Study of the influence of location of substitutions on the surface energy of dioctahedral smectites

The surface energy of some clays belonging to the smectite group has been calculated starting from crystal structures and combining a partial charge model with the computation of the lattice energy. The dioctahedral smectites studied here include montmorillonite; beidellites; and nontronite. One of the differences between these clays is the location of the substitution in the octahedral sheet or in the tetrahedral one. Another is the possibility of vacancies in cis- or trans-octahedral positions. These locations and vacancies have an effect on the distortion of the crystal framework and therefore on the surface energy. Calculated surface energies of the solid samples increase in the order beidellites > montmorillonite > nontronite. The bond energy between the interlayer cation and the layer appears to follow the same order and to depend both on the nature of the most electropositive elements of the layer and on their location. The trends obtained provide elements for an analysis of data related to interlayer enlargement.

In dioctahedral smectites, substitutions can occur in tetrahedral or octahedral positions and in cis- or trans-octahedral positions. A set of four idealized smectites has been built in order to represent the whole set of natural samples. The calculation of solid surface energy has been performed following a calculation, which uses the equalization of electronegativity method (partial charge calculation) and Ewald summation. The order of surface energy is as follows: cis-beidellite > cis–trans-beidellite > montmorillonite > nontronite. Two main reasons appear; the octahedral substitution increases the surface energy. Cis-substitutions have also an effect. However, Fe atoms decrease the surface energy.Figure optionsDownload as PowerPoint slide