Water and ion sorption in a series of cross-linked AMPS/PEGDA hydrogel membranes

Charge density of ion exchange membranes (IEMs) profoundly influences water and ion sorption. However, fundamental studies of water and ion sorption in charged membranes over a wide range of charge densities are not widely available, which makes it difficult to establish fundamental structure/property relations in such materials. In this study, a series of homogeneous cross-linked uncharged and sulfonated hydrogel membranes were prepared using poly(ethylene glycol diacrylate) (PEGDA) copolymerized with 2-acrylamido-2-methylpropanesulfonic acid (AMPS). Equilibrium water uptake and ion content (Na+, Cl−) in the membranes were determined as a function of external NaCl concentration (0.01–1.0 mol/L). As polymer fixed charge density increased, both water and counter-ion (Na+) sorption increased. Co-ion (Cl−) sorption decreased markedly with only modest increases in polymer fixed charge density, suggesting that even low levels of fixed charges exclude co-ions significantly. However, Cl− sorption became independent of charge density at higher levels of charge density. Experimental values for sorbed Cl− concentrations deviated from those obtained via ideal Donnan model due to thermodynamic non-idealities of ions in the solution and membrane phases. [Display omitted] •Low levels of added fixed charges exclude co-ions significantly.•Ideal Donnan model cannot quantitatively describe experimental data due to non-idealities of ion activity coefficients.•Membrane ion activity coefficients are correlated with water uptake and fixed charge concentration.