Theoretical study of ion solvation at the water liquid–vapor interface

Molecular dynamics calculations are reported for several ions at the liquid–vapor interface of water. The transition from the bulk to the interface region is investigated from structural, energetic, and dynamical points of view by calculating ion–water geometries, radial distribution functions, solvent molecular reorientation times, solvent polarization fluctuations, and solvation free energy as a function of distance from the interface. It is shown that ions tend to keep most of the structural and dynamical properties of their first solvation shell intact as they are moved into the interface, and that the tendency for negative adsorption (positive free energy of adsorption) is associated with weaker and fewer long range interactions. A comparison of some of the molecular dynamics results to predictions of simple continuum models is discussed, showing generally poor quantitative agreement.