Effect of Layer Charge Density on Hydration Properties of Montmorillonite: Molecular Dynamics Simulation and...

Four kinds of Ca-montmorillonite with different layer charge density were used to study the effect of charge density on their hydration properties by molecular dynamics simulation and experiments. The research results of Z-density distribution of water molecules, H_w (hydrogen in water molecules), and Ca in the interlayer of montmorillonite show that the hydration properties of montmorillonite are closely related to its layer charge density. If the charge density is low, the water molecules in the interlayers are mainly concentrated on the sides of the central axis about -1.3 Å and 1.5 Å. As the charge density increases from 0.38_semi-cell to 0.69_semi-cell, the water molecules are distributed -2.5 Å and 2.4 Å away from the siloxane surface (Si-O), the concentration of water molecules near the central axis decreases, and at the same time, Ca²⁺ appears to gradually shift from the vicinity of the central axis to the Si-O surface on both sides in the montmorillonite layer. The simulation results of the radial distribution function (RDF) of the Ca-H_w, Ca-O_w (oxygen in water molecules), and Ca-O_t (the oxygen in the tetrahedron) show that the Ca²⁺ and O_w are more tightly packed together than that of H_w; with the increase of the charge density, due to the fact that the negative charge sites on the Si-O surface increase, under the action of electrostatic attraction, some of the Ca²⁺ are pulled towards the Si-O surface, which is more obvious when the layer charge density of the montmorillonite is higher. The results of the RDF of the O_t-H_w show that with the increase of charge density, the number of hydrogen bonds formed by O_t and H_w in the interlayers increase, and under the action of hydrogen bonding force, the water molecules near the central axis are pulled towards the two sides of Si-O surface. As a result, the arrangement of water molecules is more compact, and the structure is obvious. Correspondingly, the self-diffusion coefficient shows that the higher the layer charge density, the lower the self-diffusion coefficient of water molecules in interlayers is and the worse the hydration performance of montmorillonite. The experimental results of the experiments fit well with the above simulation results.