Cation and Water Structure, Dynamics, and Energetics in Smectite Clays: A Molecular Dynamics Study of Ca-Hectorite
N Loganathan and AO Yazaydin and GM Bowers and AG Kalinichev and RJ Kirkpatrick, JOURNAL OF PHYSICAL CHEMISTRY C, 120, 12429-12439 (2016).
The incorporation of Ca2+ into smectite minerals is well-known to have a significant effect on the welling behavior and mechanical properties of this environmentally and; technologically important group of materials. Relative to common alkali cations such as Na+, and Cs+, Ca2+ 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 H-2 and Ca-43 NMR studies of the smectite mineral, hectorite, show that the molecular scale interlayer dynamics is quite different with Ca2+ than with alkali cations. Classical molecular dynamics (MD) simulations presented here use a newly developed hectorite model with a disordered, distribution of Li+/Mg2+ substitutions in the octahedral sheet and provide new insight into the origin of the effects of Ca2+ 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 Ca- montmorillonite and are more negative than for Cs- and Na-hectorite due to the higher hydration energy of Ca2+. The coordination environments of Ca2+ 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 Ca2+ to the surface. The Ca2+ 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 Ca2+ is approximately 6 angstrom thick. Quantification of these interactions provides a basis for understanding intercalation of Ca2+ by organic species and smectite minerals.
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