Electrolytes under inhomogeneous nanoconfinement: Water structuring-mediated local ion accumulation

The behaviors of aqueous electrolytes confined in nanoscale spaces impact a broad range of biological processes and industrial applications. Our current microscopic understanding of confined electrolytes relies primarily on ideal model systems featuring homogeneous nanoconfinement. Here, we investigate the structure and dynamics of various electrolytes subject to inhomogeneous nanoconfinement, i.e., confined in two-dimensional nanochannels with gradually varying local channel heights, by means of molecular dynamics simulations. Our results reveal unexpected local ion accumulation in the inhomogeneous space occurring at boundaries between coexisting structured water phases, including trilayer, four-layer, and bulk-like waters. This contrasts markedly with the intuition that hydrated ions are more favorable to weakly confined regimes due to a steric exclusion effect. We further show that the location and intensity of the water structuring-mediated ion accumulation are sensitive to the nanochannel's geometry and surface wettability. The revealed anomalous ion behaviors in inhomogeneous nanoconfinement should help to improve our understanding of the microscopic mechanism underlying the operation of biological ion channels and to develop functional nanofluidic devices.