Early stage of swelling process of dehydrated montmorillonite through molecular dynamics simulation

The early stage of the swelling process of dehydrated montmorillonite (MMT) is crucial to understand the mechanism of swelling. In this study, molecular dynamics simulations and analyses are presented to examine the conditions under which water molecules adsorb into the MMT interlayer in the early stages. Dehydrated (no interlayer water molecule - 0 W) and slightly hydrated (one and two water molecules per unit cell -1 W and 2 W) models of montmorillonite in a free water box are first built out with three different compensated cations (Ca²⁺, K⁺, or Li⁺). Simulations are then carried out at 298 K and 324 K using the CLAYFF force field. All results show that no water molecules can be adsorbed into dehydrated MMT interlayers at those temperatures. The MMT swelling process starts with existing initial interlayer water molecules, which allows more water molecules to adsorb into the interlayer to cause MMT swelling. The size and the valence of the compensated cations have a significant effect on the swelling behavior of MMT, and the Ca-MMT has a stronger swelling ability than K-MMT and Li-MMT. However, the swelling capacity of Ca-MMT is inhibited in saltwater compared to that in pure water.