Free energy profiles for Na+ and Cl- adsorption onto water/NaCl crystal interfaces evaluated by molecular dynamics simulation

The potential of mean force (PMF) of solute Nai and Cl- ions approaching water/NaCl(001) and water/NaCl(011) interfaces is calculated by using classical molecular dynamics (MD] simulations. It is found that (1) Na+ and Cl- ions adsorb an the crystal surface either directly (direct adsorption) or with a water molecule interposed between the surface and themselves (solvent-separated adsorption), (2) both ions are adsorbed more stably on the NaCl surfaces under solvent-separated conditions in most cases, and they adsorb on NaCl(001) more easily than on NaCl(011) in the case of direct adsorption, and (3) Na+ adsorbs on the NaCl surfaces more easily than Cl- in the case of direct adsorption, but direct adsorption of Cl- on NaCl (011) is impossible. These results indicate that the solute-surface and solvent-surface interactions are strongly affected by the lattice arrangement of the surface such that the PMF profiles largely depend not only on the size of solute ions but also on the lattice arrangement.