Dynamics of Water Adsorption onto a Calcite Surface as a Function of Relative Humidity

Abstract

We have developed a molecular dynamics scheme in order to understand the dynamics of water adsorption on calcite surfaces as a function of relative humidity. In contrast to previous studies where either a monolayer or bulk water was assumed to cover the surface, we observe the formation of two to three prominent layers of water depending on the relative humidity. Due to the fact that these simulations are at room temperature, the distribution of water molecules on the surface is inhomogeneous and nonuniform. Our simulation results agree well with recent grazing incidence X-ray diffraction studies. The free energy of adsorption of a single water molecule onto the bare calcite (101̄4) surface is predicted to be −10.6 kcal/mol at room temperature while the enthalpy for the same process is −21.3 kcal/mol. The time scale for the bare calcite surface to become in dynamic equilibrium with water vapor at 100% relative humidity is determined to be close to 6 ns, and the adsorption follows a BET (Brunauer−Emmet−Teller)-like isotherm, in that multilayers form. From our orientational distribution functions, we are able to determine the existence of three binding modes for water. The mobility of water adsorbed on the calcite surface is greater in directions parallel to the surface. Motion perpendicular to the suface is slower. The diffusivity of water significantly increases with increasing relative humidity.