Simulations of Clay Mineral Swelling and Hydration: Dependence upon Interlayer Ion Size and Charge

Abstract

The dependence of the crystalline swelling and hydration properties of clay minerals on interlayer ion size and charge was investigated using molecular dynamics computer simulations of Na-, Cs-, and Sr-substituted montmorillonites. For all clays studied, layer spacings measured as a function of water content exhibited plateaus at the one-layer hydrate spacing. Calculated immersion energies exhibited minima for integer-layer hydrates up through the three-layer hydrate, with apparent global minima identified with the one-layer hydrate for Cs−montmorillonite and with the two-layer hydrates for the Na− and Sr−montmorillonites. In addition, for Sr−montmorillonite, layer spacings jumped discontinuously between one-layer and two-layer separations and showed a second plateau at the two-layer hydrate spacing. The immersion energy curve for Sr−montmorillonite showed similar discontinuities. These results provide evidence for a constant water content swelling transition between one-layer and two-layer spacings. This...