Selective ion transport in uniform sub-nm pores in polymer membranes

Ions of the same valency and charge exhibit limited differences in their transport in bulk media but may display strong ion-specific effects in nanoscale environments. Polymers with precisely-sized nm scale pores offer the possibility of developing new understanding regarding nanoscale ion transport, and eventually, tailoring species selectivity. They represent a qualitatively different approach relative to dense polymer membranes. Here, we examine transport of ions in polymer membranes with well-defined sub-1 nm pores. The membranes are charged hydrated nanoporous polymers produced by crosslinking self-assembled lyotropic mesophases with direct hexagonal and gyroid nanostructures. This work addresses the role of hydration, pore geometry and anion identity on transport using a combination of electrochemical and NMR measurements, and molecular simulation. Our results highlight the role of hydration separate of morphology, the role of pore size separate of hydration, and the role of local heterogeneity on ion transport. In particular, our results point to the role of hydration enthalpy in yielding orders of magnitude differences in ion transport under confinement. These results provide new insight that can be exploited to develop mechanisms for tailoring ion selectivity in nanoporous polymers and shed new light on transport under nanoscale confinement in the presence of charge.