A theoretical investigation of water adsorption on the compressed (001) surface of kaolinite under pressure

Abstract

The interaction of kaolinite and water has been extensively investigated by scholars in geomechanics and high-pressure physics fields. As well known, the effects of pressure on the adsorption capacity of clay mineral cannot be ignored. In the present paper, the adsorption behaviors of water molecules with different coverage (0 ≤ Θ ≤ 1 ML) on the kaolinite (001) surface under the pressure range of 0–10 GPa were examined using density functional theory (DFT). The atomic structure of kaolinite has undergone phase transitions under high pressure. The transition pressures of kaolinite occurred at 4.93 GPa (kaolinite-I → kaolinite-II) and 8.23 GPa (kaolinite-II → kaolinite-III), respectively. The results revealed that the hollow sites of kaolinite-Ⅰ, Ⅱ, and Ⅲ (001) surface were the most stable adsorption sites for water molecules at different coverages. The order of adsorption capability for three phases kaolinite was kaolinite-Ⅰ > kaolinite-Ⅱ > kaolinite-Ⅲ. In the pressure range of 0∼10 GPa, the adsorption energies of water molecules on the three phases of kaolinite (001) surfaces increased with increasing coverage (0 ≤ Θ < 1/2 ML) and then decreased with increasing coverage (1/2 < Θ ≤ 1 ML). For three phases kaolinite, the adsorption energies decreased with increasing pressure at 1/16 and 1/4 ML, while the changes of adsorption energies at other coverages with increasing pressure didn't exhibit obvious consistent regularity. Further exploration was performed on the structure and electronic properties change within the adsorption process by studying the lattice relaxation, and electronic density of states (DOS) of the adsorption system of (001) surface and water molecules.