Reactivity, Ionic Structure, and Solvation at The Air-Water Interface

Towards this goal, we present a series of vibrational spectroscopy studies of the air-water interface, mainly using a surface-bound azide probe that is sensitive to changes in the local environment such as hydration, electric field, and ion-coordination. Three main results will be highlighted. First, we show that the collective solvation environment at the air-water interface is intermediate between that of bulk organic solvents and pure water. Some level of tunability of this environment is possible with mixed charged monolayers of surfactants, and some tunability is afforded by mixed solvents in the bulk. Second, we show that raising the salt concentration in the bulk, even up to 5 M, does not affect the vibrational frequency of the surface-bound azide. This supports the existence of a small layer of water at the interface, even in extremely high salt concentrations. Third, we present results on the click chemistry of the surface bound azide, and show that surprisingly the initial rate of the reaction is ~10 times faster compared to the bulk, but only if the subphase solvent composition is appropriately tuned. We will also relate these results to hydrolysis kinetics of an organic triflate, which is surprisingly pH-insensitive at the interface. These results provide benchmarks for theories that invoke electric fields, solvation, and extreme acid-base chemistry to explain some of the unusual microdroplet and interfacial reactivity.