Ion hydration near air/water interfaces and the structure of liquid surfaces

Abstract

Negative adsorption of ions, commonly observed at air-water interfaces, is examined in terms of models of restricted polarization of the solvent by ions at the interface and the structure of the liquid interface. The Born and other models of ionic hydration are applied to evaluate the self-energy of the ion arising in the region of solvent near its interface and in the vacuum or vapour beyond. The adsorption energy of an ion varies substantially with distance from the liquid interface so that a distribution of ions arises as a function of distance from the interface. Integration of this distribution gives an expression, and results, for the ionic surface excess. The diffuse-layer potential, which an unequal distribution of cations and anions give rise to, gives a contribution to the surface potential of the electrolyte solution at finite concentrations. Structural aspects of the liquid interface at which ions are negatively adsorbed are discussed in terms of Stefan's ratio and the superficial excess entropies of various liquid surfaces. These entropies are related to the cohesive energy densities of the bulk liquids. Ion solvent-structure co-sphere interactions with structured interfaces will lead to specificity of negative adsorption of ions.