Anion Transport and Selectivity in Ordered Nanoporous Polymers with 1 nm Scale Charged Pores

Solvated ions of the same valency and charge exhibit minor differences in bulk transport but may display strong ion-specific effects in nanoscale environments. Investigating such effects is challenged by the heterogeneous nature of conventional nanostructured membranes, which can smear out underlying structure-property correlations. We use a combination of electrochemical impedance spectroscopy, two-dimensional infrared spectroscopy, NMR relaxometry, and molecular dynamics simulations to systematically investigate anion transport in nanoporous polymers with uniform charged 1 nm scale pores. The pores are water-containing channels formed by lyotropic self-assembly of positively charged amphiphilic monomers that are then cross-linked to produce a highly ordered nanoporous polymer. Across a series of monovalent anions, we observe strong correlations of activation energy and conductivity with hydration enthalpy; more strongly hydrated species have higher conductivity and lower activation energies. These effects originate from differences in pore-wall interactions and solvation shell behavior, with more weakly hydrated species showing larger departures from their bulk behavior in their water coordination and activation energy in the membrane. Our results indicate that pore confinement amplifies the impact of water contributions to ion motion. Specifically, the ability to maintain hydration shell waters and concomitantly to avoid interactions with hydrophobic pore wall patches leads to significant differences in transport and to ion-specific trends that are unexpected in nanoporous materials. These results provide insight into ion transport in highly confined and hydration-limited geometries and suggest a mechanism by which ion selectivity can be explicitly manipulated.