Nonlinear Optical Studies of Molecular Adsorption and Solvation at Solid/Liquid and Liquid/Liquid Interfaces

Abstract

Interfacial solvation is responsible for promoting biological phenomena in vivo including protein folding, solute transfer across membranes and enzymatic activity. The specific solvation interactions responsible for these and other processes can be both cooperative and complex. Because many cellular processes rely on interfacial effects, understanding how forces at an interface influence a solute will give insight into how molecules behave within these cellular bodies. The studies presented here are focused on isolating how these solvation interactions vary systematically with the identity of the solute and solvent at an interface.

The interfaces probed in these experiments varied from weakly to strongly associating interfaces defined as such by the identity of the solvent used to form the silica/liquid interface. Findings from strongly associating interfaces gave rise to surprising results from both the silica/methanol and silica/ethanol interfaces. The silica/ethanol interface forms a very polar interface as probed by the solute p-nitroanisole (pNAs). At the silica/methanol interface, a very nonpolar region was probed by several solutes sensitive to solvent polarity.

The findings from the silica/methanol interface, led us to the research completed in the final chapter of this thesis. Data obtained from these measurements described the interfacial solvation and adsorption behavior of two solutes, pNAs and p-nitrophenol (pNP). Several silica/liquid interfaces were used in this study including, water, dimethyl sulfoxide (DMSO), acetonitrile (ACN), n-hexane, decane, cyclohexane, and methyl-cyclohexane. The two solutes are sensitive to solvent polarity and show similar solvatochromic behavior in bulk solvents. The solutes sample different interfacial polarities at the same silica/liquid interfaces according to SHG spectra obtained. pNAs is shown to be more sensitive to solvent identity at an interface than pNP, but less surface active. The sensitivity of pNAs to solvent identity at a silica/liquid interface is attributed to the solute's higher solubility in the solvents than pNP's solubility in the same solvents. On average, pNP has ~10 kJ/mol more adsorption energy at the measured interfaces than pNAs, and this too can be attributed to the inability of pNP to sufficiently solvate in many of the alkane solvents, forcing the solute out of solution and into the interface.