Simulations reveal heterogeneous structure in ion-exchange membranes

Well-designed polymer membranes can be useful for selective ion transport. Such membranes typically consist of charged polymers with hydrophobic backbones, possibly crosslinked to control water uptake, swollen with water and neutralized by counterions. The question arises: what governs ion affinity for such membranes: size exclusion? solvation effects? or Donnan equilibrium, in which concentrated mobile counterions with reduced translational entropy tends to exclude identical ions? Such membranes are often sketched as homogeneous mixtures; atomistic simulations reveal otherwise, and can be used to investigate ion exclusion mechanisms. For example, biomimetic pore molecules like PAP5 can be shown to exclude ions by solvation shell stripping. But size exclusion requires precise dimensional control, which makes it difficult to select among ions. As a model membrane, we simulated lamellar phases of sulfonated PS-PMB diblock copolymers. The PSS block forms a nanostructure of water-filled pores, with bound charge groups decorating the walls and counterions concentrated close by. Consequently, Donnan analysis can only be applied locally, at the pore center where solvation conditions resemble bulk solution. Practically, ion clustering near the wall greatly reduces the effectiveness of ion exclusion from entropy reduction, suggesting that pores must be small for ions to be excluded. Evidently, membrane nanostructure depends on chain architecture; more polar backbones may form more finely nanostructured membranes. Recently, we have investigated polysulfone-based membranes, and measured the size of water threads and droplets within, relevant to judging whether water transport in such membranes is better described as diffusive or collective.