Organization of Water and Atmospherically Relevant Ions and Solutes: Vibrational Sum Frequency Spectroscopy at the Vapor/Liquid and Liquid/Solid Interfaces

Abstract

The nature of water's hydrogen-bonding network is a vital influence on the chemistry that occurs at interfaces, but a complete understanding of interfacial water has proven elusive. Even-order nonlinear optical spectroscopies, such as vibrational sum frequency generation (VSFG) spectroscopy and heterodyne detected phase-sensitive sum frequency generation (PS-SFG) spectroscopy, are inherently surface specific. With the advent of advances in these spectroscopic techniques, researchers can now explore many long-standing questions about the dynamics and structures present at the vapor-water and water-solid interfaces. Of special interest to the atmospheric chemistry community is the accommodation of ions and solutes by water's hydrogen-bonding network. A better understanding of how ions and solutes behave in hydrogen-bonded water has afforded a fresh perspective of aqueous aerosols, because the interactions involved therein drive phenomena such as the hydrolysis of atmospheric chemical species. In this Account, we present work from our laboratory focusing on applying VSFG and the recently developed PS-SFG techniques to probe the perturbation of water's hydrogen-bonding network at the vapor-water interface by a variety of ions and solutes. We also present very recent results from our laboratory on the direct observation of the adsorption of ions at the water-CaF(2) interface. We begin by discussing the influence of ions and solutes on interfacial water structure. Results for halide salts and the acid analogs on interfacial water structure are shown to be quite different, as would be expected from differences in surface tension measurements that have been known for a long time. Also examined are systems with the largely polarizable molecular anions nitrate (NO(3)(-)), sulfate (SO(4)(2-)), carbonate (CO(3)(2-)), and bicarbonate (HCO(3)(-)).These systems feature more complicated influences on interfacial water structure than halide-containing solutions; however, our conventional VSFG results for both nitrate and sulfate solutions are in agreement with recent PS-SFG results and molecular dynamics simulations. We also discuss recent PS-SFG work on carbonate and bicarbonate systems in which the accommodation of the bicarbonate ion at the vapor-water interface is in stark contrast to the carbonate results. Perturbation of interfacial water by solutes is examined for solutions of dimethyl sulfoxide and methylsulfonic acid. PS-SFG results for these systems are striking: they illustrate the dramatic changes that interfacial water molecules undergo in the presence of solutes that are not observed with conventional VSFG. Finally, we discuss direct sulfate ion adsorption for the aqueous sodium sulfate-CaF(2) interface, with the goal of elucidating water behavior at this surface.