New view of the surface of salt solutions with implications for atmospheric chemistry

It has been known for almost a century that simple inorganic salts raise the surface tension of water. Interpretation of this phenomenon via the Gibbs adsorption equation has led to the generally accepted picture of ions being repelled from the air/solution interface. Here, we report results from molecular dynamics simulations of a series of sodium halide air/solution interface. In all cases, the simulations reproduce the experimentally observed increase in surface tension relative to pure water. Analysis of the simulations reveals that the small, nonpolarizable fluoride anion is excluded from the surface in accord with the traditional picture. However, all the larger, polarizable halide anions (Cl-, Br-, and I-) are presented at the surface. Moreover, bromide and iodine exhibit surfactant activity, i.e. their concentration at the surface is higher than in the bulk. On the basis of the simulations a molecular picture of hydrogen bonding in the interfacial region, Which can be tested by surface sensitive spectroscopic experiments, is developed. The novel microscopic view of the interfacial structure of aqueous salt solutions is in a stark contrast with the traditional belief (which, incidentally, is not supported by any direct measurements with molecular resolution) that the surface of aqueous electrolytes is devoid of ions. The result of the present simulations have also direct implications for reactivity of aqueous sea salt aerosols in the marine boundary layer, as well as for bromine chemistry on Arctic snow packs during the polar sunrise. (author)