Structure and Dynamics of the Water/MgO Interface

Abstract

A pairwise additive potential energy expression for the water/MgO interaction was obtained by fitting the parameters to ab initio electronic structure energy data, computed using correlation-corrected periodic Hartree−Fock (PHF) theory, at selected adsorbate/surface geometries. This potential energy expression was used in molecular dynamics and Monte Carlo simulations to elucidate the water/MgO interaction. Energy minimization reveals a nearly planar adsorbate/surface equilibrium geometry (−15° from the surface plane with the hydrogens pointing toward the surface oxygens) for an isolated water on perfect MgO (001), with a binding energy of 17.5 kcal/mol; subsequent PHF calculations on this system confirmed that this is a potential minimum. Rate constants for desorption (kdsorb), intersite hopping (khop), intrasite rotation (krot), and intrasite flipping (kflip) were estimated for an isolated water on the surface using simple transition state theory. The computed rates (at T = 300 K) are kdsorb = 1.1 × 105 s-1, khop = 3.7 × 1010 s-1, krot = 5.7 × 1011 s-1, and kflip = 4.6 × 1011 s-1. The motion of a single water on the surface is described by an effective diffusion constant (Deff = 8.0 × 10-6 cm2/s), computed from the surface rate constants combined with Monte Carlo simulations. The structure of the liquid water/MgO interface was determined from simulations with 64 and 128 water molecules on the surface. Simulations (at T = 300 K) of the two-dimensional water overlayers reveal a densely packed first layer, Z(Ow−surf) = 2−3 Å, with one water per surface magnesium, with a nearly equal distribution of water molecules aligned −17° and +30° with respect to the surface plane. A more diffuse second layer exists, Z(Ow−surf) = 4−5.5 Å, with a much broader distribution of water angular orientations with respect to the surface plane. The region Z(Ow−surf) > 6 Å resembles bulk water, with the density profile approaching a constant as a function of distance above the surface and a uniform distribution in water/surface angular orientations. At the water/vacuum interface (top of the multilayer) the waters assume a “planar orientation” (0° with respect to the surface plane). During the timescale of these simulations very little interlayer exchange of water molecules occurs between the first monolayer (n = 1) and the additional overlayers (n ≥ 2). In contrast, the water molecules in the multilayers (n ≥ 2) display motion similar to bulk liquid water at this temperature.