Adsorption of water on NaCl(001). I. Intermolecular potentials and low temperature structures

Water adsorption on the NaCl(001) surface has been extensively studied both theoretically and experimentally during recent years. Here we investigate it using intermolecular potentials derived from intermolecular perturbation theory (IMPT). The water-water interactions are described by the recently developed ASP-W4 potential. For the water-NaCl surface, repulsion parameter s were developed using IMPT, and C-6 dispersion coefficients were calculate d using coupled Hartree-Fock perturbation theory. The binding energy betwee n a single water molecule and the NaCl surface is found to be 40 kJ mol(-1) . A stable tetramer can form on the surface, similar to the water tetramer in the gas phase. At a coverage of one water molecule per NaCl unit, there are several different water monolayer structures with approximately the sam e energy. Some have all the water oxygens located close to Na+ ions, but ot hers have some water molecules located above the Cl- ions. The latter are f arther from the surface, and are hydrogen bonded to water molecules sited a bove the Na+ ions. Structures were also located with 1.5 water molecules pe r NaCl unit. Their binding energy per water molecule was similar to the str uctures with one water molecule per NaCl unit. Experimentally water monolay er structures with (1x1) and (c4x2) unit cells have been suggested. Our res ults support the conclusion that owing to the small energy differences betw een different monolayer structures, several different types of monolayer st ructure can coexist on the surface at low temperatures. (C) 1999 American I nstitute of Physics.