STRUCTURE AND ENERGETICS OF THE WATER NACL(100) INTERFACE/

A series of correlation-corrected periodic Hartree-Fock (PHF) calculat ions have been performed to evaluate the structure of a single layer o f water adsorbed on NaCl(100). This work was motivated by differing ex perimental observations which assign the water/NaCl interface structur e as either a monolayer, with a single adsorbate binding site, or a 4x 2 bilayer model. Quantum mechanical binding energies were computed for several adsorbate/surface geometries corresponding to 1x1 and 2x1 mon olayer structures and 4x2 bilayer structures. The calculations indicat e that the binding energy per water molecule for the monolayer and bil ayer models are very similar; the estimated PHF and correlation-correc ted binding energies are 10 and 14 kcal/mol, respectively, for both mo dels. When measured per unit surface area the 4x2 bilayer is energetic ally favored because it has a 50% greater packing density than the mon olayer. The computed binding energies are consistent with experiment. These data show that the monolayer structure may be stable (or metasta ble) at low water coverages, but as the coverage is increased the bila yer structures become more favorable. The quantum mechanical data impl y that the structure of the water/NaCl interface will be very sensitiv e to the sample preparation and experimental techniques used. The calc ulated binding energies for the 4x2 water bilayer geometries reveal th e existence of many local minima on the potential energy surface which could result in the domaining of water on the NaCl(100) surface.