Ions at aqueous interfaces: From water surface to hydrated proteins

The surfaces of aqueous solutions are traditionally viewed as devoid of inorganic ions. Molecular simulations and surface-selective spectroscopic techniques show, however, that large polarizable anions and hydronium cations can be found (and even enhanced) at the surface and are involved in chemistry at the air/water interface. Here, we review recent studies of ions at the air/water interface and compare from this perspective water with other polar solvents. For water, we focus in particular on the surface behavior of its ionic product (i.e., hydronium and hydroxide ions). We also investigate the feasibility of dielectric models for the description of the protein/water interface, in analogy to the air/water interface. Little correlation is found between these two interfaces in terms of ion segregation. Therefore, we suggest a local model of pairing of ions from the solution with charged and polar groups at the protein surface. We also describe corresponding results of experimental studies on aqueous model systems.