Molecular dynamics simulation of Mg-montmorillonite (clay) hydrates was performed at T=300 K in order to elucidate the structure and dynamics properties of interlayer water molecules and magnesium ions. The water molecules are divided into two layers in the clay and only a few water molecules are absorbed on the surface of the clay with hydrogen bonds, and the water molecules at different distribution sites are in dynamic equilibrium. The calculations reveal that the confined water molecules are of 24% fewer H-bond coordination numbers per molecule than the molecules in the bulk liquid water. The simulated self-diffusion coefficient of the interlayer water, D, is 5.355×10^-10 m²·s^-1, which is about 1/4 of that for the bulk water molecules. It is found that the confined magnesium cations form one layer and the coordination number with water molecules is about 6. The effect of temperature on the structure and dynamic properties of the interlayer water molecules were investigated. The local densities of the water layers decrease, and the self diffusion coefficient of water in the X-Y direction increases with the increase of temperature. When the temperature reaches above 600 K, similar to the supercritical water, the hydrogen bonds between the interlayer water molecules are broken up, and diffusion coefficient reaches a maximum.

Key words: molecular dynamics simulation, montmorillonite, hydrate, self-diffusion