Surface structure of aqueous electrolyte solutions probed by UV second harmonic generation

A qualitatively new understanding of the nature of ions at the liquid water surface is emerging. Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements. The latter indicate that anions generally approach closer to the surface than do cations, but since the surface tension of electrolyte solutions generally increases with concentration, inorganic ions must be depleted in the interfacial region. These experimental findings have traditionally been rationalized by simple continuum models with the conclusion that the outermost liquid layer of the water surface is essentially devoid of ions with a monotonically increasing electrolyte concentration towards the bulk.;This simple picture has recently been challenged; first by chemical kinetics measurements, where the chemical reaction dynamics occurring on aqueous sea salt particles, ocean surfaces, and laboratory aerosol particles could not be explained without invoking surface reactions involving ions, subsequently by theoretical molecular dynamics simulations using polarizable models, and most recently, by new surface sensitive experimental observations. The MD simulations predict a non-monotonic surface distribution with anions enhanced in the outermost liquid layer but depleted in the adjacent sublayer, where the cations are, in turn, enhanced. Surface-sensitive experiments seem to be consistent with this description.;Here I describe surface specific UV second harmonic generation (SHG) experiments, directly probing the anion concentration at the surface. Surface enhancement is found for both dilute and concentrated electrolyte solutions, and is due to two completely different adsorption mechanisms with different scales of adsorption energies. The high concentration enhancement is apparently results from the high polarizability of the anions, consistent with the reaction dynamics experiments and MD simulations, whereas the dilute concentration enhancement is attributed to the elusive Jones-Ray effect with a currently undetermined molecular origin. Besides describing the SHG experiments, this thesis presents an overview of the nature of the interfacial structure of electrolyte solutions and gives a detailed description of the new picture that is emerging largely as a result of the measurements presented herein.