Medium-range ordering of interfacial water at charged clay interfaces: From layering to network connectivity

The microscopic structure of water at charged mineral-electrolyte interfaces governs a wide range of physicochemical properties yet remains elusive beyond molecular densities and orientations. Using molecular dynamics simulations, we combine conventional short-range descriptors with hierarchical clustering of the hydrogen-bond (H-bond) network to quantify medium-range ordering of interfacial water at montmorillonite-NaCl interfaces. This dual framework distinguishes the layered arrangement imposed by surface electrostatics from the cooperative connectivity that develops within this scaffold. Our results reveal that strong orientational constraints in the first hydration layer suppress the formation of extended clusters, favoring quasi-planar two-dimensional motifs, whereas beyond this constrained zone, cooperative structures partially recover as three-dimensional networks through water-water H-bonds and counterion coordination. By systematically varying electrolyte concentration and surface charge density, we identified a fixed spatial boundary at ∼7.8 Å from the surface based on the spatial heterogeneity of the medium-range H-bond network. Within this framework, interfacial water structure reflects the complementary but competing roles of the mineral surface and solvated counterions: layering imposed by surface electrostatics creates the scaffold, while ion distributions regulate where and how medium-range networks form or decay. The hierarchical clustering approach developed here offers a transferable means to probe cooperative water structures across diverse solid-liquid interfaces, with implications for geochemistry, electrochemistry, and biomolecular interactions.