Vibrational Sum Frequency Spectroscopic Investigations of Carboxylic Acid- Containing Polyelectrolytes at the Oil-Water Interface
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The boundary between two immiscible fluids is an important location for the adsorption and assembly of polyelectrolytes. A good description of the fundamental properties of polyelectrolytes at such interfaces is a prerequisite to understanding a multitude of important phenomena in biological, environmental, and industrial systems. This dissertation examines the molecular-level details involved in the adsorption and assembly of carboxylic acid-containing polyelectrolytes at the oil-water interface. Vibrational sum frequency spectroscopy, a surface-selective spectroscopic technique, was used to obtain vibrational spectra of oriented polyelectrolytes at the carbon tetrachloride-water interface. Studies that vary both polyelectrolyte and solution conditions are presented that identify the factors that dictate whether or not polyelectrolytes will adsorb to the oil-water interface and that describe the specifics of the adsorption process. First, the role of the position of ionizable groups along the polyelectrolyte backbone is examined. This is accomplished through studies of syndiotactic poly(methacrylic acid) (sPMA) and isotactic poly(methacrylic acid) (iPMA). Both sPMA and iPMA rapidly adsorb to the interface as highly ordered layers, but only sPMA is able to accumulate to the interface over time. Studies exploring the role of polyelectrolyte charging are presented next. Molecular weight studies of sPMA as a function of pH show the effect of the number of charges per polyelectrolyte chain on interfacial adsorption and assembly. Results show that charge accumulation on polymer chain segments plays a major role in polyelectrolyte behavior at the oil-water interface. Subsequently, studies of both iPMA and sPMA with Ca2+ and K+ are presented to demonstrate the influence of the presence and identity of ions on polyelectrolyte interfacial behavior. These ions only induce the adsorption of iPMA to the oil-water water interface under solution conditions when it is not normally surface active. The identity of the cation is important to the adsorption process. Lastly, studies of two different peptoid polymers emphasize the role of intermolecular interactions between adsorbed polymer chains in polyelectrolyte interfacial assembly. It is shown that favorable electrostatic interactions between negatively charged carboxylate and positively charged amine groups are important in ordered monolayer assembly. This dissertation includes both published and unpublished co-authored materials.