Multicomponent space-charge transport model for ion-exchange membranes

A multicomponent space-charge transport model for ion-exchange membranes was developed where the membrane structure was modeled as an an-ay of cylindrical pores with a uniform distribution of fixed-charge sites on the pore walls. Ion/fixed-charge site electrostatic interactions, electric-field-induced water dipole orientation ion-hydration free-energy changes during ion partitioning and concentration-dependent transport parameters were consider-ed in the analysis. The model predicted experimental concentration vs. time data accurately for Donnan dialysis separations with a DuPontNafion 117 cation-exchange membrane, where the membrane separated a dilute H2SO4 solution from an aqueous mixture of either Cs2SO4 + Li2SO4 or Cs2SO4 + Na2SO4. Both computer predictions and experimental measurements showed that the alkali metal cation with the larger hard-sphere radius (lower surface charge density) was selectively absorbed in and transported across the membrane during a multicomponent separation. The cation/cation transport permselectivity was less than the selectivity for equilibrium uptake due to slow ion transport near the pore wall, where discrimination between like-charge cations was greatest.