Influence of fixed charge concentration and water uptake on ion sorption in AMPS/PEGDA membranes

Abstract

Water content and charge density strongly influence ion sorption in charged membranes. However, isolating the individual effect of each parameter on ion sorption can be difficult due to experimental challenges associated with independently controlling these two variables during membrane synthesis. This frustrates development and validation of fundamental models and structure/property relationships. In this study, three series of uncharged and negatively charged hydrogel membranes were prepared using poly(ethylene glycol) diacrylate (PEGDA) as a cross-linker and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as an ionic monomer. Water content and charge density were controlled by varying AMPS content and PEGDA chain length. Ion sorption in these materials was measured using aqueous NaCl solutions at concentrations ranging from 0.01 to 1.0 M. At constant charge density, salt sorption increased as water content increased. At fixed water content, salt sorption decreased as charge density increased. Salt sorption in membranes with the highest water content and charge density was predicted using the Donnan/Manning model, which accounts for solution and membrane phase thermodynamic non-idealities. However, this approach could not predict sorption in weakly charged and uncharged membranes, presumably due to additional membrane phase thermodynamic non-idealities associated with ion solvation and uncharged polymer segments. • High ion activity coefficients in uncharged membranes suggest ions have a much greater affinity for aqueous solution than swollen, uncharged membranes. • The Donnan/Manning model predicts experimental sorption well for highly hydrated and/or highly charged membranes. • Model predictions agree poorly with experimental data for membranes that are less hydrated, weakly charged, or uncharged due to phenomena not accounted for in Manning's theory (e.g., dielectric exclusion and non-idealities introduced by polymer segment-water-ion interactions).