Abstract
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 Å. 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 Å, 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 Å. The research results can provide a theoretical basis for the efficient application of Na-MT at the molecular and atomic levels.
Highlights
Montmorillonite (MT) is a typical layered aluminosilicate clay mineral and is abundant in mud stone and shale rock [1]
We found that the polarity of Ca-MT sheets was strengthened when the charge density increased, so the interlayer particles were more likely to be drawn to tetrahedral surfaces [29], Shen et al drew a similar conclusion [30]
When octahedral substitution sites increase in Na-MT with high layer charge, the hydrogen bonding force between both sides of Na-MT sheets and HW (H in H2 O) are weakened because of the long-range spatial effect, which results in H2 O no longer being trapped in the central axis of the interlayer space [43]
Summary
Montmorillonite (MT) is a typical layered aluminosilicate clay mineral and is abundant in mud stone and shale rock [1]. Regarding the impact of crystal structure on the hydration properties of MT, Qiu et al studied the effect of charge density on the hydration properties of Ca-MT by combining molecular dynamics simulation and experimental methods. It can be seen from above that the related factors (such as the types of interlayer cations, environmental conditions and crystal structure) affecting the hydration properties of MT have been studied by molecular simulation. A molecular dynamics simulation was performed to study the influence mechanism of layer charge density and isomorphic substitution sites on the spatial distribution of H2 O and Na+ in the interlayer domain of Na–MT. Na–MT with different crystal chemical properties at the atomic and molecular levels
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