Hydration-Induced Transition from Site-Controlled to Diffusion-Controlled Ion Transport in Montmorillonite Interlayers

Ion transport in clay minerals is governed by hydration (interlayer water content), yet the underlying molecular mechanisms remain insufficiently understood. Specifically, it is unclear whether water simply reduces migration barriers or fundamentally alters the nature of transport. Here, we combine density functional theory (DFT) with two-dimensional potential energy surface (PES) mapping to investigate the mobility of Na$^+$ and K$^+$ in montmorillonite interlayers as a function of water content. In water-free conditions, the energy landscape is strongly corrugated, exhibiting pronounced minima and large migration barriers (∼several eV) that localize ions at specific adsorption sites, defining a site-controlled regime. Upon hydration, migration barriers decrease nonlinearly, and the energy landscape progressively flattens. At sufficiently high water content, well-defined adsorption sites disappear, signaling a transition to a diffusion-controlled regime. We further demonstrate that the topology of the tetrahedral framework acts as a physical sieve, proving as decisive for ion localization as the electrostatic charge distribution. While Na$^+$ mobility remains sensitive to the local structural vacancy arrangement, the larger K$^+$ ion experiences a spatially averaged potential due to steric restrictions that decouple its transport from local lattice heterogeneity. These findings reveal that hydration does not merely enhance mobility but fundamentally alters the topology of the energy landscape, acting as a molecular “switch” between localized and delocalized ion behavior in confined nanoporous materials.