Structure and energetics of the water/NaCl(100) interface

A series of correlation-corrected periodic Hartree-Fock (PHF) calculations have been performed to evaluate the structure of a single layer of water adsorbed on NaCl(100). This work was motivated by differing experimental observations which assign the water/NaCl interface structure as either a monolayer, with a single adsorbate binding site, or a 4x2 bilayer model. Quantum mechanical binding energies were computed for several adsorbate/surface geometries corresponding to 1x1 and 2x1 monolayer structures and 4x2 bilayer structures. The calculations indicate that the binding energy per water molecule for the monolayer and bilayer models are very similar; the estimated PHF and correlation-corrected binding energies are 10 and 14 kcal/mol, respectively, for both models. When measured per unit surface area the 4x2 bilayer is energetically favored because it has a 50% greater packing density than the monolayer. The computed binding energies are consistent with experiment. These data show that the monolayer structure may be stable (or metastable) at low water coverages, but as the coverage is increased the bilayer structures become more favorable. The quantum mechanical data imply that the structure of the water/NaCl interface will be very sensitive to the sample preparation and experimental techniques used. The calculated binding energies for the 4x2 water bilayer geometries reveal the existence of many local minima on the potential energy surface which could result in the domaining of water on the NaCl(100) surface.