Interdiffusion of exchanging counterions in poly(perfluorosulfonic acid) membrane

The kinetics of forward (Li+/Na+mem <=> Mn+aq) and reverse (Mn+mem <=> Li+/Na+aq) exchanges of metal ions(Mn+ ) Ag+, Cs+, Ba2+, and Eu3+) in the poly(perfluorosulfonic acid) membrane (Nafion-117) equilibrated with a well-stirred aqueous salt solution were experimentally measured using radioactivity tagged counterions. A numerical solution of the Nernst-Planck (N-P) equation for interdiffusion was used to interpret the kinetics of these exchanges. The experimentally measured kinetics of forward and reverse Na+ ion exchanges with Mn+ ions between the membrane and the equilibrating solution were found to be close to that predicted by the N-P equation. The minor differences between the experimental and predicted exchange rate profiles were attributed to change in water content of the poly(perfluorosulfonic acid) membrane in different ionic forms. The kinetics of Li+ ion exchanges with Na+ and Cs+ ions was found to be unusual as the self-diffusion coefficient D(ion)(m) of Li+ ions in the membrane were quite different in the forward and reverse exchanges. The values of D(ion)(m) for Li+ ion, obtained by the N-P equation, were found to be 2 x 10-6 and 0.2 x 10-6 cm2 s-1in Li+mem <=> Na+/Cs+aq and Na+/Cs+mem <=> Li+aq exchanges, respectively. The drastic differences in diffusion mobility of Li+ ions during forward and reverse exchanges was attributed to weak electrostatic interactions of Li+ ions with the fixed exchange sites as indicated by the reported equilibrium constant Kex of the Li+-exchange reaction in Nafion membrane. D(ion)(m) is high in the forward exchange due to minimum retardation in diffusion mobility of Li+ ions by the electrostatic forces in the membrane. In reverse exchange, Li+ diffusion mobility is retarded due to its slow replacement of counterions from the membrane.