(449bt) Influence of Fixed Charge Group Concentration on Ion Sorption in Ion Exchange Polymers

Authors: 
Kamcev, J., The University of Texas at Austin
Freeman, B. D., University of Texas at Austin
Paul, D. R., The University of Texas at Austin
Controlling ion sorption and transport in polymer membranes is critical for efficient operation of a number of membrane-based technologies such as reverse osmosis, electrodialysis, reverse electrodialysis, and fuel cells. Often, membranes for these processes are made from polymers having ionizable functional groups (fixed charge groups) bound to their backbone (i.e., ion exchange membranes, ionomers, etc.). Sorption of charged species (e.g., ions) in such materials is profoundly influenced by the fixed charge groups, due to Donnan exclusion. Consequently, the concentration of fixed charge groups can be used to tune, to an extent, sorption of ions in charged membranes. However, changes in the concentration of fixed charge groups are typically accompanied by changes in membrane water content (i.e., swelling), which also substantially influences ion sorption in membranes. Decoupling these two effects is critical for developing fundamental understanding of the influence of membrane fixed charge concentration on ion sorption in charged membranes, ultimately leading to strategies for rational design of high performance materials.

In this study, positively and negatively charged membranes having different fixed charge group concentrations but similar water content were synthesized. Ion concentrations in membranes equilibrated with NaCl solutions of varying concentration were measured using a desorption technique. Counter-ion (i.e., ion with opposite charge to that of the fixed charges) sorption increased with increasing fixed charge group concentration, since a larger number of counter-ions are required to electrically balance the fixed charge groups. Co-ion (i.e., ion with same charge as that of the fixed charges) sorption decreased with increasing fixed charge group concentration, presumably due to enhanced Donnan exclusion. However, despite similar changes in membrane fixed charge group concentrations, co-ion sorption in the negatively charged membranes was suppressed to a greater extent than in the positively charged membranes, presumably due to structural differences between the materials. The ion sorption data were interpreted using a model based on Donnan theory and Manningâ??s counter-ion condensation theory. The model predicted the experimental ion sorption data in the positively charged membranes with no adjustable parameters. However, agreement between model and experimental data for the negatively charged membranes was poor, presumably due to membrane inhomogeneities introduced during polymerization. Use of the Donnan/Manning model for predicting ion sorption in inhomogeneous materials is discussed.