The interlayer configuration

The interlayer configuration in two-layer hydrate of Mg-beidelite [74] is consistent with results of previous simulations of Mg-smectites [66, 67], The Mg2+ cations are situated at the midplane solvated by three water molecules each above and below in agreement with a number of experimental studies [15, 34], Nonsolvating water molecules form H-bonds with surface O atoms of beidelite. However, water molecules in Mg-beidelite show a greater tendency to occupy ditrigonal cavities in the siloxane surface than in the case of Mg-montmorillonite. 

Figure 1. Plan view of the interlayer configuration in Li(H20)3-beidellite, with the MC simulation cell outlined.

In the above configurations for interlayering and interlayer ion distribution, only type (c) represents an assemblage of two phases. The other forms are solid solutions. 

In conclusion, definite information on the density of adsorbed interlayer water cannot be derived from the bulk density measurements. However, such measurements supplement adsorption isotherm data, and both will be helpful in the interpretation of x-ray structure analysis of the configuration of water molecules and exchange cations in the interlayer space. 

Figure 7.48 (a) The 4.6-net sheet in [d-Co(en)3][AI3P4Oj6] 3H20. The [3.3.3] propellane-like chiral motif (A configuration) is also shown, (b) The Co(en)33+ cation with A configuration occluded in the interlayer region. Reproduced with permission from [91]. Copyright (2003) Wiley-VCH... 

Tildesley, 1990). Next, structural relaxation is performed based on MC trial moves of all the atoms in the supercell and acceptance or rejection of the generated configurations according to the Metropolis criterion. This hydrogenation of the a-Si layers results in a layered a-Si H/c-Si superstructure with structurally stable interfaces. The simulation cell is then cut in half and the periodic boundary conditions (PBCs) are removed in the direction normal to the plane of the a-Si H/c-Si interface. Additional c-Si atomic layers are placed at the appropriate interlayer distance at the bottom of the resulting simulation cell and are held fixed to simulate contact with an infinite rigid substrate. MC equilibration is used to distribute the hydrogen in the presence of the free surface. Finally, the surface is relaxed further by MD at the temperature of interest. 

Another interesting aspect of the simulation results concerns the location of the counterions in the interlayer space. The (001) perspective of simulated Cs+-mont-morillonite (Fig. 8- 4) shows that, at a water content [ 150 g H20 kg-1] approximately corresponding to a monolayer, the cations always adopted locations such that they were aligned with the center of a siloxane ditrigonal cavity on the basal surface of one clay layer and with the base of an SiC>4 tetrahedron on the basal surface of the opposing clay layer. At lower water contents this configuration was not stable, and the interlayer counterions tended to align themselves with the centers of siloxane cavities on the basal surfaces of both clay layers. 

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