First-principles study on the effect of pressure on the adsorption of H2O on the Mg-montmorillonite (010) surface

Abstract

The effect of pressure on H2O adsorption on the montmorillonite surface is a crucial issue in geomechanics and high-pressure physics. In this paper, the effect of pressure on adsorption capability were calculated based on density-functional theory (DFT) for pressures ranging from 0 to 18 GPa, and the adsorption energies and structures of the H2O molecules were analyzed with different surface coverages (0 < Θ ≤ 1.0 ML). Firstly, the clean (010) surface of Mg-montmorillonite was modelled at different pressures from an atomic perspective, then the calculations demonstrated that single H2O molecules were stably adsorbed on the top, bridge, hollow and interlayer bridge sites, independent of pressure. However, the adsorption energies at different sites were significantly influenced by the pressure and showed different trends. For the hollow and bridge sites, the adsorption energy increased within the pressure range 0–2 GPa but decreased within the pressure range 2–18 GPa. Conversely, for the top sites, the adsorption energy decreased with increasing pressure within the range 0–2 GPa but increased with increasing pressure within the range 2–18 GPa. For increasing coverage with H2O molecules, the adsorption energy was almost constant at 0 GPa but decreased within the range 0 < Θ ≤ 1.0 ML at 2, 10, and 18 GPa. Furthermore, the adsorption energies for the bridge sites all increased with increasing coverage but decreased with increasing coverage for the interlayer bridge sites at the four pressures. In addition, other characteristics of the H2O and Mg-montmorillonite (010) surface were discussed, including bonding, changes in the electronic density of states, the charge distribution, and the structure of the atomic interlayer at different pressures.