Molecular dynamics simulation of the influence of doping on the water absorption characteristics of kaolinite

Abstract

The large amount of water adsorbed by clay minerals in argillaceous rocks upsets the balance of the microstructure, causing large deformation and failure of the rock surrounding deep mining roadways when they encounter water. The many impurities in natural clay minerals, and the electronegativity of the system caused by doping, greatly promote the adsorption of water. Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods were both used to simulate the water adsorption characteristics of kaolinite to study the effects of doping elements of different valency and the pressure on water molecules in kaolinite particles, and the extent to which these affect the adsorption conformation, relative concentration distribution, adsorptive capacity, adsorption site, the heat of adsorption and adsorption energy of the water molecules on the surface of kaolinite particles. The molecular mechanism of microscopic interactions between doped kaolinite and water molecules is illustrated. It was found that at a temperature of 25 °C and a pressure of 10 MPa, water molecules are adsorbed on the surface of kaolinite in two layers. The heat of adsorption is 1.899–2.128 kJ/mol, which is far below the critical value of 42 kJ/mol for chemical adsorption. The interaction between kaolinite and water molecules is typical of physical adsorption. The absorption energy is negative, and the system is thermodynamically stable after kaolinite absorbs water. The variation of adsorption energy values is similar to the variation pattern of absorptive capacity, and both increase logarithmically with increasing pressure; doping causes both to decrease. Doping enhances the interaction between the kaolinite surface and water molecules, promoting the adsorption of water molecules on the kaolinite surface. This work explores the adsorption behavior of water molecules on the doped kaolinite surface from a microscopic perspective, and provides a theoretical foundation for further research on the deterioration of engineering properties of kaolinite-like clay minerals in contact with water.