Calcium hydration on montmorillonite clay surfaces studied by Monte Carlo simulation

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Abstract
Monte Carlo computer simulations were used to investigate the interlayer structure of Ca-montmorillonite for hydration levels ranging from the dry clay to a three-layer system. Our model of montmorillonite consists of negative charge sites in both the octahedral and tetrahedral layers. Our results are compared with X-ray diffraction data from Ca-clays, which have suggested an interlayer structure similar to Mg-clays. However, the Ca 2+ co-ordination number in these systems and in bulk liquid is a subject of ongoing debate. Our results support an eightfold water solvation shell for calcium, even within the confines of a clay pore. The Ca 2+ co-ordination number decreased at lower water content, as the water layer became constrained and less like bulk liquid. Additionally, Ca 2+ solvation shells included surface oxygen atoms at low water content, although this may be a result of the Ca-O interaction potentials.