USING VIBRATIONAL SUM-FREQUENCY-GENERATION SPECTROSCOPY TO EXPLORE THE ROLE OF SOLVENT ORGANIZATION IN DETERMINING ION LOCATIONS NEAR SILICA SURFACES

Abstract

Chemical processes occurring at liquid–solid interfaces are fundamental to applications in fields such as energy storage and nanofluidic transport. In this thesis I establish that the general framework used to describe and understand these systems, the electrical double-layer model, is insufficient in describing interfacial electrolyte solutions in polar, aprotic organic solvents. Using vibrational sum-frequency-generation (VSFG) spectroscopy, a nonlinear optical technique that is indispensable for exploring interfacial organization and dynamics, I study different polar aprotic solvents at silica interfaces. These studies highlight the importance of the organization of such solvents in dictating the interfacial distribution of ions. In the first part of this dissertation, I compare electrolyte experiments in acetonitrile (MeCN) and propionitrile (EtCN) to determine how an increase in alkyl chain length can influence solvent organization at a liquid–solid (LS) interface, and thereby influence the interactions of ions with the interface. In the second part of the dissertation, I focus on a solvent mixture of EtCN and deuterated MeCN at a silica interface. VSFG data for solutions with different molar ratios of the two solvents indicate that there is preferential partitioning of each liquid at this surface. In the third part of this dissertation, I examine the effects of solvent chirality on the organizational behavior at an LS interface, and consequently on the effects of this organization on ion partitioning. The key result of my research is that a polar, aprotic, organic solvent’s structure, chirality, and mixing with other solvents, can drive the partitioning of ions in interfacial electrolyte systems, in contradiction to the predictions of the EDL.