Cation and Water Structure, Dynamics, and Energetics in Smectite Clays: A Molecular Dynamics Study of Ca–Hectorite

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• 作者：Narasimhan Loganathan ; A. Ozgur Yazaydin ; Geoffrey M. Bowers ; Andrey G. Kalinichev ; R. James Kirkpatrick
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：June 16, 2016
• 年：2016
• 卷：120
• 期：23
• 页码：12429-12439
• 全文大小：559K
• 年卷期：0
• ISSN：1932-7455

文摘

The incorporation of Ca²⁺ 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⁺, 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 ²H and ⁴³Ca 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 Å thick. Quantification of these interactions provides a basis for understanding intercalation of Ca²⁺ by organic species and smectite minerals.