Molecular Dynamics Simulation of Interlayer Structure and Mobility in Hydrated Na‐, Cs‐, and Ca‐Montmorillonite

Classical molecular dynamics (MD) simulations were performed to systematically investigate the swelling behavior, hydration mechanisms, and mobility of interlayer species in Na‐, Cs‐, and Ca‐montmorillonite (MMT) over a wide range of water contents. The results show that hydrated MMT exhibits an initial stepwise crystalline swelling, followed by an approximately linear osmotic swelling phase. Thermodynamic analyses demonstrate that Cs + ions effectively inhibit MMT swelling—advantageous for nuclear waste disposal—while Na + and Ca 2+ promote swelling to varying extents, with Ca 2+ having the most pronounced effect. Structural analyses reveal that interlayer water molecules preferentially form hydrogen bonds with adjacent bridging oxygen atoms; furthermore, the average hydrogen bond number in the interlayer exceeds that of bulk water, indicating stronger aggregation. At thermodynamic equilibrium, the system attains a stable state characterized by local maximum density and minimal atomic free volume, with the density of interlayer water slightly exceeding that of bulk water. The self‐diffusion coefficients of interlayer species are significantly reduced compared to aqueous solution due to confinement by the clay surfaces. Among the investigated cations, the sequence of diffusion coefficients is Cs + > Na + > Ca 2+ , reflecting differences in their hydration shell structures. These findings provide new atomistic insights into the coupled swelling, hydration, and transport phenomena in clay barriers relevant to nuclear waste disposal and geotechnical engineering.