Interaction of monovalent ions with the water liquid-vapor interface: a molecular dynamics study

Results of molecular dynamics studies on the ions Na+, F-, and Cl- near the water liquid-vapor interface are reported. The free energies required to move the ions to the interface are presented and shown to depend on the sign of the ionic charge, and not the size of the ion. F- and Cl- can approach to within 2 molecular layers of the interface without incurring a significant change in free energy, while it costs about 2.5 kcal/mole to move Na+ this same distance. The free energy differences between the cation and the anions arise from the interaction of the ions with the water molecules in the interfacial region. These water molecules are oriented with a slight preference for their molecular dipoles to point toward the liquid. Thus, the anions approaching the interface disrupt the water structure less than does the cation. The calculated free energy curves are compared with predictions of simple dielectric models. It is shown that these models do not provide a good description of ions at the water surface. The ions are found to retain their first solvation shells at the interface. The anions also retain part of their second solvation shells, while Na+ does not. As a result, a larger bulge in the water surface is observed above the anions than above Na+. The lateral mobilities of the ions increase at the interface, in qualitative agreement with predictions of hydrodynamic models.