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INTERLAYER CATION ARRANGEMENT AND HYDRATION SHELLS IN SODIUM AND CALCIUM MONTMORILLONITE OBTAINED BY MONTE CARLO SIMULATIONS
Today the interlayer structure of montmorillonite is simulated by computer experiments (e.g. Monte Carlo, MC). Early MC simulations focussed on the 1D-distribution of interlayer species perpendicular to the 2:1 layers and reproduced the well- established layer-wise arrangement of water molecules. Recently, more attention is drawn to the precise arrangement of interlayer species in 3D. However, no detailed picture of the interlayer structure has been proposed until now.
In the present study, we placed strong emphasis on position, orientation and geometry of cationic hydration shells, but also on structure of water layers and distribution of cations on the 2:1 layer surface. We performed MC simulations of a montmorillonite at two different layer spacings (sodium one- and two-layer hydrate and calcium two- layer hydrate). The relative arrangement of the 2:1 layers was equally chosen for all three, i.e. it was independent of the degree of expansion.
In the sodium one-layer hydrate, cations have a quadratic planar hydration shell. The diagonals of the square are rotated (~15°) with respect to the crystallographic a,b axes. This is deduced from paired hydrated cations. In the sodium two-layer hydrate, some cations stay coordinated to the 2:1 layer, their hydration shell is extended to a tetragonal pyramid. The rotation angle is preserved. Sodium ions detached from the 2:1 layer are octahedrally coordinated with six water molecules. The octahedron is lying parallel to the 2:1 layers with two of its triangular sides. All the extracted bond lengths compare well with literature data.
In the calcium two-layer hydrate, the cations are coordinated to the 2:1 layer and are found in/over the pseudo-hexagonal structural element of the 2:1 layer. In many examined cases, the first hydration shell is a pentagonal pyramid with its base parallel to the 2:1 layer. This geometry is in good agreement with recent simulation data on hydrated calcium in solution (pentagonal bipyramidal coordination).
All three simulations give clear indications of a superstructure caused by the ordered distribution of interlayer cations. This is evidenced from the cation-cation radial distribution functions.
Authors:Müller, Christian and Delville, Alfred and Kahr, Günter and Plötze, Michael
Index Terms:quantitative clay analysis; Monte Carlo simulation; montmorillonite; superstructure; hydration shell; Clay; ClayGroup
Further Information:Date published: 21.06.2004