Cation and water structure, dynamics and energetics in smectite clays: A molecular dynamics study of Ca-hectorite.
The incorporation of Ca(2+) into smectite minerals is well-known to have a significant effect on the swelling behavior and mechanical properties of this environmentally and technologically important group of materials. Relative to common alkali cations such as Na+, K+, and Cs+, Ca(2+) has a larger charge/ionic radius ratio and thus interacts very differently with interlayer water molecules and the oxygens of the clay basal surface. Recent (2)H and (43)Ca NMR studies of the smectite mineral, hectorite, show that the molecular scale interlayer dynamics is quite different with Ca(2+) than with alkali cations. Classical molecular dynamics (MD) simulations presented here use a newly developed hectorite model with a disordered distribution of Li+/Mg(2+) substitutions in the octahedral sheet and provide new insight into the origin of the effects of Ca(2+) on the structure, dynamics, and energetics of smectite interlayers. The computed basal spacings and thermodynamic properties suggest the potential for formation of stable monolayer hydrates that have partial and complete water contents, a bilayer hydrate, and possible expansion to higher hydration states. The system hydration energies are comparable to those previously calculated for Caâ€“montmorillonite and are more negative than for Cs+ and N+ hectorite due to the higher hydration energy of Ca(2+). The coordination environments of Ca(2+) change significantly with increasing interlayer hydration, with the extent of coordination to basal oxygens decreasing as the number of interlayer molecules increases. On external (001) surfaces, the H2O molecules closest to the surface are adsorbed at the centers of ditrigonal cavities and bridge Ca(2+) to the surface. The Ca(2+) ions on the external surface are all in outer-sphere coordination with the basal oxygens of the surface, and the proximity-restricted region with a significant number of Ca(2+) is approximately 6 Å thick. Quantification of these interactions provides a basis for understanding intercalation of Ca(2+) by organic species and smectite minerals.