Monte Carlo Molecular Simulation of the Hydration of K-Montmorillonite at 353 K and 625 bar

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Monte Carlo molecular simulations of the hydration of K-saturated Wyoming-type montmorillonite at constant stress in the NPzzT ensemble and at constant chemical potential in the grand canonical muVT ensemble, under basin-like conditions of 353 K and 625 bar, show a strong tendency of the K+ ions to adhere to the siloxane surface, forming predominant inner-sphere complexes with tetrahedral oxygen atoms and adsorbed water molecules. Simulations in the grand canonical ensemble predict that none of the K-montmorillonite hydrates, the one-, two-, and three-layer hydrates, are stable in this environment of high depth, temperature, and pressure. The most nearly stable configuration corresponds to the one-layer hydrate, characterized by a d001 spacing of 12.75 A, the adsorbed water being 60 molecules/layer or 180.83 mg of H2O/g of clay, an internal energy of -22.73 kcal/mol, an interlayer density of 0.365 g/mL, and a pressure tensor, Pzz, of 1999.9 bar. The interlayer structure consists of two close layers of water molecules 0.50 A from the midplane, with broad shoulders on the sides, the protons oriented toward the midplane and the siloxane surfaces, and the K+ ions close to the clay surfaces and on the interlayer midplane.