Ion-specific symmetry-breaking at nanoconfined water-hydrocarbon interfaces

We hypothesize that in nanoconfined biphasic fluids, ion-specific interactions can induce spontaneous interfacial symmetry breaking, even under geometrically and chemically symmetric confinement, by modulating the balance between surface-water, surface-hydrocarbon, and hydrocarbon-water interfacial tensions. While the direction of symmetry breaking is stochastic, the magnitude of the resulting asymmetry is reproducible and governed by the ion-specific interactions.

All-atom molecular dynamics simulations are performed on water-n-dodecane mixtures confined between graphene nanochannel walls. Salt identity, salt concentration, and surface wettability are systematically varied. Interfacial composition, hydrogen bond networks, water orientation, ion localization, hydrocarbon conformational statistics, and interfacial tensions are quantified. An Eyring-type kinetic framework is employed to analyze asymmetric hydration barriers governing ion transport across the two interfaces.

Despite symmetric surfaces and initial conditions, the confined system evolves toward a stable equilibrium with pronounced interfacial asymmetry, where one interface becomes water-rich and the opposing interface hydrocarbon-rich. This spontaneous symmetry breaking is strongly ion-specific. The observed asymmetry is accompanied by ion-induced restructuring of hydration shells, reorganization of hydrogen bond networks, directional water orientation, and ion-mediated perturbations of hydrocarbon conformations. Asymmetric hydration free energy barriers are reflected in differences in ion localization across the two interfaces. At a coarse-grained level, this behavior is consistent with an interfacial free energy balance, wherein electrolyte-induced modulation of surface-water and water-hydrocarbon tensions drives the system toward an asymmetric configuration. These results establish a molecular mechanism by which ion specificity governs interfacial organization in nanoconfined multiphase systems, with implications for nanoscale transport, separation, and soft-matter interfaces.