Molecular modeling of initiation of interlayer swelling in Na-montmorillonite expansive clay

Expansive soils are found in many parts of the world. A major constituent of these clays is the mineral montmorillonite. Molecular interactions between clay, cations, and water play an important role in swelling. Molecular models of Na-montmorillonite clay are constructed, and solvation studies are conducted using molecular dynamics. Analysis of molecular trajectories, conformations, and interaction energies reveal key mechanisms that initiate interlayer swelling. In the initially dry interlayer, the interlayer collapses due to strong attractive interactions between Na ions and clay sheets. The van der Waals (vdW) radii of the oxygen atoms further reduce available spacing, thus preventing water molecules from entering the interlayer. Further, it was found that clays, when slightly hydrated, allowed flow of water molecules into the interlayer. The speed of water molecules in the interlayer is found at various time intervals, and the speed decreases with time. The simulations elucidate that Na ions have strong attractive interactions with water molecules and are important initially for attracting water molecules into the interlayer and initiate swelling. Subsequently, after the formation of the hydration shell, flow of water molecules continues, potentially through the hydrogen bond network. These studies provide a clear insight into the molecular mechanisms at the beginning of swelling.