Understanding the effect of morphology of hydrophobic polymers on ion selectivity

Ion exchange membranes (IEM) have been used to separate ions from water, and to separate different types of ions. However, the selective transport of ions of the same charge, such as chloride and nitrate, is not optimized. In our previous work, we showed that coating IEM with a layer of polypyrrole, a conductive polymer, is effective in improving the selectivity between nitrate and chloride. We hypothesize that further improvement of ion selectivity can be achieved by controlling polypyrrole structure, such as tuning the polymer morphology and charge distribution. We use molecular dynamics to investigate the effect of polypyrrole morphology on the selective transport of same-charged ions. We elucidate how polypyrrole morphology influences the partition and diffusion of same-charged ions in the membrane by analyzing the ion diffusion, the dynamic pore size of the polymers, ion hydration and interaction energy landscape. The research sheds light on how to tune morphology of polymeric coatings to achieve high ion selectivity between same-charged ions.