Ion Permeation in Nanoscale Films: Fundamental Limitation and Evaluation of Dielectric Properties

Nanoscale films play a central role in biology and osmotic separations. Their water/salt selectivity is often regarded as intrinsic property, favoring thinner membranes for faster permeation. Here we highlight and quantify a fundamental limitation arising from the dependence of ion self-energy on film thickness, governed by its ratio to Bjerrum length. The resulting relation factors out this dependence from intrinsic ion permeability, which agrees well with available data and enables evaluation of dielectric properties of ultrathin films, advancing understanding of ion transport in membranes.