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

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 transition was further characterized using pressure versus layer-spacing isotherms. The isotherms showed a loop structure with peaks that correlated with the coordination number of the interlayer ion. Integration of the Sr−clay isotherms yielded Gibbs energy curves with minima at both one-layer and two-layer spacings. The results indicate that the mechanism of swelling and hydration depends upon the interlayer ion charge. For monovalent ions studied, swelling and hydration were coincidental, with water occupying all available interlayer volume. In contrast, expansion of Sr−montmorillonite, driven by formation of a first and partial second hydration shell for Sr2+, occurred initially without the filling of the interlayer volume. Trapping in metastable expanded states was also observed for Sr−montmorillonite and identified as a possible mechanism for layer-spacing hysteresis.