Structure of bound water layer on montmorillonite surface: The role of trans- and cis- vacant sites

Bound water on the surface of clay minerals determines the nature of a wide class of phenomena, from the diffusion of interlayer cations to synthesizing prebiotic macromolecules during the early Earth. However, the nature of forming the bound water layer's spatial structure and properties has yet to be fully investigated. Many factors, including peculiarities of structural charge distribution, surface adsorption centers, and OH-groups of the octahedral sheet of the clay minerals, can affect it in natural clays. In this work the role of hydroxyl ligands and isomorphic substitutions of the octahedral sheet in trans- and cis- vacant sites in the formation of the bound water layer structure on the basal surface of montmorillonite was studied by the Density Functional Theory. It was found out that in the case of single isomorphic substitution in the octahedral sheet OH-groups in the cis-vacant sites provide higher adsorption energy of water molecules on the basal surface as compared to OH-groups in the trans-vacant sites. The double isomorphic substitution in the octahedral sheet creates a mutual enhancement effect due to the difference in the structural arrangement of the OH-groups. This leads to an increase in the interaction with adsorbed water molecules. The maximum increase in adsorption energy is observed for the case with isomorphic substitutions of Mg-cis and Fe-trans in the octahedral sheet. The results of the theoretical study contribute to explaining the experimental data available in the literature.