Membrane potential across low-water-content charged membranes: Effect of ion pairing

In the present paper, we systematically examined the ion-pairing effect in low-water-content charged membranes. Cation- and anion-exchange membranes with various water contents and homogeneous fixed-charge distribution were prepared by radical copolymerization and then characterized by membrane potential measurements. The experimental results were analyzed by our recently developed theoretical model (Yamamoto, R.; Matsumoto, H.; Tanioka, A. J. Phys. Chem. B 2003, 107, 10615), which is based on the Donnan equilibrium, the Nernst-Planck equation for ion flux, and the Fuoss formalism for ion-pair formation between the fixed-charge group and the counterion in the membrane. The theoretical predictions agreed well with the experimental results for both cation- and anion-exchange membranes. This supported the belief that the ion-pairing effect was substantial in a low-water-content membrane system. Our theoretical analysis also showed the following results: (i) the dielectric constant in the membrane, epsilon(r), was smaller than the value in bulk water, (ii) the center-to-center distance of the ion pair, a, was independent of the water content of the membranes, and (iii) the charge effectiveness of all membranes, Q, was small (< 0.35).