(384a) Ion Sorption and Transport in Ion Exchange Membranes

One avenue for improvement of such processes as electrodialysis or reverse-electrodialysis  is to design more effective membranes, but this requires knowledge of how membrane structure affects the processes of ion sorption and transport within anion and cation exchange membranes.  Despite the long history of literature on such studies and the commercial use of electrodialysis, the current state of understanding of ion sorption and transport in charged polymers is not sufficient to make adequate connections between these fundamental issues and the structure (chemical and physical) of polymer membranes.  We have embarked on a long-term program to improve this understanding and have established techniques to characterize both cation and anion exchange membranes by measuring coion and counterion sorption, salt permeation, membrane conductivity, etc. versus external salt solution concentration.  This work will eventually involve studies of how the polymer structure, charge density, water content, etc. affect the ion sorption and transport behavior.  However, for this presentation, we will focus on a complete characterization of a pair of commercial cation and anion exchange membranes that have similar structures (crosslinked styrenic) except for the nature of the fixed ions attached to  the polymer.  These membranes have been used to develop our techniques and will serve as benchmarks for future structure-property studies.  The presentation will show how the fixed ions affect the sorption and the diffusion of the coions and the counterions of the salt, sodium chloride.  By combining ion sorption data with salt permeability and membrane electrical resistance, we deduce the individual diffusion coefficients of sodium ions and of chloride ions within the membrane as a function of the external salt concentration over the range from 0.01 to 1.0 M.