UC Irvine

Studying ion transport in nanoporous materials is crucial to a wide range of energy and environmental technologies, including ion-selective membranes, drug delivery systems, and supercapacitors. Many of these applications rely on non-aqueous solvents or non-traditional interfaces, such as hydrophobic surfaces. However, classical descriptions of solid-liquid interfaces fail to adequately describe ion and electrical potential distributions in these non-traditional systems. This dissertation examines what consequences occur when these non-traditional solid-liquid interfaces are placed under nanoconfinement. Specifically, ion transport measurements were performed in a variety of nanodevices, including nanopipettes and nanopores, which acted as model systems for probing non-traditional solid-liquid interfaces. My first project details the breakdown in classical electrical double-layer models when moving from water to an organic solvent. This project demonstrates that in a model system, acetonitrile at a silica interface, the solvent molecules organize in a well-defined lipid-bilayer-like structure. This solvent organization in turn dictates the ionic distribution at the interface, which stands in stark contrast to traditional models, where ion distribution is determined by native surface charge. A combined approach of ion-transport measurements in nanopipettes, surface-selective spectroscopy, and molecular dynamics simulations were used to probe the acetonitrile-silica interface in a variety of salts, including LiClO4, NaClO4, LiBF4, and LiPF6. These findings emphasize the importance of including solvent molecules and ions explicitly in descriptions of solid/liquid interfaces. My second and third project examine hydrophobic interfaces in nanopores. Single hydrophobic nanopores are ideal model systems for studying nanoconstricted hydrophobic surfaces and wetting/dewetting transitions at the nanoscale. In these projects, ion current measurements in single silicon nitride nanopores containing a hydrophobic-hydrophilic junction were performed in a variety of salt types and concentrations. The results show that transport properties in these devices are highly dependent on the size, hydration strength, and concentration of the solvated ions. Large, polarizable anions, such as bromide and iodide, facilitate pore wetting, with an unusual dependence on electrolyte concentration – higher concentrated solutions more easily wet the devices, in contrast to bulk surface tension trends. Experimental results were supplemented with molecular dynamic simulations that revealed key characteristics of the asymmetric nanopore system. These results are essential for designing nanoporous systems that are selective for ions of the same charge as well as understanding the fundamental role of ion hydration on the properties of solid-liquid interfaces.