Why are trace amounts of chloride so highly surface-active?

Abstract

On many metals, small quantities of chloride are known to be adsorbed at potentials well below the potential of zero charge and to influence other electrochemical processes. In order to understand this behavior, we have investigated the adsorption of a single Cl− ion from aqueous solution by a combination of density functional theory and classical molecular dynamics, taking Au(111) as a model surface. While in the vacuum, Cl is adsorbed directly on the surface in the 3-fold hollow sites, in aqueous solution the optimal position lies a fraction of an Ångstrom towards the solution, where the ion is almost fully solvated and mobile in the direction parallel to the surface. As a result, the adsorption of a single ion is exothermic by about -0.6 eV, even at the potential of zero charge. This adsorption behavior is limited to low coverages, because the adsorbates repel each other and because full solvation shells can only form at low coverage.