The adsorption of soil water significantly affects the physical and mechanical properties of soil. This process mainly occurs at the clay mineral-water interface, and different functional groups on the surfaces of clay minerals have certain influences on the water adsorption behavior. Therefore, studying the adsorption behavior of water on clay mineral surfaces is crucial for understanding the clay-water interaction mechanism.

Dong Yi, a research professor at the Institute of Rock and Soil Mechanics, has long been dedicated to research on the mechanics and engineering of multiphase geomaterials, including the microscopic interaction mechanisms of clay minerals and adsorption in unsaturated soils. Focusing on the topic of water adsorption behavior on the external surfaces of kaolinite, he employed molecular dynamics and metadynamics methods, based on a proposed thermodynamic framework, to study properties such as matric potential energy, adsorption structure, and adsorption sequence during water adsorption. The results show that, owing to protonation and deprotonation processes, the edge surfaces of kaolinite exhibit stronger adsorption characteristics than the basal surfaces (the lowest matric potential energy of the edge surfaces can reach ?1.86 GPa, lower than ?0.92 GPa of the basal surfaces). Adsorbed water mainly forms two structures, H-up and H-down, and this difference is mainly caused by hydrogen bonding on the mineral surfaces. Water molecules preferentially adsorb on the edge surfaces, followed by the basal surfaces. Combined with the experimentally measured adsorption isotherm of Georgia kaolinite and analyzed using a modified BET model, the adsorption isotherm of Georgia kaolinite was decomposed into edge-surface adsorption and basal-surface adsorption, verifying this adsorption sequence. These results provide a microscopic mechanistic explanation of the adsorption process for research on the water retention and mechanical properties of clay minerals.

The above research was funded by the National Natural Science Foundation of China under grants NSFC-41702338, NSFC-51779254, and NSFC-42277178, and the related results were published in the paper "Water adsorption on kaolinite basal and edge surfaces" in the internationally authoritative chemistry journal Langmuir.

Paper link: https://doi.org/10.1021/acs.langmuir.2c03282

Fig. 1 Adsorption sequence of water on the external surfaces of kaolinite and microscopic SEM scanning  

Fig. 2 Calculated results of the lowest matric potential energy for adsorption on the edge and basal surfaces of Georgia kaolinite

Fig. 3 Analysis of the adsorption isotherm of Georgia kaolinite and molecular dynamics simulation verification of the lowest potential energy of the basal and edge surfaces