Linear and nonlinear optical studies of molecular adsorption to silica/liquid interfaces

Abstract

Adsorption mechanisms at buried interfaces are difficult to predict a priori, with many interactions to consider including those between the substrate and solvent, the substrate and adsorbate, and the solvent and adsorbate. Studies described in this thesis examine the roles these variables have on controlling interfacial behavior, including molecular adsorption and aggregation at solid/liquid interfaces. Specifically, second harmonic generation (SHG) was employed to characterize adsorption environments and adsorption energies at different silica/liquid interfaces, due to the technique's surface specificity. Additionally, time resolved fluorescence was used to quantify emission lifetimes within these same interfacial regions. By systematically changing the substrate, solvent identity, and adsorbate functionality, the impact of each contribution was identified and quantified. Initial studies examined the role played by interfacial pH in controlling adsorption. Above pH 5, silica surfaces become negatively charged and promote two distinct adsorption mechanisms. Adsorption due to these mechanisms requires very long equilibration times (>3hrs). Subsequent experiments studied the role played by solvent identity on interfacial solvation. At a methanol/silica interface a non-polar interfacial environment was produced, independent of solute choice. Non-polar solvents conversely create polar interfacial solvation environments. At these different solid/liquid interfaces, similarly structured coumarin dyes, C151 and C152, were examined. Slight changes in structure lead to differing behaviors at the surface, C151 terminates at monolayer coverage while C152 shows clear signs of multilayer formation. This observation is explained by the difference in hydrogen bonding opportunities for each adsorbate: C151 can accept and donate H-bonds while C152 can only accept H-bonds, resulting in more degrees of freedom for C152 at an interface and thus the possibility of aggregation.