Interlayer Cation Arrangement and Hydration Shells in Sodium and Calcium Montmorillonite obtained by Monte Carlo Simulations
Creators: name: Müller, Christian Jörg
name: Delville, Alfred
name: Kahr, Günter
name: Plötze, Michael
Publisher: name: Zenodo
Year: 2004

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

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