Effect of Crystal Chemistry Properties on the Distribution Characteristics of H2O and Na+ in Na-Montmorillonite Interlayer Space: Molecular Dynamics Simulation Study

At monolayer hydration state, the spatial distribution of H2O and Na+ in the interlayer of Na-montmorillonite (Na-MT) with different crystal chemistry properties was investigated by the molecular dynamics simulation method. The simulation results show that when layer charge density increases, H2O will move and form hydrogen bonds with O in tetrahedral surfaces (Ot) at a distance of 1.676 ± 0.043 A. The impact of isomorphic substitution on the relative concentration of H2O depends largely on the layer charge density of Na-MT, when layer charge density is high, H2O move obviously to both sides of Na-MT sheets with the increase of octahedral substitution ratio. Nevertheless, Na+ coordinate with Ot at a distance of 2.38 A, and the effect of isomorphic substitution ratio on the diffusion of Na+ is opposite to that of H2O. The mobility of both H2O and Na+ decreases with the increase of layer charge density or tetrahedral substitution ratio. The radial distribution function of Na-Ow (O in H2O) shows that the coordination strength between Na+ and Ow decreases with the increase of layer charge density or tetrahedral substitution ratio, and Na+ are hydrated by four H2O at a Van der Waals radius of 2.386 ± 0.004 A. The research results can provide a theoretical basis for the efficient application of Na-MT at the molecular and atomic levels.