Mixtures of Polyelectrolytes and Surfactants at the Oil/Water Interface
Date
2019-09-18
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Publisher
University of Oregon
Abstract
Life itself would not exist without chemical modification of aqueous surfaces. As humanity explores and designs ever-more-complex interfacial systems, multicomponent polymer/surfactant (P/S) mixtures could increase the functionality of designed interfaces. Many applications involve an oil phase, but understanding of P/S assembly at oil/water interfaces is lacking. Most conventional techniques are unfeasible or impossible at buried interfaces.
This dissertation uses a non-invasive and interface specific technique – vibrational sum frequency (VSF) spectroscopy – to study the fundamental forces that control coadsorbing P/S systems. Because the VSF response is nonlinear, the net orientation of the participating dipoles can be determined from the phase relationships of the resultant data. VSF spectroscopy is powerful, but not all-knowing; many corroborating techniques are used to construct a holistic model of P/S behavior.
The first three chapters of this dissertation introduce the system of interest and the techniques by which it will be explored. Chapter III introduces the benefits of selective deuteration, and the information contained within VSF spectra is explained and demonstrated for a simple surfactant interface.
Chapter IV looks critically at previous literature conclusions regarding a model P/S system. The oil/water adsorption is compared with what is known previously about adsorption at the air/water interface. Ultimately, it is found that similar electrostatic effects lead to ordered adsorption at both interfaces, but two conclusions from air/water are not replicated: the formation of multilayers and the persistence of polymer adsorption in the presence of micelles. A more robust interfacial pictures is constructed, which demonstrates the wealth of information obtained from vibrational spectroscopies.
Chapter V explores a carboxylate-containing polyelectrolyte which has been used previously to model environmental humic acid molecules. The low charge-density causes hydrophobic forces to play a much larger role in P/S coadsorption. The adsorbed polymer’s interfacial structure depends strongly on polymer concentration. At first, only hydrophobic P/S interactions cause modest coadsorption, but once a threshold polymer concentration is surpassed, specific electrostatic interactions uncoil adsorbed polyelectrolyte. Electrostatic effects are identified as the foremost contributor to the system’s enhanced surface activity.
This dissertation includes both published and unpublished co-authored materials.
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Keywords
Nonlinear spectroscopy, Oil/water interface, Polyelectrolytes, Surfactants