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

Abstract

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, Hw (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 0.38semi-cell to 0.69semi-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-Hw, Ca-Ow (oxygen in water molecules), and Ca-Ot (the oxygen in the tetrahedron) show that the Ca2+ and Ow are more tightly packed together than that of Hw; 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 Ot-Hw show that with the increase of charge density, the number of hydrogen bonds formed by Ot and Hw 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.

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

“…Therefore, the relative concentration of H 2 O decreases with the increase of layer charge. This conclusion fits well with our previous study [29].…”

“…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. 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 also drew a similar conclusion [30]. Wungu et al compared the effect of isomorphic substitution or a non-isomorphic substitution on the geometric structure and electronic properties of MT using DFT calculations.…”

“…According to the research purposes and crystal structure of natural Na-MT [29], the established semi-cell layer charge of Na-MT was determined as 0.38, 0.50, 0.58 and 0.70 in this paper. The corresponding numbers of Na + which balance the negative potential of Na-MT layers is 6, 8, 9 and 11.…”

“…The result indicates that H 2 O are more tightly packed together near the tetrahedral surfaces in the interlayer domain of Na-MT. The reason is that with the increase of the layer charge density of Na-MT, the polarity of Na-MT surfaces increases, therefore, H 2 O are more likely attracted by stronger hydrogen bonding force and eventually form hydrogen bonds with O t at the distance of 1.676 ± 0.043 Å away from tetrahedral surfaces (Figure 8, Na-MT4 0-11 is shown) [29,30]. Na + decreases.…”

Microscopic Distribution of Chemical Constituents in the Interlayer Space of OTAC Intercalated Montmorillonite Complex:Molecular Simulation Study

“…According to the simulation results, we can further prove the rationality of conclusions and predict the experimental results. [2,21,22] Up to now, molecular simulation methods have been successfully used in the study of MMT and achieved many amazing research results, [14,23,24] such as the distribution and hydration characteristics of inorganic cation in MMT interlayers, [25][26][27][28] the distribution and kinetic characteristics of various kinds of atoms in organic modifiers and the arrangement of the organic modifiers in MMT interlayers. [29][30][31][32] At present, the research on organic MMT by molecular simulation mainly focuses on the effect of carbon chain length and the amount of modifier on the structure of organic MMT and the arrangement of organic modifier in MMT interlayers.…”