FREE-ENERGY PROFILES FOR NA+ ADSORPTION ON A METAL-ELECTRODE

Monte Carlo statistical mechanics simulations have been used to determine free energy profiles for the approach of a sodium ion to a model metal electrode in aqueous solution and in tetrahydrofuran (THF). The ion and 200-300 solvent molecules are explicitly represented in a periodic cell bounded in the +/- z directions by soft walls that are polarizable via image charges. Statistical perturbation theory provided the free energy profiles from the wall out 8-10 angstrom into the solvent. In both cases, a single minimum is found corresponding to the sodium ion with its first shell of solvent molecules in contact with the wall. At least for the case of an uncharged electrode, the traditional model of a layer of adsorbed water between the solvated ion and the electrode appears incorrect. The structure of the interfacial solvent and the changes in solvation of the ion as it approaches the wall are also considered in detail.