A molecular-level equilibrium partition coefficient model has been tested using uptake data for aqueous NaCl-NaBr and NaCl-NaNO3 mixtures in Neosepta AMX and Selemion AMV anion-exchange membranes. The model considers Gibbs hydration energy changes associated with the transfer of an ion to the membrane pore fluid and accounts for dielectric saturation effects and the orientation of solvent dipoles within a membrane pore due to the strong electric field generated by the membrane’s ion-exchange sites. Membrane structure parameters required for the computer program were obtained from experimental measurements.
Membrane-phase anion concentrations predicted by the model were matched against experimentally measured anion concentrations for NaCl-NaBr and NaCl-NaNO3 mixtures at a fixed total external salt concentration of 0.1 M. Predicted and measured membrane counterion concentrations differed by an average of 4%. For multicomponent absorption, the results indicated that the anion with the larger hard sphere radius was preferentially absorbed by the membrane.