Anomalous Diffusion of Water and Ions in Swelling Clays

Molecular transport in swelling-clay interlayers is commonly described by a single long-time diffusion coefficient, implicitly assuming Fickian dynamics. Here, molecular dynamics simulations extending to ∼300 ns are used to characterize water and Na+ transport in sodium montmorillonite (Na-MMT), from near-collapsed (<1W) to free-swelling (4W) conditions. Mean-squared displacements are combined with the non-Gaussian parameter, self-intermediate scattering function, and van Hove self-correlation function to resolve the evolution from heterogeneity-dominated to Fickian dynamics. Both water and Na+ exhibit pronounced subdiffusion under strong confinement, with the strongest anomalous behavior around the one-water-layer (1W) state. However, subdiffusion is predominantly transient: a finite crossover to Fickian transport is resolved from 1W outward, while only the near-collapsed state remains persistently caged over the accessible simulation window. Confinement acts primarily by extending the lifetime of dynamical heterogeneity rather than by establishing a distinct asymptotic transport law. Residence-time analysis links this slowing to increasingly persistent local adsorption and hydration environments: Na+ remains trapped within lateral adsorption basins under strong confinement, while the large variation of its crossover time below and through 1W occurs without inner-/outer-sphere exchange. The crossover time depends strongly on pore size and species, reaching 0.1–1 ns at 1W, whereas Fickian behavior is recovered within tens of picoseconds at larger spacings. Consequently, trajectories typical of previous simulations may probe only the transient regime and yield apparently anomalous transport even when the long-time dynamics are Fickian. Comparison with persistent subdiffusion in other layered adsorbing materials shows that subdiffusive scaling under nanoconfinement does not imply a universal long-time transport mechanism; its interpretation requires resolving the underlying relaxation time scale.