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| - Dynamics of Water Adsorption onto a Calcite Surface as aFunction of Relative Humidity
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| - We have developed a molecular dynamics scheme in order to understand the dynamics of water adsorptionon calcite surfaces as a function of relative humidity. In contrast to previous studies where either a monolayeror bulk water was assumed to cover the surface, we observe the formation of two to three prominent layersof 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 resultsagree well with recent grazing incidence X-ray diffraction studies. The free energy of adsorption of a singlewater molecule onto the bare calcite (101̄4) surface is predicted to be −10.6 kcal/mol at room temperaturewhile the enthalpy for the same process is −21.3 kcal/mol. The time scale for the bare calcite surface tobecome in dynamic equilibrium with water vapor at 100% relative humidity is determined to be close to 6ns, 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 modesfor 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 increasingrelative humidity.
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