Molecular Dynamics Study of the Effect of Layer Charge and Interlayer Cations on Swelling of Mixed-Layer Chlorite-Montmorillonite Clays

M Rahromostaqim and M Sahimi, JOURNAL OF PHYSICAL CHEMISTRY C, 124, 2553-2561 (2020).

DOI: 10.1021/acs.jpcc.9b10919

Understanding the properties of clay minerals is crucial to various phenomena in geophysics and environmental science and engineering. Although about 70 percent of all clays are of mixed-layer (ML) types, such as illite-montmorillonite (I-MMT) and chlorite-montmorillonite (CH- MMT) clays, the vast majority of the previous experimental and simulation studies were focused on pure clays. This paper reports on a study of important properties of ML clays by molecular dynamics simulations. One goal of the study is to understand the differences between the behavior of CH-MMT and pure chlorite and MMT. Another goal is to understand the effect of cations on the swelling behavior of the ML clays, for which we use Na+, K+, and Cs+. In both CH-CH and CH-MMT, the strong octahedral substitution of the chlorite layer results in, respectively, increasing both polarization and adsorption of water near, and onto the clay surface. The latter reduces hydration of the interlayer cations and, consequently, swelling of the CH-CH and CH-MMT clays, a conclusion that is supported by the computed density profiles and the radial distribution functions. Compared with the octahedral substitutions, the interlayer cation concentration and tetrahedral substitutions are shown to have a substantially weaker effect on swelling, whose pattern is also a function of the type of the interlayer cation. We find that the differences in the size and hydration energy of the cations have strong implications for the distribution of the interlayer species and, thus, their swelling, and that the higher the hydration energy and the smaller the atomic radius of the cation, the more swelling of the clay interlayers occurs. We also show that the layer substitution, the location, density, type of interlayer cations, and asymmetry of the ML clays play important roles in swelling of ML clays.

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