Mechanism research on surface hydration of kaolinite, insights from DFT and MD simulations

Abstract

The surface hydration is one of the main reasons for the difficulty of coal slurry water sedimentation and dewatering, the Density functional theory (DFT) and Molecular dynamics (MD) simulations on the surface hydration of kaolinite were investigated in this study. The results indicate that the adsorption of single water molecule on kaolinite (0 0 1) surface and (001¯) surface mainly by forming hydrogen bonds, and the adsorption energies of single water molecule on different initial positions of kaolinite (0 0 1) surface are −72.12 to −19.23 kJ/moL, less than that of kaolinite (001¯) surface with −19.23 to −5.77 kJ/moL, which mean that the water molecule preferentially adsorption on kaolinite (0 0 1) surface. The binding force between kaolinite surfaces and water molecules decreased with the increase of water coverage rate (or the number of water molecules), the hydrogen-bond interaction on kaolinite/water interface gradually weakened, and the water molecules on hydrophilic kaolinite surfaces can be gradually formed 3 layers of water molecules. The hydrated film of kaolinite surface is mainly composed of about 3 layers of water molecules, and its thickness is about 8–10 Å. The hydration mechanism of kaolinite surface is mainly that the water molecules at kaolinite interface are adsorbed on surface with hydrogen bonds, and a hydrated film composed of multiple water molecules is gradually formed on kaolinite surface with the increase of water coverage rate.