Effects of layer-charge distribution on swelling behavior of mixed-layer illite-montmorillonite clays: A molecular dynamics simulation study

One of the intriguing properties of clay minerals is swelling, which is considered an essential characteristic for various geological processes, environmental science, and engineering. Almost all previous experimental and simulation studies focused on the understanding of the swelling behavior of pure clays. The fact is that about 30 percent of all clays are a pure type, and virtually 70 percent of clays are the mixed-layer one that swelling mechanism is still elusive. In this research, molecular dynamics simulations were used to exclusively investigate the effect of layer charge distribution on the crystalline swelling and hydration behavior of the most common types of mixed clays, so-called illite-montmorillonite mixed-layer clay (I-Mt MLC) with Na+ and K+ counterions in the presence of different water concentrations. Based on the results, we found that the layer charge density and charge location of both illite and Mt significantly influenced the I-Mt clays' swelling behavior. Regardless of Mt charge density, the presence of highly charged illite led to powerful interactions of counterions with the surface sheet, resulting in less swelling behavior of the I-Mt system. Also, the various polarization power of clay surface due to the difference in charge distribution of octahedral sheets had a significant influence on water molecules and interlayer cations interaction with clay surface, which affected the swelling behavior of I-Mt clays. Overall, our findings demonstrated the new insight into I-Mt clays' swelling behavior, which is a function of charge distribution of both illite and Mt.