Ion transport in crosslinked AMPS/PEGDA hydrogel membranes
Ion exchange membranes (IEMs) are key components to water purification [e.g., electrodialysis (ED)] and energy generation [e.g., fuel cells, reverse electrodialysis (RED)] applications. IEMs are also being explored for other membrane-based techniques, such as reverse osmosis (RO), pressure-retarded osmosis (PRO), and membrane-assisted capacitive deionization (CDI). Membrane performance (e.g., conductivity and permeability) in these applications is sensitive to membrane transport (water/ion sorption and diffusion) characteristics. However, much remains unknown about the influence of polymer membrane architecture on water and ion transport properties critical to high performance membranes. In this study, a series of homogeneous cross-linked uncharged and sulfonated hydrogel membranes with various IEC values were prepared. Equilibrium water uptake, ion sorption, and ion diffusion coefficients in the membranes were determined as a function of external salt concentration. Co-ion sorption decreased markedly as IEC increased slightly, suggesting that even low levels of fixed charges exclude co-ions significantly.
However, co-ion sorption became independent of charge density at higher IEC values due to non-idealities in the solution and membrane phases. Counter-ion and co-ion diffusion coefficients are mainly governed by water content in the membrane. Meares’ model predictions for counter-ion and co-ion diffusion coefficients agree reasonably with the experimental values. Minor deviations for co-ion diffusion coefficients were evident in more highly charged membranes. These discrepancies might be a result of the interactions omitted by Meares’ model (e.g., fixed charges-ion, ion-ion, etc.). The effects of water content and charge density on ion sorption were also isolated. Membranes with similar water content but different charge densities and similar charge density but different water uptake values were synthesized and characterized. At constant charge density, the sorbed mobile salt concentration increases as membrane water content increases. At fixed water content, mobile salt sorption decreases as charge density increases due to stronger Donnan exclusion effect. Salt sorption in the membranes with the highest water content or charge density could be predicted after accounting for the non-idealities in solution and in the membrane. However, this approach fails for less hydrated or weakly charged membranes due to more pronounced thermodynamic non-idealities introduced by the uncharged polymer segments.