Alkali Metal Chloride Sorption in Cross-linked Polymer Membranes
Eui Soung Jang, Donald R. Paul**, Benny D. Freeman*
McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, TX, 78758, U.S.A.
Various alkali metal chloride concentrations in cross-linked polymer membranes were experimentally determined to investigate the effect of fixed charge group on ion sorption in polymers. Cross-linked poly(ethylene glycol)diacrylate hydrogels (XLPEGDA) were chosen as control uncharged polymers and negatively charged cross-linked poly(2-acrylamido-2-methylpropane sulfonic acid-r-diethylene glycol dimethacrylate) (XL(AMPS-DEGDMA)) was chosen as a charged counterpart. Three alkali metal chlorides (LiCl, NaCl, and KCl) were chosen to study the significance of ion size and its hydration state on ion sorption in hydrated polymers.
The equilibrium membrane ion concentrations were measured using a desorption technique in the range of 0.01-1.0 mol L-1 external aqueous alkali metal chloride solutions. In the uncharged polymer, the concentrations of cations and anions in the membrane were equal to one another due to electro-neutrality. The order of salt sorption coefficient isotherms in XLPEGDA was: KCl > LiCl > NaCl, which does not correspond to the order of ion hydration size, i.e., Li+ > Na+ > K+. These results suggest that ion sorption in uncharged XLPEGDA is influenced by both ion hydration and polymer-ion specific interactions.
The charged polymer sorbed a significantly greater amount of cations (i.e., counter-ions) than anions (i.e., co-ions or mobile ions) due to the presence of fixed charge group attached to the polymer backbone (i.e., sulfonate group). The order of mobile salt sorption coefficient isotherms in XL(AMPS-DEGDMA) was: KCl > NaCl > LiCl, in contrast to the order observed in uncharged polymers. These results suggest that ion sorption in XL(AMPS-DEGDMA) is influenced mainly by the hydration tendency of counter-ions. Furthermore, based on the ion sorption measurements, electrolyte activity coefficients in the polymers were calculated to quantify the thermodynamic non-ideality of the ion-polymer-water mixing system. All of these findings assist in characterizing and understanding ion sorption properties of polymers as part of a long-term effort to improve fundamental understanding regarding ion uptake and ion transport in both charged and uncharged polymers.
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