Interfacial electrostatics of confined aqueous electrolytes: A new frontier for tuning water permeability and ion selectivity

The behavior of water and salt ions in nanoscale environments departs fundamentally from bulk expectations, with important consequences for membrane separations, biological ion channels, and electrochemical energy technologies. A key origin of these departures is the pronounced suppression and anisotropy of the dielectric response of water arising from confinement-induced constraints on water ordering. The resulting anisotropic dielectric screening within nanopores has far-reaching consequences: the description by traditional models such as the Born energy of solvation no longer holds, ion–ion interactions, including ion pairing, are altered, ion-pore interactions are amplified, and electrostatic interactions become longer ranged and stronger in magnitude. Consequently, electrostatically driven ion selectivity can persist at pore sizes far larger than those achievable through steric or hydrogen bonding mechanisms alone. Further, the confinement geometry, atomic roughness, and electronic polarization and quantum effects jointly shape the resulting potential energy landscape and transport behavior. Recent experiments and simulations of electrolyte transport in sub-nm pores in graphene nanocapillaries and carbon nanotubes demonstrate several intriguing possibilities, including the electronic properties of the pore wall influencing water structuring, ion pairing, and nanoscale water and ion transport. This perspective argues that rational control of interfacial electrostatics offers a powerful and underexplored design lever for simultaneously enhancing water permeability and ion selectivity across a broad range of pore sizes.