Adsorption of Na+ and Cl− at the charged water–platinum interface

Abstract

The adsorption of Na⁺ and Cl⁻ at the charged water–platinum interface as a function of external voltage is investigated by molecular dynamics computer simulation. Generally, although the water structure is significantly affected by a strong external electric field, the structure of the ion–water complex at the surface is much less affected. At electric field values comparable to those found in experimental systems, Cl⁻ is ‘‘contact adsorbed’’ on the metal and is mainly solvated by the water layer adjacent to the metal. In contrast, the small Na⁺ is solvated equally well by both adsorbed water molecules and water molecules outside the inner layer. At higher electric field values that are close to the upper end of what is believed to exist in electric double layers, both ions lose part of their hydration shell. The dynamic of the ion motion towards the metal up to about one solvent layer from the surface is in reasonable agreement with experimentally known ion conductivities at low electric fields, but it overestimates the ionic mobility at a high electric field. This can be rationalized in terms of the effect of the electric field on the hydration shell residence time. The final step toward contact adsorption at the very high electric field is instantaneous for Na⁺, but involves a barrier for Cl⁻.