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

Abstract

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 parameters were developed using IMPT, and C6 dispersion coefficients were calculated using coupled Hartree–Fock perturbation theory. The binding energy between 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 same energy. Some have all the water oxygens located close to Na+ ions, but others have some water molecules located above the Cl− ions. The latter are farther from the surface, and are hydrogen bonded to water molecules sited above the Na+ ions. Structures were also located with 1.5 water molecules per NaCl unit. Their binding energy per water molecule was similar to the structures with one water molecule per NaCl unit. Experimentally water monolayer structures with (1×1) and (c4×2) unit cells have been suggested. Our results support the conclusion that owing to the small energy differences between different monolayer structures, several different types of monolayer structure can coexist on the surface at low temperatures.