Insight into the Hydration Mechanism of Illite Surfaces: A DFT Study

Abstract

Water-sensitive clay minerals are the key factors influencing the physical characteristics of mud shale reservoirs. The nature of macroscopic responses such as softening and loosening, crushing and disintegration, and deterioration of bearing capacity of mud shale in geotechnical engineering and underground rock engineering under water-rich environments is a reflection of the intrinsic microscopic clay mineral unit cell–water interactions. In this paper, the periodic density functional theory (DFT) is applied to systematically study the reactive sites, adsorption energy, charge transfer, and bonding characteristics of the water molecule on the common exposed surfaces (001) and ($00\overline{1}$) of illite. The results indicated that the most reactive adsorption sites for the water molecule on the illite (001) surface were the K⁺ ions and the O atoms adjacent to the lattice substitution ion Al³⁺. On the illite ($00\overline{1}$) surface, the reactive sites were the O atoms of the surface silicon-oxygen rings. The water molecule can be stably adsorbed on both the illite (001) surface and ($00\overline{1}$) surface, with the water molecule tending to be preferentially adsorbed on the illite (001) surface. The adsorption mechanisms of the water molecule on different illite surfaces show slight differences. The adsorption of the water molecule on the illite (001) surface is driven by both hydrogen bond and electrostatic attraction, whereas water molecule adsorption occurs on the illite ($00\overline{1}$) surface only through hydrogen bond. In the process of water molecule adsorption on both the illite (001) and ($00\overline{1}$) surfaces, there is charge transfer as well as interatomic bonding.