Sodium chloride sorption in sulfonated polymers for membrane applications

Ion sorption measurements were conducted to characterize sodium and chloride sorption in sulfonated polymers of interest for membrane applications. An uncharged polymer, based on cross-linked poly(ethylene glycol diacrylate), was included for comparison. The charged polymers included a sulfonated polysulfone random copolymer and two phase separated sulfonated styrenic pentablock copolymers. The sodium and chloride ion concentrations sorbed in these polymers were measured after polymer films had been equilibrated with 0.01-1.0molL ^-1 aqueous NaCl solutions at neutral pH. For the sulfonated polymers, cation sorption was measured using a polymer ashing technique and flame atomic absorption spectrophotometry. Anion sorption was measured using a desorption technique and ion chromatography. Polymer charge, i.e., sulfonation, influenced ion sorption. In general, salt sorption increased as water sorption increased in all polymers considered. In the uncharged polymer, the molar concentrations of sorbed sodium and chloride were equal to one another, as expected. However, the sulfonated polymers sorbed much more sodium than chloride ions due to ion exchange. The experimental mobile salt (i.e., anion) sorption data were fit to a model containing contributions from both Donnan exclusion and simple salt partitioning to the overall salt uptake by the polymer. In general, the influence of Donnan exclusion on anion (i.e., mobile salt) sorption in these cation exchange materials is reduced as water uptake increases, presumably due, in part, to dilution of the hydrated polymer's sulfonate groups.