Water Combination Bands and Mixed Systems at the Oil/Water Interface
Datum
2021-09-13
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University of Oregon
Zusammenfassung
Surface chemistry is fundamental to life. The interface between oil and water is a unique environment home to vital chemical phenomena, from reactions occurring at our cell membranes to the functionality of industrial products we consume. Understanding the behavior of molecules at the surface between oil and water is necessary for building upon fundamental aspects of this ubiquitous surface; however, detailed investigations of the molecular level of the oil/water surface are limited due to experimental challenges of probing a buried interface. Vibrational sum frequency (VSF) spectroscopy is non- invasive surface specific nonlinear spectroscopic technique that can probe the population and orientation of molecular vibrations at interfaces. The work presented in these studies utilizes VSF and surface tensiometry methods to investigate two main areas of surface chemistry at the oil/water surface: 1) the presence and implications of weak VSF signal from combination bands of surface water molecules, and 2) intermolecular interactions between charged and nonionic surface adsorbates in polymer-surfactant and co-surfactant mixed systems.
These studies find the spectral presence of both the stretch + libration and bend + libration combination modes of interfacial water molecules are very sensitive to surface charge, as is expected from surface water vibrational responses. Due to the broad nature of these modes, their presence in the surface water spectrum can have significant effects on the VSF analysis of C-H and C-D vibrational modes from surface adsorbates as their alkyl vibrational modes interfere with the signal from water combination bands. Studies investigating the surface structure of mixed systems containing nonionic polymer and charged surfactants reveal minimal interfacial interactions between charged surfactants with surface-active nonionic polymers; however, surface-inactive nonionic polymers, such as polyacrylamide, are synergistically drawn to the surface through charge-dipole interactions with charged surfactants. Studies investigating co-surfactant mixtures of nonionic Span-80 with anionic surfactants indicate competitive adsorption between the two surfactant species, as Span-80’s attraction to the oil phase is stronger than any intermolecular interactions occurring between Span-80 and the anionic surfactants.
This dissertation includes both published and unpublished co-authored material.