Spectroscopic studies of noncovalent interactions at interfaces and their effects on interfacial structure, organization, and association
Gobrogge, Eric Andrew
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Studies described in this dissertation used linear and nonlinear optical methods to examine the effects of noncovalent forces on molecular structure, organization and reactivity at solid/liquid, solid/vapor and liquid/vapor interfaces. These studies address three general questions: 1) Solvent structure at solid/liquid interfaces; 2) Solute adsorption to chemically tailored solid and liquid interfaces; and 3) Partitioning of binary solvents at solid/liquid and solid/vapor interfaces. 1) Solvent structure at solid/liquid interfaces. Vibrational sum frequency spectroscopy (VSFS) was used to study the interfacial organization of different alkanes and alcohols at the silica/liquid interface. Results showed that solvent organization depended sensitively on both interactions with the interface and on the solvent's molecular structure. Silica/methanol and silica/ethanol interfaces were also compared in order to determine why ethanol gives a VSFG spectrum but methanol does not. 2) Solute adsorption to chemically tailored interfaces. VSFS and fluorescence spectroscopy were used to characterize and analyze the effectiveness of silica substrates functionalized specifically to promote adsorption of organic analytes in aqueous solutions through catch and release chemistry. VSFS has also been used to study cooperative adsorption at aqueous/vapor interfaces to explore how insoluble surfactants can increase near-surface concentrations of soluble species. 3) Binary solvent partitioning at solid/liquid and solid/vapor interfaces. VSFS was used to study molecular organization at silica/binary solvent interfaces where the binary solvent consisted of acetonitrile and methanol in varying mole fractions. The vibrational spectra indicated that while methanol adsorbed ideally at the silica/vapor interface, acetonitrile accumulated in excess relative to the vapor phase composition. At the silica/liquid interface, methanol appeared to remain strongly associated with the surface until an acetonitrile mole fraction of 0.85 was reached. At higher mole fractions, interfacial acetonitrile adopts an antiparallel bilayer organization. This binary mixture was also compared to various other binary mixtures at the silica/vapor and silica/liquid interfaces.