Ion sorption and transport in ion exchange membranes : importance of counter-ion condensation
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Due to their ability to selectively transport charged species (e.g., ions), ion exchange membranes (IEMs) are required for efficient operation of various membrane-based technologies for producing clean water and energy. Examples include electrodialysis, reverse electrodialysis, and fuel cells, among others. IEMs are also actively explored for use in processes that have not traditionally used them (e.g., reverse osmosis, forward osmosis, batteries, etc.). A molecular understanding of the relationship between polymer structure and water/ion transport properties could lead to new strategies for rational design of high performance membranes, improving efficiencies of membrane-based technologies and catalyzing their use in novel applications. However, despite the sustained relevance of ion exchange membranes, such molecular level understanding remains largely incomplete. This study is aimed at elucidating the main factors governing ion transport in ion exchange membranes. Fundamental models for equilibrium ion sorption and concentration gradient driven ion transport (i.e., salt permeability coefficients) in IEMs were developed using ideas from the polyelectrolyte literature (i.e., Manning’s counter-ion condensation theory). The framework presented in this dissertation accurately predicted, for the first time to the best of our knowledge, experimental equilibrium ion sorption and salt permeability results in a series of commercial IEMs with no adjustable parameters. The modeling results were used to establish a connection between polymer structure and ion transport properties. The influence of fixed charge group concentration on equilibrium ion sorption in IEMs was also investigated. A series of cation and anion exchange membranes having different fixed charge group concentrations but similar water content were synthesized. Equilibrium membrane ion concentrations were experimentally measured. Co-ion sorption in the membranes decreased with increasing fixed charge group concentration, presumably due to enhanced Donnan exclusion. However, the extent of co-ion sorption decrease in the cation exchange membranes was greater than that in the anion exchange membranes, despite similar changes in fixed charge group concentration, presumably due to polymerization induced phase separation in the cation exchange membranes. The experimental ion sorption data were interpreted within the framework of the ion sorption model presented in this study, and relations between membrane properties and equilibrium ion sorption in such materials were developed.