On the Nature of Ions at the Liquid Water Surface

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, wherein the microscopic picture then has to be inferred by applying theoretical models. Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions. This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations. Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging.