interfacial geometry
has an effect on DDAPS’s interfacial behavior, as well as the interactions between
DDAPS and co-additives. DDAPS alone adsorbs to the planar oil/water interface
with a net conformational ordering of its alkyl tails that is unaffected by the presence
of co-additives. At the droplet oil/water interface, however, the co-additives have a
more pronounced effect on DDAPS adsorption and net ordering and play an essential
role in nanoemulsion stabilization. Therefore, formulations incorporating DDAPS in
nanoemulsion formation necessitates co-additives such as salts or other surfactants.
70
Surface activity of DDAPS with various co-additives
The surface activity of surfactant solutions can provide insight into their emul-
sifying and oil dispersing capabilities. For example, highly surface-active species
will readily adsorb to an interface and reduce the interfacial tension (IFT) consider-
ably, making them effective emulsifiers.[149, 171, 172] While zwitterionic surfactant
DDAPS at a concentration of 0.05 mM lowers the IFT of hexadecane and water from
∼55 mN/m[173] to 20.669 ± 1.021 mN/m (dashed horizontal line in Figure 6.1), the
presence of co-additives is expected to further reduce the IFT.
Figure 6.1 shows the IFT of 0.05 mM DDAPS in conjunction with varying amounts
of NaCl (a, blue markers) and MgCl2 (b, green markers). The salt concentrations
were increased from 0.01 mM (○) to 0.1 mM (□), and finally to 1 mM (△). The
horizontal dashed line present in all IFT plots represents the equilibrated IFT of
0.05 DDAPS alone, without any co-additives. This line serves as a visual guide for
comparing the surface activity of DDAPS alone versus DDAPS with co-additives.
Figure 6.1. IFT as a function of time for solutions of 0.05 mM DDAPS with
varying NaCl (a, blue markers) and MgCl2 (b, green markers) concentrations at a
hexadecane/water interface. The salt concentrations were varied from 0.01 mM (○),
0.1 mM (□), and 1 mM (△). The dashed horizontal line indicates the equilibrated
IFT value of 0.05 mM DDAPS alone, absent of any salt additives. The molecular
structure of DDAPS is shown above the IFT panels.
From Figure 6.1a, it can be seen that both salts lower the IFT, but to different
degrees. In solutions containing DDAPS+NaCl, the IFT was not reduced significantly
beyond that exhibited by DDAPS alone. This is in contrast (Figure 6.1b) to solutions
71
containing DDAPS+MgCl2 where the IFT decreases substantially, specifically with 1
mM MgCl2. IFT reduction in the presence of salts is a phenomena attributed to the
screening effect, which allows more surfactants to adsorb by decreasing charge-charge
repulsion between neighboring surfactant head groups.[150, 174, 175] The impact
charge screening has on the surface activity is more pronounced with MgCl2 than
NaCl, likely due to the differences in ionic strength and valency of the cations.
In a study investigating DDAPS on foam stability, Varade et al. showed that
the effects monovalent salts (including NaCl) have on DDAPS’s surface activity is
relatively small compared to the effects on ionic surfactants, which supports our
IFT results.[150, 151, 175] For ionic surfactants, charge screening allows for more
surfactant molecules to populate the interface because one ion is electrostatically
attracted to the surfactant head group, whether it be positive or negatively charged.
This reduces the electrostatic repulsion between neighboring surfactant head groups
and allows surfactants to pack more tightly, ultimately increasing the population
of surfactants at the interface. In the case of zwitterionic surfactants, while there
are attractive interactions between the Na+ and Cl− ions with the sulfonate and
quaternary ammonium regions, respectively, on the DDAPS head group, there are
also repulsive interactions between ions and head group regions with like charges
(e.g., Na+ and the quaternary ammonium regions). The net result is that monovalent
salts have minimal effect on DDAPS surface activity.[150, 151] MgCl2 on the other
hand, reduces the IFT to a greater degree than NaCl. This can be attributed to the
sulfonate region of the DDAPS head group having a greater binding affinity to the
divalent Mg2+ cation over the monovalent Na+ cation.[176] This affinity, specific to
divalent cations, can further screen neighboring sulfonate groups, increase surfactant
packing, and allow for more DDAPS to adsorb–all of which explains the reduction in
IFT at higher MgCl2 concentrations.
In addition to salts, mixed or co-surfactant systems have also been shown to
interact synergistically to achieve a greater reduction in the IFT than the individual
components.[177–179] Thus, we investigate how the surface activity of DDAPS+SDS
solutions compares to that of DDAPS+DTAB. Figure 6.2 contains the IFT traces of
0.05 mM DDAPS with increasing amounts of d-SDS (a, pink markers) and d-DTAB
(b, purple markers). The co-surfactant concentrations were increased from 0.01
mM (○) to 0.1 mM (□), and finally to 1 mM (△). Again, the horizontal dashed
line denotes the equilibrated IFT of 0.05 DDAPS alone, serving as a visual guide.
We note that the use of deuterated SDS and DTAB here is simply to maintain
experimental consistency between our tensiometry and spectroscopic measurements
72
(to be discussed later).
Figure 6.2. IFT as a function of time for solutions of 0.05 mM DDAPS with
varying SDS (a) and DTAB (b) concentrations at a hexadecane/water interface. The
surfactant concentrations were varied from 0.01 mM (○), 0.1 mM (□), and 1 mM
(△). The dashed horizontal line indicates the equilibrated surface tension value of
0.05 mM DDAPS alone, absent of any co-surfactants.
Clearly, co-surfactants have a greater impact on the surface activity of the system
than the salts, which is inherently due to the addition of more surfactants (i.e. d-
SDS and d-DTAB) in solution. While salts can enhance the adsorption of DDAPS
through charge screening, the salts themselves do not directly affect the IFT because
they do not disrupt the hydrogen-bonding network at the interface, as surfactants
do.[180, 181] Co-surfactants, on the other hand, may encourage more DDAPS to
adsorb via synergistic interactions, and also play a direct role in reducing the IFT. By
comparing the effects d-SDS and d-DTAB has on the surface activity of the mixture,
it is clear that with d-SDS as the co-surfactant, there is a much greater reduction in
the IFT overall. The IFT of DDAPS+d-SDS solutions decreases progressively as the
concentration of d-SDS increases from 0.01 mM to 1 mM.
In the presence of d-DTAB, however, the reduction in the IFT does not become
significant until d-DTAB is at a concentration of 1 mM (dark purple △). The differ-
ence in surface activity between DDAPS+d-SDS versus DDAPS+d-DTAB indicates
stronger synergistic effects when an anionic surfactant is present. This can be due to
d-SDS being more surface active than d-DTAB, effectively increasing the total number
of surfactants at the interface.[182] The zwitterionic head group of DDAPS provides
separate hydrophilic regions that can interact differently with d-SDS versus d-DTAB.
The interactions between DDAPS+d-SDS results in a tighter packing of the alkyl
73
tails than DDAPS+d-DTAB, subsequently allowing more surfactants to adsorb and
further reduce the IFT. It is possible that steric hindrance from the –(CH2CH2CH2)–
between the DDAPS quaternary ammonium and sulfonate head groups prevent d-
DTAB from interacting effectively with the DDAPS sulfonate group.[150, 161] This
hypothesis is revisited in later sections using our surface spectroscopic technique.
Adsorption and net ordering of DDAPS at a planar oil/water interface
IFT measurements provide valuable information on the relative population of ad-
sorbed surfactants, but do not provide molecular-level details about the net ordering
of adsorbed species. To obtain such a perspective, we employ vibrational sum fre-
quency spectroscopy (VSFS) to measure the vibrational resonances of the DDAPS
alkyl tails at the planar interface. Figure 6.3 shows VSFS spectra measured in the CH
stretching region of DDAPS at a planar CCl4/water interface. The DDAPS concen-
trations were increased from 0.05 mM (light gray ○) to 0.5 mM (gray □) and finally
to 5 mM (dark gray △). All spectra were collected in the SSP polarization scheme,
which probes dipole components perpendicular to the interface. The resulting peaks
are characteristic of CH vibrational resonances, which are summarized in Table 6.1
and consistent with phase-sensitive SFG results conducted at the air/water interface
by Mafi et al.[163, 183] Details on the fitting routine and exact fit parameters can
be found in Table A.18 of the Appendix and elsewhere.[33, 34, 65, 184] The inset in
Figure 6.3 shows the d+/r+ ratio as a function of DDAPS concentration. The d+/r+
ratio allows relative net ordering of surfactants to be quantified.[54, 55, 59] A higher
(lower) d+/r+ ratio indicates surfactant tails that are less (more) ordered.
Table 6.1. Characteristic CH resonances at an oil/water interface.[12, 185]
Peak assignment Notation Wavenumber (cm−1)
CH2 symmetric stretch (s.s.) d
+ 2846
CH +3 symmetric stretch (s.s.) r 2874
CH2 asymmetric stretch (a.s.) d
− 2916
CH2 s.s. Fermi resonance (F.R.) d
+
FR 2890-2930
CH3 s.s. Fermi resonance (F.R.) r
+
FR 2933
CH3 asymmetric stretch (a.s.) r
− 2962
74
Figure 6.3. VSFS measurements of DDAPS solutions at a planar CCl4/water in-
terface. The DDAPS concentrations varied from 0.05 mM (light gray ○), 0.5 mM
(gray □), and 5 mM (dark gray△). All spectra were collected with the SSP polariza-
tion scheme, probing molecular dipole moments perpendicular to the interface. The
dashed vertical lines denote the CH +2 (d ) and CH3 (r
+) symmetric stretches. The
inset depicts the d+/r+ ratio as a function of DDAPS concentration.
As the bulk DDAPS concentration increases from 0.05 mM to 0.5 mM, there is an
increase in overall SF intensity, as well as a decrease in the d+/r+ ratio (Figure 6.3,
inset), which is indicative of more tightly packed alkyl tails.[12, 54, 55, 59] Increasing
the DDAPS concentration another magnitude from 0.5 to 5 mM yields minimal change
in the overall SF intensity, but the d+/r+ ratio continues to decrease. Once an interface
is saturated with surfactants, there are less gauche defects along the alkyl tails, which
decreases the CH2 s.s. (d
+) signal and increases the CH3 s.s. (r
+). In Figure 6.3,
comparing the intensities of the d+ and r+ peaks of 0.5 and 5 mM DDAPS, we
observe no change in the d+ amplitude, but an increase in the r+ amplitude. This
result illustrates that the interface is not yet completely saturated with DDAPS,
but is becoming more populated, which induces a stronger net alignment of the r+
resonances (and hence, an increased r+ signal). If the interface resembled that of
a fully packed DDAPS monolayer absent of any gauche defects, we would expect a
decrease in the d+ intensity, as the dipoles causing the d+ signal would cancel. In
other words, the CH2 symmetric stretches are in a centrosymmetric environment and
75
thus, SF-inactive. This effect is not observed for the concentrations of DDAPS we
investigated.
Now that we have developed an understanding of how DDAPS behaves at a planar
oil/water interface on its own, we can investigate how co-additives such as salts and
other surfactants influence its interfacial properties. As mentioned above, co-additives
can cause synergistic effects that enhance the overall surface activity of the system,
which can affect the packing and arrangement of interfacial species. Further, the
importance of these co-additives will be underlined in later sections where we discuss
the formation of nanoemulsions with DDAPS.
Figure 6.4. VSFS measurements of DDAPS and co-additive solutions at a planar
CCl4/water interface. The co-additives include NaCl (a, blue), MgCl2 (b, green),
d-SDS (c, pink), and d-DTAB (d, purple). The DDAPS concentration was fixed at
0.05 mM while the co-additive concentrations varied from 0.01 mM (○), 0.1 mM (□),
and 1 mM (△). All spectra were collected with the SSP polarization scheme, probing
molecular dipole moments perpendicular to the interface. The dashed vertical lines
denote the CH2 (d
+) and CH3 (r
+) symmetric stretches.
Now that we have an understanding of how DDAPS alone orients itself at an
oil/water interface, we begin to add co-additives and measure their effects. Figure
6.4 shows VSFS spectra collected in the CH stretching region of DDAPS solutions in
76
conjunction with different co-additives, including NaCl (a, blue), MgCl2 (b, green), d-
SDS (c, pink), and d-DTAB (d, purple). The DDAPS concentration was fixed at 0.05
mM, while the concentrations of co-additives varied between 0.01 mM (○), 0.1 mM
(□), and 1 mM (△). All spectra shown here were collected with the SSP polarization
combination. Since the primary focus of this study is to probe the DDAPS alkyl tails,
deuterated co-surfactants (i.e., d-SDS and d-DTAB) were used to prevent spectral
overlap of the DDAPS CH resonances with those of the co-surfactants.
While the overall surface activity of the system varies from the type and concen-
tration of the co-additive, the net ordering of the DDAPS tails does not significantly
change. From Figure 6.4, it can be seen that the CH spectral features representing
the DDAPS tails are very similar across co-additives and concentrations, especially in
the presence of MgCl2. With d-SDS and d-DTAB, the SF intensity from the DDAPS
alkyl tails decreases with increasing co-additive concentration. Fit parameters can be
referenced in the Appendix, Tables A.19–A.22. Recall that IFT results above show
that the lowest IFT is achieved when the co-surfactants are in excess, as the density
of adsorbed surfactants is highest. Thus, the decrease in SF intensity observed here
demonstrates that the co-surfactants (i.e., d-SDS or d-DTAB) are replacing DDAPS
at the interface when they are in excess.
77
Figure 6.5. d+/r+ ratio of DDAPS and various co-additives at both interfaces. For
VSFS measurements at the planar interface, the co-additives were NaCl (blue △),
MgCl2 (green ▽), d-SDS (pink ◁), and d-DTAB (purple ▷). For VSFSS measure-
ments at the droplet interface, the co-additives were d-SDS (pink ○), h-SDS (pink
●), d-DTAB (purple □), and h-DTAB (purple ■).
The influence of each co-additive on DDAPS net conformational ordering at the
planar oil/water interface is minimal, as demonstrated by the similar d+/r+ ratios in
Figure 6.5 (open triangles). Note the d+/r+ ratios calculated from VSFSS data will
be discussed in later sections. The differences in SF amplitude for some mixtures, are
minor and do not suggest that DDAPS’s interfacial net ordering is dependent on the
type or concentration of co-additives. The insights gained from these planar studies
will aid in our investigation of these mixed systems on nanoemulsion formation and
how co-additives contribute to emulsion stability.
Zwitterionic DDAPS on nanoemulsion formation
We now investigate how DDAPS and co-additives interact to stabilize oil droplets
in water. Kinetically stable oil droplets dispersed in water (i.e., nanoemulsions) can be
created by ultrasonicating small amounts of oil into an aqueous solution of emulsifiers
or surfactants. Nanoemulsions are generally ∼100s of nanometers in diameter and
the droplet distribution should have a polydispersity index < 0.3 to be considered
monodispersed and appropriate for applications.[149, 171, 172] Common surfactants
such as SDS and DTAB are very effective emulsifiers and oil dispersants on their own.
Interestingly, zwitterionic DDAPS alone (0.1–5 mM) does not form nanoemulsions.
Despite reducing the IFT of hexadencane/water by ∼34 mN/m (Figure 6.1) and its net
orientation at the planar CCl4/water interface suggesting a closely packed monolayer
(Figure 6.3), DDAPS is not effective at stabilizing nanoemulsions unless co-additives
are also present.
Figure 6.6 summarizes droplet diameter (a) and ζ-potential (b) measurements for
oil-in-water emulsions stabilized by 0.05 mM DDAPS and varying concentrations of
different co-additives. The co-additives include NaCl (blue ◇), MgCl2 (green △),
SDS (pink ○), and DTAB (purple □) ranging from 0.01 to 1 mM. The emulsion
diameters increased with increasing concentrations of salt, especially in the presence
of MgCl2, but were not as affected by increasing concentrations of co-surfactants.
Under all concentrations of SDS and DTAB, the nanoemulsion diameters remained
below 500 nm, but at high concentrations of NaCl and MgCl2, the diameters
78
enter the size regime of ‘macroemulsions’ (i.e., diameter ∼1-100 µm). This is
important to note for two reasons: 1) emulsions classified under this category are
only weakly kinetically stable and thus, governed by different energetics compared
to nanoemulsions; 2) the theory behind sum frequency scattering from spherical
droplets begins to break down for larger particle diameters.[14, 17] With these two
notions in mind, we do not probe oil droplets larger than 800 nm in diameter with
our VSFSS technique.
Figure 6.6. Diameter (a) and ζ-potential (b) measurements of oil-in-water emulsions
stabilized by 0.05 mM DDAPS and varying concentrations of NaCl (blue ◇), MgCl2
(green △), SDS (pink ○), and DTAB (purple □).
The ζ-potentials of the different emulsions are shown in Figure 6.6b. In the pres-
ence of NaCl and MgCl2, the ζ-potentials are negative overall and similar across all
salt concentrations. Recall that DDAPS is a zwitterionic surfactant and its head
79
group has a net neutral charge. Thus, the negative ζ-potentials are likely due to the
adsorption of hydroxyl ions to an interface,[151, 186, 187] which is consistent with
ζ-potentials of DDAPS alone measured at the hexane/water interface.[151] Addition-
ally, it is also possible that the sulfonate portion on DDAPS extends farther away
from the droplet surface, placing it closer to the slip plane, and making the nega-
tive charge more sensitive to ζ-potential measurements. With the addition of SDS
as a co-surfactant, the ζ-potentials become more negative and conversely, with the
addition of DTAB, the ζ-potentials become more positive. This indicates that the co-
surfactants are adsorbed to the droplet surface and assisting DDAPS with stabilizing
the nanoemulsions. We hypothesize that the added salts contribute to stabilizing the
nanoemulsions by allowing more DDAPS to adsorb to the droplet surface. This idea
cannot be discussed fully with ζ-potential measurements alone, and is investigated
more deeply with our VSFSS technique in the following section.
DDAPS in the presence of salts at the droplet interface
At the planar oil/water interface, we found through IFT and VSFS measurements
that DDAPS adsorption and net ordering change only marginally in the presence of
various salt concentrations. This result suggests that these co-additives have little
influence over DDAPS’s interfacial behavior. However, regarding the formation and
stabilization of nanoemulsions, NaCl and MgCl2 play an essential role, as DDAPS
alone cannot form nanoemulsions. 6.7 depicts the VSFSS spectra of nanoemulsions
coated by DDAPS in the presence of NaCl (a, blue) and MgCl2 (b, green), measured
in the CH stretching region using the SSP polarization scheme. This provides infor-
mation on the net alignment of the DDAPS alkyl tails, specifically dipole components
perpendicular to the droplet surface. The DDAPS concentration was 0.05 mM, and
the salt concentrations were either 0.01 mM (○) or 0.1 mM (□). The dashed vertical
lines denote the CH2 (d
+) and CH (r+3 ) symmetric stretches. Fit parameters are
provided in Tables A.23 and A.24 in the Appendix.
80
Figure 6.7. VSFSS spectra of nanoemulsions stabilized by DDAPS in the presence
of NaCl (a, blue) and MgCl2 (b, green) in the CH stretching region, using the SSP
polarization scheme. The DDAPS concentration was fixed at 0.05 mM and the salt
concentrations were increased from 0.01 mM (○) to 0.1 mM (□). The dashed vertical
lines denote the CH2 (d
+) and CH (r+3 ) symmetric stretches.
Unlike the VSFS spectra collected at the planar oil/water interface (Figure 6.4),
the presence of NaCl and MgCl2 at the droplet interface significantly influences the
adsorption of DDAPS, which contributes to the nanoemulsion stability. With both
NaCl (6.7a) and MgCl2 (6.7b) as co-additives, a higher concentration of either salt
results in an enhancement in the SF intensity of the DDAPS tails. This enhancement
can be ascribed to a greater population of DDAPS at the droplet surface. Because
the d+/r+ ratio decreases from low to high concentrations of salt, the DDAPS tails
81
are adopting a more ordered conformation with less gauche defects.
The differences in adsorption behavior of various DDAPS solutions at the planar
versus droplet oil/water interface is likely due to the differences in surface areas. For
example, a surfactant concentration that would constitute maximum surface coverage
at the planar interface may not have the same result at the nanoemulsion interface,
since spherical droplets have a higher surface area-to-volume ratio. Therefore, the
same concentration of surfactants and co-additives at the two interfaces can result in
very different interfacial phenomena. At the planar interface, the introduction of a salt
will have a less pronounced effect on DDAPS adsorption if the interface is already
well populated. However, for a given amount of oil that is dispersed as droplets,
the droplet surface area is much larger and results in surfactants that are less densely
packed. Thus, the screening effect from added salts will have a more significant impact
on the interfacial surfactant population, which is reflected in the VSFSS spectra. The
increase in DDAPS number density at the droplet interface explains the need for salts
to stabilize the nanoemulsions. Additionally, this result highlights that the system’s
energetics play an important role in understanding surfactant interfacial properties.
Synergy between DDAPS and co-surfactants at the droplet interface
The addition of salts as co-additives was shown above to enhance the adsorption
of DDAPS to the nanoemulsion surface. We now investigate how co-surfactants such
as SDS and DTAB influence DDAPS adsorption and net ordering at the droplet
interface. Figure 6.8 depicts VSFSS spectra of the CH stretching region measuring
the surfactant alkyl tails. The nanoemulsions were stabilized by 0.05 mM DDAPS
and either 0.01 mM (○/●), 0.1 mM (□/■), or 1 mM (△/▲) SDS (Figure 6.8a and
8b, pink) or DTAB (Figure 6.8c and 8d, purple) as co-surfactants. The open markers
denote the use of deuterated co-surfactants, such that all CH resonances detected are
attributed to DDAPS; while the filled markers denote the use of hydrogenated co-
surfactants. In the presence of hydrogenated co-surfactants (i.e., h-SDS or h-DTAB),
we note that the measured SF response is from the alkyl tails of both DDAPS and
the co-surfactant. All spectra were collected with the SSP polarization scheme, which
probes dipoles containing a component perpendicular to the droplet surface. For exact
fit parameters, see Tables A.25–A.28 in the Appendix.
82
Figure 6.8. VSFSS spectra of nanoemulsions coated by DDAPS in conjunction with
d-SDS (a), h-SDS (b), d-DTAB (c), and h-DTAB (d). The concentration of DDAPS
was fixed at 0.05 mM, while the co-additive concentrations were varied from 0.01 mM
(○/●), 0.1 mM (□/■), and 1 mM (△/▲). The open markers denote deuterated
co-surfactants and the filled markers denote hydrogenated co-surfactants. All spectra
were collected with the SSP polarization scheme, probing molecular dipole moments
perpendicular to the interface. The dashed vertical lines denote the CH2 (d
+) and
CH3 (r
+) symmetric stretches.
The VSFSS spectra of DDAPS in conjunction with d-SDS (Figure 6.8a) and d-
DTAB (Figure 6.8c) do not show large changes in the SF amplitude with increasing
co-surfactant concentration, as was seen with increasing salt concentrations (Figure
6.7). The behavior of DDAPS+d-DTAB at the droplet surface is similar to the planar
surface, with DDAPS having similar adsorption across different d-DTAB concentra-
tions. From the slight decrease in d+/r+ ratios (Figure 6.5, purple □), the DDAPS
alkyl tails are becoming more ordered with increasing concentration of d-DTAB, al-
beit the effect is subtle. However, with d-SDS as the co-surfactant, there is a more
pronounced decrease in the d+/r+ ratio with increasing concentration of d-SDS (Fig-
ure 6.5, pink ○). Thus, for the DDAPS+d-SDS system, we conclude that there is a
higher population of surfactants at the droplet surface allowing for tighter packing
of the alkyl tails, which is consistent with the high surface activity of this system
83
(Figure 6.2).
Figures 6.7b and 6.7d are VSFSS measurements containing the contributions from
the co-surfactant alkyl tails, as they are no longer deuterated. This allows us to de-
termine the adsorption behavior of h-SDS and h-DTAB in conjunction with DDAPS.
Because the bulk DDAPS concentration was fixed throughout this study, changes to
the SF intensity in Figures 6.7b and 6.7d can be attributed to the increasing concentra-
tions of h-SDS and h-DTAB, respectively. In the presence of h-SDS, the SF intensity
progressively increases from 0.01 mM h-SDS (●) to 1 mM h-SDS (▲), which indicates
an increase in adsorption of h-SDS, DDAPS, or both due to cooperative adsorption.
The increased adsorption of total surfactants allows for a more tightly packed sur-
factant layer at the droplet surface, which is reflected in a lower d+/r+ ratio (Figure
6.5, pink ●). With h-DTAB as the co-surfactant however, the change in SF intensity
is less pronounced which indicates that increasing the bulk concentration of h-DTAB
does not greatly affect the overall surfactant packing. This result further supports
our hypothesis that for the mixed DDAPS+SDS system, there is a greater interfacial
population of surfactants than mixed DDAPS+DTAB system. Additionally, because
the surface activity of DDAPS+DTAB is not as strong as DDAPS+SDS, we do not
observe a large decrease in the d+/r+ ratio for the former mixed system as we do for
the latter.
Conclusions
In this chapter, we have demonstrated that is it possible for some surfactants,
specifically zwitterionic DDAPS in this case, to exhibit different interfacial properties
depending on the surface geometry. Although DDAPS alone is surface active and
readily adsorbs to the planar oil/water interface, it cannot stabilize nanoemulsions
without the aid of co-additives such as salts or other surfactants. Moreover, the
molecular structure of DDAPS at the planar interface is largely invariant with co-
additives, but DDAPS adopts tighter packing with co-additives at the droplet surface.
The results from this work highlight the importance of accounting for the interfacial
geometry and system energetics when analyzing surfactant properties. For example,
DDAPS on its own would not be effective at dispersing oil for remediation purposes,
as it cannot stabilize tiny oil droplets (kinetically stable). Thus, the oil droplets will
eventually coalesce back into the large oil slick (thermodynamically stable) before
any oil can be degraded. However, in a mixed system containing salts and other
surfactants (which is likely the case in a natural environment), DDAPS’s oil dispersing
84
performance is enhanced. This insight is valuable for scientists developing surfactant
mixtures for various applications via rational design. By developing an understanding
of the synergy between DDAPS and other species, formulations for cleaning products,
cosmetics, and oil dispersants can reduce the amount of harsh chemicals used by
incorporating more DDAPS as a less toxic alternative, all while preserving the efficacy
of the product.
85
CHAPTER VII
SUMMARY AND OUTLOOK
This dissertation is a compilation of work investigating the interfacial phenomena
of nanoemulsions, surfactant/polymer self-assembly, and colloidal stability. Experi-
mental techniques spanning dynamic light scattering, ζ-potential measurements, in-
terfacial tensiometry, and surface-specific spectroscopy were employed to reveal valu-
able insights into surface chemistry. Chapter III demonstrated that the interfacial
behavior of PEI can be tuned by adjusting the solution pH. Under acidic conditions
PEI is not surface-active, but under basic conditions the hydrophobicity of PEI facil-
itates its adsorption to the nanoemulsion surface. Building up from a single polymer
layer, the work discussed in Chapter IV focuses on multi-polymer layering around
nanoemulsion surfaces. Additional pH-dependent properties were observed, further
underlining the utility of polymer layers as surface modifiers. Polymer adsorption
plays a crucial role in colloidal stability via steric effects. The system studied in
Chapter V provided a unique avenue to investigate colloidal stability via steric stabi-
lization, absent of electrostatic repulsive forces. The nonionic polymer PNVA forms
layers of strongly bonded, rigid polymer networks that stabilizes the droplet sur-
face. This level of molecular detail was made possible with our VSFSS technique.
Continuing the work that probes droplet surfaces coated by neutral (or net neutral)
species, Chapter VI investigates the interfacial behavior of zwitterionic DDAPS in
the presence of various co-additives. The work in this chapter sheds light on how the
geometry and energetics of the interface can influence surfactant behavior.
Interfacial phenomena is an area of research that is continually expanding with
the development of instrumentation, technology, and surface-active molecules. The
diversity and sheer number of applications that involve any type of interface (e.g.,
solid/air, liquid/air, liquid/liquid) or colloid (e.g., emulsions, foams, aerosols) necessi-
tates the advancement of surface-specific characterization techniques accompanied by
appropriate analysis methods. As researchers push the boundaries of spectral regions
and chemical systems that can be studied with sum frequency scattering spectroscopy,
the complexity of the interface grows leaving creative and challenging fundamental
questions to be investigated. Moreover, with developments such as two-dimensional
electronic SFG spectroscopy,[188, 189] SFG imaging,[190, 191] and dual resonant SFG
spectroscopy,[192] there is no doubt the outlook of colloid and interface science is a
bright one, pun very much intended.
86
APPENDIX
Fitting parameters for all spectra shown in Chapter III
Table A.2. Fitting parameters and d+/r+ ratios for h-SDS and PEI/h-SDS spectra
shown in Figure 3.5b. Nanoemulsions were either stabilized with 1 mM h-SDS or 5.2
mM PEI and 1 mM h-SDS at constant pH 3.
Peak assignment Parameter h-SDS alone PEI/h-SDS
Amplitude 0.05 ± 0.005 0.07 ± 0.005
CH2 Phase 0 0
symmetric Lorentizian linewidth 2 2
stretch (d+) Peak position 2850 ± 1 2851 ± 1
Gaussian linewidth 5 ± 1 8 ± 1
Amplitude 0.03 ± 0.004 0.06 ± 0.006
CH3 Phase 0 0
symmetric Lorentizian linewidth 2 2
stretch (r+) Peak position 2875 ± 6 2873 ± 1
Gaussian linewidth 19 ± 7 13 ± 2
Amplitude 0.05 ± 0.005 0.08 ± 0.004
CH3 Phase 0 0
asymmetric stretch Lorentizian linewidth 2 2
& Fermi resonance Peak position 2935 ± 2 2935 ± 1
Gaussian linewidth 20 ± 2 24 ± 1
Non-resonant Amplitude 0.05 ± 0.006 0.03 ± 0.003
contribution Phase 0 0
d+/r+ ratio 1.7 ± 0.2 1.2 ± 0.1
87
Table A.3. Fitting parameters for PEI/d-SDS spectra shown in Figure 3.6a for pH
7.5, 8, 10, and 11. PEI and d-SDS concentrations were fixed at 5.2 mM and 0.1 mM,
respectively.
Peak
assignment Parameter pH 7.5 pH 8 pH 10 pH 11
Amplitude 0.04 ± 0.04 0.09 ± 0.01 0.1 ± 0.001 0.1 ± 0.001
CH2 Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch Peak position 2850 ± 5 2851 ± 2 2851 ± 2 2850 ± 2
Gaussian 19 ± 5 26 ± 0.6 27 ± 0.4 28 ± 0.3
Amplitude 0.02 ± 0.005 0.008 ± 0.04 0.009 ± 0.003 0.01 ± 0.002
CH3 Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch Peak position 2880 ± 26 2890 ± 5 2894 ± 4 2900 ± 4
Gaussian 29 ± 12 14 ± 6 11 ± 5 17 ± 6
Amplitude 0.02 ± 0.005 0.04 ± 0.007 0.03 ± 0.002 0.02 ± 0.002
CH3 Phase 0 0 0 0
asymmetric Lorentzian 2 2 2 2
stretch Peak position 2935 ± 9 2964 ± 3 2956 ± 2 2953 ± 2
Gaussian 28 ± 9 63 ± 1 48 ± 3 30 ± 5
N.R. Amplitude 0 0.02 ± 0.001 0.04 ± 0.001 0.05 ± 0.001
contribution Phase 0 0 0 0
88
Table A.4. Fitting parameters for PEI/d-SDS and PEI/h-SDS spectra shown in
Figure 3.6b for pH 11. PEI and d-SDS/h-SDS concentrations were fixed at 5.2 mM
and 1 mM, respectively.
Peak
assignment Parameter PEI/d-SDS PEI/h-SDS
Amplitude 0.05 ± 0.008 0.09 ± 0.002
CH2 Phase 0 0
symmetric Lorentzian 2 2
stretch Peak position 2851 ± 2 2852 ± 3
Gaussian 27 ± 2 26 ± 4
Amplitude 0.009 ± 0.008 0.05 ± 0.003
CH3 Phase 0 0
symmetric Lorentzian 2 2
stretch Peak position 2885 ± 7 2875 ± 2
Gaussian 32 ± 3 28 ± 6
CH Amplitude 0.007 ± 0.003 0.04 ± 0.0023
asymmetric Phase 0 0
stretch & Fermi Lorentzian 2 2
resonance Peak position 2935 ± 9 2935 ± 3Gaussian 27 ± 5 32 ± 3
Non-resonant Amplitude 0.02 ± 0.007 0.03 ± 0.003
contribution Phase 0 0
89
Table A.5. Fitting parameters for PEI/d-DTAB spectra shown in Figure 3.8a for
pH 8, 10, and 11. PEI and d-DTAB concentrations were fixed at 5.2 mM and 0.1
mM, respectively.
Peak
assignment Parameter pH 8 pH 10 pH 11
Amplitude 0.03 ± 0.04 0.08 ± 0.007 0.1 ± 0.007
CH2 Phase 0 0 0
symmetric Lorentzian 2 2 2
stretch Peak position 2854 ± 10 2852 ± 2 2853 ± 2
Gaussian 19 ± 7 25 ± 2 22 ± 2
Amplitude 0.02 ± 0.02 0.03 ± 0.006 0.03 ± 0.007
CH3 Phase 0 0 0
symmetric Lorentzian 2 2 2
stretch Peak position 2891 ± 14 2881 ± 2 2889 ± 2
Gaussian 25 ± 16 56 ± 4 67 ± 2
Amplitude 0.04 ± 0.002 0.04 ± 0.004 0.03 ± 0.004
CH3 Phase 0 0 0
asymmetric Lorentzian 2 2 2
stretch Peak position 2938 ± 6 2943 ± 3 2945 ± 5
Gaussian 56 ± 3 28 ± 3 32 ± 8
Non-resonant Amplitude 0 0 0
contribution Phase 0 0 0
90
Table A.6. Fitting parameters for PEI/d-DTAB and PEI/h-DTAB spectra shown
in Figure 3.8b for pH 11. PEI and d-DTAB/h-DTAB concentrations were fixed at
5.2 mM and 1 mM, respectively.
Peak
assignment Parameter PEI/d-DTAB PEI/h-DTAB
Amplitude 0.01 ± 0.001 0.05 ± 0.001
CH2 Phase 0 0
symmetric Lorentzian 2 2
stretch Peak position 2849 ± 3 2852 ± 1
Gaussian 25 ± 3 26 ± 2
Amplitude 0.009 ± 0.008 0.02 ± 0.001
CH3 Phase 0 0
symmetric Lorentzian 2 2
stretch Peak position 2881 ± 4 2870 ± 2
Gaussian 29 ± 2 28 ± 4
CH Amplitude 0.02 ± 0.001 0.04 ± 0.0023
asymmetric Phase 0 0
stretch & Fermi Lorentzian 2 2
resonance Peak position 2940 ± 2 2938 ± 2Gaussian 30 ± 3 31 ± 4
Non-resonant Amplitude 0.005 ± 0.009 0.002 ± 0.001
contribution Phase 0 0
91
Fitting parameters for all spectra shown in Chapter IV
Table A.7. Fit parameters for VSFSS spectra of nanoemulsions stabilized by either
0, 0.1, or 1 mM d-DTAB, in conjunction with 5 mM PSS, shown in Figure 4.4.
Alkane CH Stretches
d-DTAB + 5 mM PSS (pH 7)
Peak Assignment Parameter 0 mM d-DTAB 0.1 mM d-DTAB 1 mM d-DTAB
Amplitude 0.03 ± 0.002 0.04 ± 0.003
Phase 0 0
CH2 symmetric Lorentzian N/A 2 2
stretch (s.s.) Peak position 2852 ± 1 2848 ± 2
Gaussian 13 ± 2 11 ± 2
Amplitude 0.02 ± 0.002 0.02 ± 0.002
Phase 0 0
CH3 symmetric Lorentzian N/A 2 2
stretch (s.s.) Peak position 2877 ± 2 2879 ± 3
Gaussian 50 ± 3 36 ± 4
Amplitude 0.03 ± 0.002 0.03 ± 0.003
Phase 0 0
CH2 asymmetric Lorentzian N/A 2 2
stretch (a.s.) Peak position 2909 ± 1 2908 ± 3
Gaussian 13 ± 1 16 ± 2
Amplitude 0.01 ± 0.001 0.02 ± 0.004
Phase 0 0
CH3 asymmetric Lorentzian N/A 2 2
stretch (a.s.) Peak position 2975 ± 2 2980 ± 3
Gaussian 88 ± 5 73 ± 6
Non-resonant Amplitude
contribution Phase N/A
0 0
0 0
Aromatic CH Stretches
d-DTAB + 5 mM PSS (pH 7)
Peak Assignment Parameter 0 mM d-DTAB 0.1 mM d-DTAB 1 mM d-DTAB
Amplitude 0.03 ± 0.003
Phase 0
Aromatic CH Lorentzian N/A N/A 2
stretch 1 Peak position 3020 ± 10
Gaussian 40 ± 5
Amplitude 0.05 ± 0.007
Phase 0
Aromatic CH Lorentzian N/A N/A 2
stretch 2 Peak position 3059 ± 2
Gaussian 7 ± 2
Non-resonant Amplitude 0
contribution Phase N/A N/A 0
92
Table A.8. Fit parameters for VSFSS spectra of nanoemulsions stabilized by 0.1
mM d-DTAB, 5 mM PSS, and either 0, 1, or 10 mM cationic PEI at pH 3, shown in
Figure 4.6).
Alkane CH Stretches
0.1 mM d-DTAB + 5 mM PSS + cationic PEI (pH 3)
Peak Assignment Parameter 0 mM PEI 1 mM PEI 10 mM PEI
Amplitude 0.1 ± 0.0009 0.07 ± 0.001
Phase 0 0
D2O stretch + libration Lorentzian 5 5 N/A
Peak position 2782 ± 2 2784 ± 2
Gaussian 65 ± 8 60 ± 10
Amplitude 0.06 ± 0.008 0.07 ± 0.009 0.09 ± 0.002
Phase 0 0 0
CH2 symmetric Lorentzian 2 2 2
stretch (s.s.) Peak position 2840 ± 1 2841 ± 1 2850 ± 2
Gaussian 20 ± 2 20 ± 2 12 ± 1
Amplitude 0.03 ± 0.005 0.03 ± 0.005 0.04 ± 0.003
Phase 0 0 0
CH3 symmetric Lorentzian 2 2 2
stretch (s.s.) Peak position 2882 ± 3 2882 ± 4 2880 ± 1
Gaussian 11 ± 4 17 ± 4 20 ± 2
Amplitude 0.03 ± 0.005 0.03 ± 0.001 0.04 ± 0.003
Phase 0 0 0
CH2 asymmetric Lorentzian 2 2 2
stretch (a.s.) Peak position 2910 ± 2 2909 ± 2 2914 ± 1
Gaussian 9 ± 2 10 ± 2 13 ± 1
Amplitude 0.01 ± 0.003 0.02 ± 0.002 0.01 ± 0.004
Phase 0 0 0
CH3 asymmetric Lorentzian 2 2 2
stretch (a.s.) Peak position 2975 ± 1 2976 ± 1 2980 ± 2
Gaussian 86 ± 4 86 ± 5 85 ± 3
Non-resonant Amplitude 0 0 0
contribution Phase 0 0 0
Aromatic CH Stretches
0.1 mM d-DTAB + 5 mM PSS + cationic PEI (pH 3)
Peak Assignment Parameter 0 mM PEI 1 mM PEI 10 mM PEI
Amplitude 0.01 ± 0.002 0.04 ± 0.004 0.06 ± 0.003
Phase 0 0 0
Aromatic CH Lorentzian 2 2 2
stretch 1 Peak position 3020 ± 1 3020 ± 2 3023 ± 2
Gaussian 71 ± 3 21 ± 4 29 ± 7
Amplitude 0.02 ± 0.006 0.08 ± 0.006 0.1 ± 0.009
Phase 0 0 0
Aromatic CH Lorentzian 2 2 2
stretch 2 Peak position 3059 ± 1 3060 ± 2 3062 ± 1
Gaussian 60 ± 3 15 ± 1 14 ± 1
Non-resonant Amplitude 0.006 ± 0.003 0.02 ± 0.003 0.03 ± 0.001
contribution Phase 0 0 0
93
Table A.9. Fit parameters for VSFSS spectra of nanoemulsions stabilized by 0.1
mM d-DTAB, 5 mM PSS, and either 0, 1, or 10 mM nonionic PEI at pH 10, shown
in Figure 4.8.
Alkane CH Stretches
0.1 mM d-DTAB + 5 mM PSS + nonionic PEI (pH 10)
Peak Assignment Parameter 0 mM PEI 1 mM PEI 10 mM PEI
Amplitude 0.03 ± 0.002 0.05 ± 0.003 0.02 ± 0.001
Phase 0 0 0
D2O stretch + libration Lorentzian 5 5 5
Peak position 2785 ± 4 2790 ± 5 2790 ± 5
Gaussian 40 ± 4 43 ± 3 ± 30 ± 2
Amplitude 0.02 ± 0.0007 0.03 ± 0.002 0.05 ± 0.004
Phase 0 0 0
CH2 symmetric Lorentzian 2 2 2
stretch (s.s.) Peak position 2853 ± 1 2850 ± 1 2852 ± 1
Gaussian 14 ± 1 16 ± 2 21 ± 1
Amplitude 0.02 ± 0.001 0.02 ± 0.006 0.01 ± 0.006
Phase 0 0 0
CH3 symmetric Lorentzian 2 2 2
stretch (s.s.) Peak position 2880 ± 1 2879 ± 3 2880 ± 2
Gaussian 70 ± 4 64 ± 4 60 ± 4
Amplitude 0.01 ± 0.001 0.009 ± 0.005 0.02 ± 0.005
Phase 0 0 0
CH2 asymmetric Lorentzian 2 2 2
stretch (a.s.) Peak position 2916 ± 2 2914 ± 5 2930 ± 7
Gaussian 12 ± 3 20 ± 3 26 ± 5
Non-resonant Amplitude 0.02 ± 0.0005 0.02 ± 0.002 0.01 ± 0.002
contribution Phase 0 0 0
Aromatic CH Stretches
0.1 mM d-DTAB + 5 mM PSS + nonionic PEI (pH 10)
Peak Assignment Parameter 0 mM PEI 1 mM PEI 10 mM PEI
Amplitude 0.02 ± 0.003
Phase 0
Aromatic CH Lorentzian N/A N/A 2
stretch 1 Peak position 3026 ± 6
Gaussian 42 ± 3
Amplitude 0.05 ± 0.003
Phase 0
Aromatic CH Lorentzian N/A N/A 2
stretch 2 Peak position 3058 ± 2
Gaussian 10 ± 2
Non-resonant Amplitude
contribution Phase N/A N/A
0.02 ± 0.005
0
94
Inputs for DLVO calculations and fitting parameters for all spectra
shown in Chapter V
Table A.10. Parameters used to calculate the Hamaker constant for hexadecane-in-
water nanoemulsions from Equation 27.
Variable Description Value
kb Boltzmann constant 1.380649×10
−23 m2 kg s−2 K−1
T Temperature (K) 297.15
εhexadecane Dielectric constant of hexadecane 2.09
nhexadecane Refractive index of hexadecane 1.43
εwater Dielectric constant of water 78
nwater Refractive index of water 1.33
Table A.11. Concentrations of SDS used and the corresponding Debye length cal-
culated from Equation 28.
SDS concentration (mM) Ionic strength (mM) Debye length
0.01 0.01 96.1
0.1 0.1 30.4
0.5 0.5 13.6
1 1 9.6
2.5 2.5 6.1
Table A.12. Parameters used to calculate the interaction pair potentials in Figure
5.5d using extended DLVO theory for nanoemulsions stabilized by SDS/PNVA.
Variable Description Value
ν Volume of ones solvent (water) molecule 2.99×10−20 m3
ϕp Volume fraction of polymer 0.05
ϕ Volume fraction of droplets 0.01
χ Flory-Huggins constant 0.495∗
Mw Polymer molecular weight 4060000 g/mol
ρ Density of oil core 0.887×106 g/m3
Steric layer thickness†L varies (m)
∗ Flory-Huggins constant for PNVA was approximated using the constant for polyacrylamide in
water, due to the similar chemical moieties between the two polymers
† Steric layer thickness ≈ Radius of SDS/PNVA nanoemulsions − Radius of SDS nanoemulsions
95
Table A.13. Fitting parameters for spectra of h-SDS/PNVA stabilized nanoemul-
sions in the C–H stretching region, corresponding to Figure 5.6a.
Fixed [PNVA] = 5 mM Hydrogenated SDS concentration:
Peak [h-SDS] = [h-SDS] = [h-SDS] = [h-SDS] =
assignment Parameter 0.1 mM 0.5 mM 1 mM 2.5 mM
Amplitude 0.03 ± 0.005 0.06 ± 0.006 0.03 ± 0.008 0.06 ± 0.006
Water Phase 0 0 0 0
combination Lorentzian 2 2 2 2
band Peak position 2770 ± 2 2777 ± 1 2774 ± 2 2774 ± 1
Gaussian 78 ± 3 67 ± 3 70 ± 4 71 ± 2
Amplitude 0.04 ± 0.003 0.1 ± 0.003 0.4 ± 0.003 0.5 ± 0.003
CH2 Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch Peak position 2850 ± 1 2849 ± 1 2852 ± 1 2850 ± 1
Gaussian 15 ± 2 17 ± 1 14 ± 1 15 ± 1
Amplitude 0.02 ± 0.004 0.06 ± 0.005 0.07 ± 0.005 0.09 ± 0.004
CH3 Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch Peak position 2891 ± 2 2891 ± 1 2891 ± 1 2891 ± 1
Gaussian 67 ± 5 67 ± 2 62 ± 1 62 ± 1
Amplitude 0.04 ± 0.003 0.06 ± 0.003 0.3 ± 0.002 0.3 ± 0.003
CH2 Phase 0 0 0 0
asymmetric Lorentzian 2 2 2 2
stretch Peak position 2929 ± 2 2928 ± 1 2931 ± 1 2929 ± 1
Gaussian 24 ± 4 22 ± 1 23 ± 1 22 ± 1
Non-resonant Amplitude 0.05 ± 0.002 0.09 ± 0.003 0.1 ± 0.002 0.1 ± 0.002
contribution Phase 0 0 0 0
96
Table A.14. Fitting parameters for spectra of d-SDS/PNVA stabilized nanoemul-
sions in the C–H stretching region, corresponding to Figure 5.6b.
Fixed [PNVA] = 5 mM Deuterated SDS concentration:
Peak [d-SDS] = [d-SDS] = [d-SDS] = [d-SDS] =
assignment Parameter 0.1 mM 0.5 mM 1 mM 2.5 mM
Amplitude 0.02 ± 0.005 0.02 ± 0.001 0.05 ± 0.002 0.07 ± 0.00
Water Phase 0 0 0 0
combination Lorentzian 2 2 2 2
band Peak position 2775 ± 2 2770 ± 3 2769 ± 1 2770 ± 3
Gaussian 71 ± 3 61 ± 2 80 ± 2 80 ± 5
Amplitude 0.01 ± 0.008 0.03 ± 0.004 0.04 ± 0.001 0.06 ± 0.002
CH2 Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch Peak position 2848 ± 2 2849 ± 2 2852 ± 1 2850 ± 1
Gaussian 27 ± 4 20 ± 2 18 ± 1 20 ± 3
Amplitude 0.02 ± 0.009 0.02 ± 0.01 0.03 ± 0.002 0.04 ± 0.003
CH3 Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch Peak position 2880 ± 1 2884 ± 2 2883 ± 2 2884 ± 1
Gaussian 76 ± 1 72 ± 1 75 ± 1 74 ± 2
Amplitude 0.04 ± 0.006 0.06 ± 0.003 0.06 ± 0.001 0.08 ± 0.003
CH2 Phase 0 0 0 0
asymmetric Lorentzian 2 2 2 2
stretch Peak position 2931 ± 1 2928 ± 2 2927 ± 1 2926 ± 1
Gaussian 29 ± 2 21 ± 2 21 ± 1 21 ± 2
Non-resonant Amplitude 0.04 ± 0.001 0.05 ± 0.002 0.05 ± 0.001 0.06 ± 0.002
contribution Phase 0 0 0 0
Table A.15. Fitting parameters for spectra of SDS (alone) stabilized nanoemulsions
in the S=O stretching region, corresponding to Figure 5.6c.
NO PNVA SDS concentration:
Peak [SDS] = [SDS] = [SDS] = [SDS] =
assignment Parameter 0.1 mM 0.5 mM 1 mM 2.5 mM
Amplitude 0.03 ± 0.01 0.1 ± 0.008 0.2 ± 0.007 0.2 ± 0.006
S=O Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch 1 Peak position 1045 ± 3 1043 ± 2 1045 ± 1 1046 ± 1
Gaussian 12 ± 4 15 ± 1 15 ± 1 12 ± 1
Amplitude 0.07 ± 0.004 0.09 ± 0.005 0.1 ± 0.004 0.1 ± 0.004
S=O Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch 2 Peak position 1095 ± 1 1091 ± 2 1090 ± 1 1089 ± 1
Gaussian 67 ± 3 62 ± 4 62 ± 2 61 ± 2
Non-resonant Amplitude 0.02 ± 0.006 0.03 ± 0.008 0.03 ± 0.006 0.05 ± 0.006
contribution Phase 0 0 0 0
97
Table A.16. Fitting parameters for spectra of SDS/PNVA stabilized nanoemulsions
in the S=O stretching region, corresponding to Figure 5.6d.
Fixed [PNVA] = 5 mM SDS concentration:
Peak [SDS] = [SDS] = [SDS] = [SDS] =
assignment Parameter 0.1 mM 0.5 mM 1 mM 2.5 mM
Amplitude 0.02 ± 0.01 0.2 ± 0.007 0.2 ± 0.008 0.3 ± 0.008
S=O Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch 1 Peak position 1045 ± 3 1043 ± 1 1044 ± 1 1045 ± 1
Gaussian 12 ± 4 12 ± 1 13 ± 1 13 ± 1
Amplitude 0.07 ± 0.004 0.07 ± 0.006 0.2 ± 0.006 0.2 ± 0.005
S=O Phase 0 0 0 0
symmetric Lorentzian 2 2 2 2
stretch 2 Peak position 1095 ± 1 1096 ± 2 1092 ± 1 1091 ± 1
Gaussian 67 ± 3 69 ± 3 65 ± 2 64 ± 2
Non-resonant Amplitude 0.02 ± 0.006 0.04 ± 0.007 0.08 ± 0.008 0.06 ± 0.007
contribution Phase 0 0 0 0
Table A.17. Fitting parameters for spectra of SDS/PNVA stabilized nanoemulsions
in the C=O stretching region, corresponding to Figure 5.6e.
Fixed [PNVA] = 5 mM SDS concentration:
Peak [SDS] = [SDS] = [SDS] = [SDS] =
assignment Parameter 0.1 mM 0.5 mM 1 mM 2.5 mM
Amplitude 0.04 ± 0.0006 0.06 ± 0.0006 0.06 ± 0.0008 0.07 ± 0.0009
Background Phase 0 0 0 0
peak 1 Lorentzian 10 10 10 10Peak position 1593 1593 1593 1593
Gaussian 45 45 45 45
Amplitude 0.01 ± 0.001 0.01 ± 0.001 0.03 ± 0.001 0.03 ± 0.002
Phase 0 0 0 0
C=O stretch Lorentzian 5 5 5 5
Peak position 1629 ± 1 1628 ± 1 1629 ± 1 1629 ± 1
Gaussian 14 ± 1 15 ± 2 20 ± 1 21 ± 2
Amplitude 0.03 ± 0.0003 0.04 ± 0.0004 0.06 ± 0.0003 0.06 ± 0.0004
Background Phase 3.14 3.14 3.14 3.14
peak 2 Lorentzian 5 5 5 5Peak position 1777 1777 1777 1777
Gaussian 106 106 106 106
Non-resonant Amplitude 0.01 ± 0.0006 0.01 ± 0.0008 0.02 ± 0.0008 0.02 ± 0.0009
contribution Phase 0 0 0 0
98
Fitting parameters for all spectra shown in Chapter VI
Table A.18. Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS alone, corresponding to Figure 6.3.
Peak [DDAPS] = [DDAPS] = [DDAPS] =
assignment Parameter 0.05 mM 0.5 mM 5 mM
Amplitude 0.8 ± 0.02 0.9 ± 0.03 0.9 ± 0.03
CH2 symmetric
Phase 0 0 0
stretch (d+) Lorentzian 2 2 2Peak position 2854 ± 1 2854 ± 1 2854 ± 1
Gaussian 10 ± 1 10 ± 1 10 ± 1
Amplitude 0.9 ± 0.03 1.2 ± 0.04 1.3 ± 0.04
CH symmetric Phase 0 0 02
stretch (r+) Lorentzian 2 2 2Peak position 2873 ± 1 2873 ± 1 2873 ± 1
Gaussian 6 ± 1 7 ± 1 7 ± 1
Amplitude 0.6 ± 0.02 0.7 ± 0.03 0.8 ± 0.03
CH Phase 0 0 02 asymmetric
stretch (d−) Lorentzian 2 2 2Peak position 2915 ± 2 2917 ± 2 2916 ± 3
Gaussian 21 ± 2 24 ± 2 23 ± 3
Amplitude 0.9 ± 0.08 1 ± 0.07 1.3 ± 0.1
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2932 ± 1 2932 ± 1 2933 ± 1
Gaussian 8 ± 1 8 ± 1 8 ± 1
Amplitude 0.3 ± 0.04 0.3 ± 0.05 0.3 ± 0.05
CH3 asymmetric
Phase 0 0 0
stretch (r−) Lorentzian 2 2 2Peak position 2977 ± 2 2978 ± 2 2979 ± 2
Gaussian 12 ± 2 13 ± 3 12 ± 3
Non-resonant Amplitude 0.08 ± 0.01 0.1 ± 0.02 0.1 ± 0.02
background Phase 0 0 0
99
Table A.19. Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and NaCl, corresponding to Figure 6.4a.
Fixed DDAPS concentration = 0.05 mM Varying NaCl concentrations
Peak [NaCl] = [NaCl] = [NaCl] =
assignment Parameter 0.01 mM 0.1 mM 1 mM
Amplitude 0.9 ± 0.03 0.9 ± 0.03 0.8 ± 0.03
CH2 symmetric
Phase 0 0 0
stretch (d+) Lorentzian 2 2 2Peak position 2854 ± 1 2854 ± 1 2854 ± 1
Gaussian 10 ± 1 10 ± 1 9 ± 1
Amplitude 1 ± 0.04 1 ± 0.04 1 ± 0.04
CH2 symmetric
Phase 0 0 0
stretch (r+) Lorentzian 2 2 2Peak position 2873 ± 1 2873 ± 1 2873 ± 1
Gaussian 7 ± 1 7 ± 1 6 ± 1
Amplitude 0.7 ± 0.03 0.7 ± 0.04 0.6 ± 0.04
CH Phase 0 0 02 asymmetric
stretch (d−) Lorentzian 2 2 2Peak position 2916 ± 3 2916 ± 3 2915 ± 3
Gaussian 25 ± 2 25 ± 2 24 ± 3
Amplitude 1 ± 0.09 1 ± 0.09 1 ± 0.1
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2932 ± 1 2932 ± 1 2932 ± 1
Gaussian 9 ± 1 9 ± 1 9 ± 1
Amplitude 0.3 ± 0.05 0.3 ± 0.05 0.3 ± 0.05
CH3 asymmetric
Phase 0 0 0
stretch (r−) Lorentzian 2 2 2Peak position 2978 ± 2 2979 ± 2 2978 ± 2
Gaussian 13 ± 3 14 ± 3 13 ± 3
Non-resonant Amplitude 0.2 ± 0.02 0.2 ± 0.02 0.1 ± 0.02
background Phase 0 0 0
100
Table A.20. Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and MgCl2, corresponding to Figure 6.4b.
Fixed DDAPS concentration = 0.05 mM Varying MgCl2 concentrations
Peak [MgCl
Parameter 2
] = [MgCl2] = [MgCl2] =
assignment 0.01 mM 0.1 mM 1 mM
Amplitude 0.8 ± 0.03 0.9 ± 0.03 0.9 ± 0.03
CH2 symmetric
Phase 0 0 0
+ Lorentzian 2 2 2stretch (d ) Peak position 2854 ± 1 2854 ± 1 2854 ± 1
Gaussian 10 ± 1 9 ± 1 10 ± 1
Amplitude 1 ± 0.04 1 ± 0.04 1 ± 0.04
CH2 symmetric
Phase 0 0 0
+ Lorentzian 2 2 2stretch (r ) Peak position 2873 ± 1 2873 ± 1 2873 ± 1
Gaussian 7 ± 1 6 ± 1 6 ± 1
Amplitude 0.7 ± 0.04 0.7 ± 0.03 0.6 ± 0.04
CH2 asymmetric
Phase 0 0 0
− Lorentzian 2 2 2stretch (d ) Peak position 2916 ± 3 2917 ± 3 2915 ± 3
Gaussian 25 ± 3 24 ± 2 23 ± 3
Amplitude 1 ± 0.09 1 ± 0.08 1 ± 0.1
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2932 ± 1 2932 ± 1 2932 ± 1
Gaussian 9 ± 1 9 ± 1 9 ± 1
Amplitude 0.3 ± 0.05 0.3 ± 0.05 0.3 ± 0.04
CH3 asymmetric
Phase 0 0 0
Lorentzian 2 2 2
stretch (r−) Peak position 2978 ± 2 2978 ± 2 2979 ± 2
Gaussian 13 ± 4 13 ± 3 13 ± 3
Non-resonant Amplitude 0.2 ± 0.02 0.1 ± 0.02 0.1 ± 0.02
background Phase 0 0 0
101
Table A.21. Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and SDS, corresponding to Figure 6.4c.
Fixed DDAPS concentration = 0.05 mM Varying d-SDS concentrations
Peak [d-SDS] = [d-SDS] = [d-SDS] =
assignment Parameter 0.01 mM 0.1 mM 1 mM
Amplitude 0.8 ± 0.03 0.8 ± 0.03 0.7 ± 0.02
CH2 symmetric
Phase 0 0 0
stretch (d+) Lorentzian 2 2 2Peak position 2853 ± 1 2853 ± 1 2853 ± 1
Gaussian 9 ± 1 9 ± 1 10 ± 1
Amplitude 1 ± 0.05 1 ± 0.04 1 ± 0.04
CH2 symmetric
Phase 0 0 0
stretch (r+) Lorentzian 2 2 2Peak position 2873 ± 1 2873 ± 1 2873 ± 1
Gaussian 7 ± 1 7 ± 1 6 ± 1
Amplitude 0.6 ± 0.04 0.6 ± 0.04 0.5 ± 0.03
CH asymmetric Phase 0 0 02
stretch (d−) Lorentzian 2 2 2Peak position 2916 ± 4 2916 ± 4 2917 ± 3
Gaussian 23 ± 4 23 ± 4 26 ± 3
Amplitude 0.9 ± 0.1 0.8 ± 0.1 0.6 ± 0.09
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2933 ± 1 2933 ± 1 2933 ± 1
Gaussian 9 ± 1 8 ± 1 8 ± 1
Non-resonant Amplitude 0.3 ± 0.02 0.3 ± 0.02 0.3 ± 0.02
background Phase 0 0 0
102
Table A.22. Fit parameters for VSFS experiments at the planar CCl4/water inter-
face with DDAPS and DTAB, corresponding to Figure 6.4d.
Fixed DDAPS concentration = 0.05 mM Varying d-DTAB concentrations
Peak [d-DTAB] = [d-DTAB] = [d-DTAB] =
assignment Parameter 0.01 mM 0.1 mM 1 mM
Amplitude 0.8 ± 0.03 0.8 ± 0.03 0.7 ± 0.03
CH symmetric Phase 0 0 02
+ Lorentzian 2 2 2stretch (d ) Peak position 2854 ± 1 2854 ± 1 2854 ± 1
Gaussian 9 ± 1 9 ± 1 8 ± 1
Amplitude 1 ± 0.04 1 ± 0.04 1 ± 0.05
CH2 symmetric
Phase 0 0 0
+ Lorentzian 2 2 2stretch (r ) Peak position 2873 ± 1 2873 ± 1 2873 ± 1
Gaussian 7 ± 1 7 ± 1 7 ± 1
Amplitude 0.6 ± 0.03 0.6 ± 0.04 0.5 ± 0.05
CH asymmetric Phase 0 0 02
− Lorentzian 2 2 2stretch (d ) Peak position 2916 ± 3 2915 ± 4 2916 ± 5
Gaussian 23 ± 3 21 ± 4 19 ± 5
Amplitude 1 ± 0.1 1 ± 0.2 1 ± 0.2
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2933 ± 1 2933 ± 1 2933 ± 1
Gaussian 9 ± 1 9 ± 1 9 ± 1
Amplitude 0.3 ± 0.04 0.3 ± 0.04 0.2 ± 0.05
CH3 asymmetric
Phase 0 0 0
stretch (r−
Lorentzian 2 2 2
) Peak position 2978 ± 2 2978 ± 2 2978 ± 3
Gaussian 13 ± 3 13 ± 3 12 ± 4
Non-resonant Amplitude 0.1 ± 0.02 0.06 ± 0.02 0.06 ± 0.02
background Phase 0 0 0
103
Table A.23. Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and NaCl, corresponding to Figure 6.7a.
Fixed DDAPS concentration = 0.05 mM Varying NaCl concentrations
Peak [NaCl] = [NaCl] =
assignment Parameter 0.01 mM 0.1 mM
Amplitude 0.06 ± 0.01 0.08 ± 0.008
CH2 symmetric
Phase 0 0
stretch (d+) Lorentzian 2 2Peak position 2849 ± 2 2847 ± 2
Gaussian 13 ± 2 10 ± 1
Amplitude 0.05 ± 0.006 0.08 ± 0.003
CH2 symmetric
Phase 0 0
stretch (r+) Lorentzian 2 2Peak position 2871 ± 3 2869 ± 2
Gaussian 14 ± 4 14 ± 2
Amplitude 0.04 ± 0.004 0.04 ± 0.004
CH asymmetric Phase 0 02
stretch (d−) Lorentzian 2 2Peak position 2914 ± 1 2914 ± 1
Gaussian 23 ± 5 25 ± 4
Amplitude 0.04 ± 0.005 0.07 ± 0.003
CH3 symmetric Phase 0 0
stretch Fermi Lorentzian 2 2
resonance (r+FR) Peak position 2931 ± 1 2930 ± 1
Gaussian 12 ± 1 12 ± 1
Amplitude 0.03 ± 0.002 0.05 ± 0.002
CH3 asymmetric
Phase 0 0
stretch (r−) Lorentzian 2 2Peak position 2985 ± 5 2985 ± 3
Gaussian 128 ± 2 125 ± 3
Non-resonant Amplitude 0.007 ± 0.002 0.01 ± 0.002
background Phase 0 0
104
Table A.24. Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and MgCl2, corresponding to Figure 6.7b.
Fixed DDAPS concentration = 0.05 mM Varying MgCl2 concentrations
Peak [MgCl ] = [MgCl ] =
assignment Parameter
2 2
0.01 mM 0.1 mM
Amplitude 0.06 ± 0.03 0.1 ± 0.002
CH symmetric Phase 0 02
stretch (d+) Lorentzian 2 2Peak position 2854 ± 1 2853 ± 1
Gaussian 13 ± 1 14 ± 1
Amplitude 0.05 ± 0.002 0.08 ± 0.004
CH2 symmetric
Phase 0 0
stretch (r+) Lorentzian 2 2Peak position 2875 ± 1 2875 ± 1
Gaussian 14 ± 2 14 ± 1
Amplitude 0.02 ± 0.003 0.05 ± 0.003
CH asymmetric Phase 0 02
stretch (d−) Lorentzian 2 2Peak position 2914 ± 1 2914 ± 1
Gaussian 18 ± 4 26 ± 3
Amplitude 0.06 ± 0.004 0.07 ± 0.003
CH3 symmetric Phase 0 0
stretch Fermi Lorentzian 2 2
resonance (r+FR) Peak position 2936 ± 1 2933 ± 1
Gaussian 14 ± 1 13 ± 1
Amplitude 0.03 ± 0.001 0.04 ± 0.002
CH3 asymmetric
Phase 0 0
stretch (r−) Lorentzian 2 2Peak position 2985 ± 2 2985 ± 2
Gaussian 126 ± 2 125 ± 2
Non-resonant Amplitude 0.02 ± 0.001 0.02 ± 0.001
background Phase 0 0
105
Table A.25. Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and d-SDS, corresponding to Figure 6.8a.
Fixed DDAPS concentration = 0.05 mM Varying d-SDS concentrations
Peak [d-SDS] = [d-SDS] = [d-SDS] =
assignment Parameter 0.01 mM 0.1 mM 1 mM
Amplitude 0.04 ± 0.002 0.05 ± 0.003 0.04 ± 0.002
CH symmetric Phase 0 0 02
+ Lorentzian 2 2 2stretch (d ) Peak position 2859 ± 1 2858 ± 1 2859 ± 1
Gaussian 13 ± 1 13 ± 1 14 ± 1
Amplitude 0.03 ± 0.004 0.04 ± 0.004 0.04 ± 0.003
CH symmetric Phase 0 0 02
+ Lorentzian 2 2 2stretch (r ) Peak position 2881 ± 1 2879 ± 1 2880 ± 1
Gaussian 12 ± 1 12 ± 2 8 ± 1
Amplitude 0.02 ± 0.002 0.03 ± 0.002 0.02 ± 0.002
CH2 asymmetric
Phase 0 0 0
− Lorentzian 2 2 2stretch (d ) Peak position 2915 ± 1 2915 ± 1 2915 ± 1
Gaussian 31 ± 6 27 ± 5 46 ± 9
Amplitude 0.05 ± 0.003 0.05 ± 0.003 0.03 ± 0.002
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2939 ± 1 2939 ± 1 2939 ± 1
Gaussian 15 ± 1 15 ± 1 11 ± 1
Amplitude 0.03 ± 0.001 0.03 ± 0.0008 0.01 ± 0.002
CH asymmetric Phase 0 0 03
− Lorentzian 2 2 2stretch (r ) Peak position 2985 ± 3 2984 ± 3 2985 ± 2
Gaussian 40 ± 7 46 ± 9 45 ± 4
Non-resonant Amplitude 0.01 ± 0.0007 0.01 ± 0.0007 0.02 ± 0.001
background Phase 0 0 0
106
Table A.26. Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and h-SDS, corresponding to Figure 6.8b.
Fixed DDAPS concentration = 0.05 mM Varying h-SDS concentrations
Peak [h-SDS] = [h-SDS] = [h-SDS] =
assignment Parameter 0.01 mM 0.1 mM 1 mM
Amplitude 0.08 ± 0.002 0.07 ± 0.002 0.08 ± 0.004
CH2 symmetric
Phase 0 0 0
Lorentzian 2 2 2
stretch (d+) Peak position 2859 ± 1 2858 ± 1 2857 ± 1
Gaussian 13 ± 1 14 ± 1 14 ± 1
Amplitude 0.06 ± 0.003 0.05 ± 0.009 0.08 ± 0.01
CH symmetric Phase 0 0 02
+ Lorentzian 2 2 2stretch (r ) Peak position 2879 ± 1 2880 ± 1 2879 ± 1
Gaussian 12 ± 1 12 ± 2 13 ± 2
Amplitude 0.06 ± 0.008 0.05 ± 0.01 0.08 ± 0.009
CH2 asymmetric
Phase 0 0 0
− Lorentzian 2 2 2stretch (d ) Peak position 2922 ± 6 2915 ± 11 2916 ± 8
Gaussian 30 ± 5 26 ± 5 26 ± 6
Amplitude 0.08 ± 0.02 0.07 ± 0.04 0.07 ± 0.05
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2939 ± 1 2937 ± 1 2938 ± 1
Gaussian 14 ± 1 14 ± 2 13 ± 3
Amplitude 0.04 ± 0.002 0.03 ± 0.001 0.02 ± 0.004
CH asymmetric Phase 0 0 03
− Lorentzian 2 2 2stretch (r ) Peak position 2985 ± 3 2985 ± 3 2984 ± 6
Gaussian 27 ± 5 30 ± 9 20 ± 12
Non-resonant Amplitude 0.02 ± 0.0008 0.02 ± 0.0009 0.03 ± 0.002
background Phase 0 0 0
107
Table A.27. Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and d-DTAB, corresponding to Figure 6.8c.
Fixed DDAPS concentration = 0.05 mM Varying d-DTAB concentrations
Peak [d-DTAB] = [d-DTAB] = [d-DTAB] =
assignment Parameter 0.01 mM 0.1 mM 1 mM
Amplitude 0.03 ± 0.003 0.03 ± 0.002 0.03 ± 0.003
CH2 symmetric
Phase 0 0 0
+ Lorentzian 2 2 2stretch (d ) Peak position 2855 ± 1 2862 ± 1 2864 ± 1
Gaussian 15 ± 1 13 ± 1 13 ± 1
Amplitude 0.03 ± 0.002 0.03 ± 0.005 0.02 ± 0.01
CH2 symmetric
Phase 0 0 0
Lorentzian 2 2 2
stretch (r+) Peak position 2878 ± 2 2882 ± 1 2882 ± 1
Gaussian 16 ± 3 10 ± 2 10 ± 3
Amplitude 0.02 ± 0.01 0.02 ± 0.002 0.02 ± 0.002
CH asymmetric Phase 0 0 02
− Lorentzian 2 2 2stretch (d ) Peak position 2919 ± 5 2915 ± 2 2914 ± 6
Gaussian 21 ± 5 26 ± 3 23 ± 6
Amplitude 0.03 ± 0.04 0.05 ± 0.002 0.05 ± 0.02
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2937 ± 2 2938 ± 1 2939 ± 2
Gaussian 13 ± 4 15 ± 1 17 ± 2
Amplitude 0.02 ± 0.0008 0.03 ± 0.0008 0.02 ± 0.0007
CH asymmetric Phase 0 0 03
− Lorentzian 2 2 2stretch (r ) Peak position 2986 ± 1 2982 ± 2 2985 ± 2
Gaussian 122 ± 2 29 ± 3 30 ± 4
Non-resonant Amplitude 0 0 0
background Phase 0 0 0
108
Table A.28. Fit parameters for VSFSS experiments on nanoemulsions stabilized by
DDAPS and h-DTAB, corresponding to Figure 6.8d.
Fixed DDAPS concentration = 0.05 mM Varying h-DTAB concentrations
Peak [h-DTAB] = [h-DTAB] = [h-DTAB] =
assignment Parameter 0.01 mM 0.1 mM 1 mM
Amplitude 0.06 ± 0.001 0.06 ± 0.002 0.05 ± 0.004
CH2 symmetric
Phase 0 0 0
+ Lorentzian 2 2 2stretch (d ) Peak position 2857 ± 1 2862 ± 1 2862 ± 1
Gaussian 13 ± 1 13 ± 1 13 ± 1
Amplitude 0.04 ± 0.003 0.05 ± 0.008 0.04 ± 0.01
CH2 symmetric
Phase 0 0 0
Lorentzian 2 2 2
stretch (r+) Peak position 2879 ± 1 2882 ± 1 2883 ± 1
Gaussian 12 ± 1 12 ± 1 12 ± 1
Amplitude 0.03 ± 0.005 0.05 ± 0.02 0.03 ± 0.03
CH asymmetric Phase 0 0 02
− Lorentzian 2 2 2stretch (d ) Peak position 2916 ± 6 2916 ± 7 2915 ± 4
Gaussian 26 ± 5 23 ± 6 25 ± 6
Amplitude 0.05 ± 0.01 0.09 ± 0.05 0.09 ± 0.07
CH3 symmetric Phase 0 0 0
stretch Fermi Lorentzian 2 2 2
resonance (r+FR) Peak position 2936 ± 1 2938 ± 2 2939 ± 3
Gaussian 13 ± 2 15 ± 2 17 ± 3
Amplitude 0.02 ± 0.002 0.04 ± 0.001 0.04 ± 0.001
CH asymmetric Phase 0 0 03
− Lorentzian 2 2 2stretch (r ) Peak position 2980 ± 1 2982 ± 1 2985 ± 1
Gaussian 69 ± 2 23 ± 2 20 ± 2
Non-resonant Amplitude 0 0 0
background Phase 0 0 0
109
REFERENCES
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