Effects of the Layer Charge Location and Interlayer Cation on Rectorite Swelling Properties: Comparison between Molecular Dynamics Simulations and Experiments

Understanding the swelling properties of clay minerals is of great significance to environmental science and engineering applications, including environmental remediation, oil and gas wellbore collapse, geological disasters, and commercial additives. The vast majority of experimental and molecular simulation studies have mainly focused on pure clays. However, 70% of clays are mixed-layer clays (MLCs), and the most frequent MLCs is illite-montmorillonite (I-MMT). In this research, grand canonical Monte Carlo and molecular dynamics simulations were used to study the effects of the layer charge location (tetrahedral and octahedral substitution), interlayer cation type (Na+, Cs+, and Ca2+), and water contents on the swelling and hydration properties of rectorite (R1 type I-MMT) and compared with the experimentally measured basal spacing by water vapor adsorption and X-ray diffraction. It turned out that the free swelling ratio was positively correlated with water contents. The order of the three cations' free expansion rate was Na+, Ca2+, and Cs+. The swelling process of rectorite (REC) was revealed from porosity changing with water contents. The surface of tetrahedral substitution (T-substitution) clay was more likely to bind to interlayer cations to form an inner-sphere surface complex (ISSC), the number of fully hydrated cations of the T-substitution clay was less than that of octahedral substitution (O-substitution), and REC was more swelling in O-substitution, which was supported by the basal spacing, atomic density distribution, and radial distribution function. The cation radius, the hydration energy, and the preference of the inner/outer-sphere surface complex (ISSC/OSSC) all affected REC swelling. Because of the presence of asymmetric charges in the REC layer, interlayer cations tended to distribute on the surface of illite and formed an ISSC when they formed 1-, 2-, and 3-layer (1, 2, and 3 W) hydrate states. When the layer charge density of illite and Mmt was constant, only the layer charge position in Mmt was changed, and the REC swelling behavior was transformed into pure Mmt. The interlayer cations and the layer charge location significantly impacted the REC swelling.