Ions at the Air/Water Interface

We present results from theoretical studies of aqueous ionic solvation of alkali halides aimed at developing a microscopic description of structure and dynamics at the interface between air and salt solutions. The traditional view has depicted the air/solution interface of simple electrolytes as being devoid of ions. However, it is now firmly established that polarizable anions, such as the heavier halides, occupy the surface of small to medium sized water clusters. Using a combination of theoretical calculations, including ab initio quantum chemistry, Car−Parrinello molecular dynamics simulations, and primarily molecular dynamics simulations based on polarizable force fields, we present a unified view of the interfacial structure of aqueous ionic clusters and bulk solutions. Indeed, we demonstrate that the heavier halogen anions have a propensity for the interface that is proportional to their polarizability. The cluster results are directly supported by existing experimental and theoretical studies, and the bulk solution results are indirectly supported by several recent experiments. The novel view of the ionic solution/air interface presented here has also implications for dynamics following photoexcitation and electron photodetachment of ions. Moreover, the present results provide insight into heterogeneous atmospheric chemistry leading to halogen release from sea salt aerosols in the lower marine troposphere and from the Arctic snowpack during polar sunrise.