First-principles calculations investigation on different coverage of H2O adsorption on the Mg-montmorillonite (0 1 0) edge surface

Abstract

Clay minerals are often cause of deformation in soft rock tunnel engineering, leading to safety problems. The deformation mechanism is closely related to the interaction between clay minerals and water molecules. Mg-montmorillonite (Mg-MMT) with good adsorbability is one of the most abundant components in clay minerals. In the present paper, the density functional theory was employed to clarify the adsorption mechanism of H2O on the Mg-MMT (010) surface at the molecular level. The adsorption structures and energies of H2O molecules on the Mg-MMT (010) surface were systematically studied for a wide coverage from 0 to 1.0 monolayer (ML). The results showed that the interlayer-bridge site was the most favorable for water molecules adsorption, and followed by the hollow, bridge, and top adsorption sites. When the coverage was 0.5 ML, the highest adsorption energy for above adsorption sites was 0.71, 0.62, 0.61, and 0.44 eV, respectively. The adsorption energy increased with increasing coverage for water molecules in range of 0 < Θ ≤ 0.5 ML, while decreased with the coverage range of 0.5 < Θ ≤ 1.0 ML. The above results implied that water molecules could be easily adsorbed on the Mg-MMT (010) surface. Besides, other properties of the H2O/Mg-MMT (010) surface system, including the atomic structures, bonding analyses, density of electronic states and charge distribution were also studied in detail for different coverages.