Molecular Structure and Bonding at Nanoemulsion Surfaces
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Date
2020-09-24
Authors
Carpenter, Andrew
Journal Title
Journal ISSN
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Publisher
University of Oregon
Abstract
The formation of nanoemulsions, nanosized oil droplets in water, has provided scientists with unique hydrophobic chemical environments capable of being leveraged for a diverse set of applications ranging from the petroleum to pharmaceutical industries. In order to stabilize nanoemulsions, the droplet surface must be stabilized. While empirical rules have been established for the kinds of chemicals able to be used for nanoemulsion stabilization, there is a lack of knowledge of the molecular details of the interface that contribute to droplet stability. This lack of knowledge is no more present than in the study of bare nanoemulsions, absent emulsifiers, where a significant negative charge of unknown origin accumulates at the droplet surface and provides the necessary stabilizing electrostatic force.
The studies detailed in this dissertation take advantage of vibrational sum-frequency scattering spectroscopy (VSFSS) to study the surfaces of nanoemulsion droplets and build up a molecularly specific picture of the droplet interface. Beginning with the creation of bare low charge nanoemulsions, an explanation for the accumulation of negative charge at bare nanoemulsion surfaces is developed that can be generalized to the charge accumulation observed at nearly all aqueous-hydrophobic surfaces. Further studies of bare nanoemulsion surfaces focus on the structure and bonding of the aqueous and hydrophobic phases, with the first direct measurements of interfacial water at the bare nanoemulsion surface being reported. These measurements have found that the chemical bonding interactions between oil and water molecules at the nanoemulsion surface is different from that of the extended planar oil-water interface. It is also found that all surfactants studied induce a structural reorganization of interfacial oil molecules.
Other studies, reported here, focus on the assembly of surfactants to nano- oil and water droplet surfaces. Surfactant alkyl chains assemble in a similar manner at the bare oil and water droplet surfaces, but in a different manner compared to the planar oil-water interface. However, despite differences in the alkyl chain assembly, further work investigating headgroup solvation and charge screening phenomena finds that these phenomena occur in similar fashions at the curved and planar oil-water interfaces.
This work contains published and unpublished co-authored material.
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Keywords
Interfacial Water, Nanoemulsions, Oil-Water Interface, Surface Spectroscopy, Surfactants, Vibrational Sum-Frequency Scattering Spectroscopy