Fundamental Studies of Ion Sorption, Diffusion and Transport in Polyelectrolyte Membranes

Charged polymer membranes are widely used for water purification applications involving control of water and ion transport, such as reverse osmosis and electrodialysis, as well as applications such as fuel cell membranes.  Efforts are also underway worldwide to harness separation properties of such materials for energy generation in related applications such as reverse electrodialysis and pressure retarded osmosis.  Improving membranes for such processes would benefit from more complete fundamental understanding of the relation between membrane structure and ion sorption, diffusion and transport properties in both cation and anion exchange membrane materials. Ion-exchange membranes often contain strongly acidic or basic functional groups that render the materials hydrophilic, but the presence of such charged groups has a substantial impact on ion (and water) transport properties.  This presentation will discuss aspects of chemistry, thermodynamics, and transport that are or may be at play in such membrane processes.  Our long-term goal is to develop and validate a common framework to interpret data from both electrically driven and concentration gradient driven mass transport in such polymers and to use it to establish structure/property relations leading to rational design of membranes with improved performance.  Questions of interest include how to properly formulate sorption and transport relationships, how to decouple effects of fixed charge density and water content on sorption and transport properties, how fixed charges affect mobility of counterions and coions, etc.  Ion sorption and permeability data were used to extract information about salt diffusion coefficients in charged membranes.  Techniques have been developed to determine concentrations of both counter-ions and co-ions in polymers.  Salt permeability, sorption and ionic conductivity data have been combined to determine individual ion diffusion coefficients in neutral, cation exchange and anion exchange polymers.  The use of models to correlate and, in many cases, predict the experimental data is discussed.