Study of self-diffusion of monovalent and divalent cations in Nafion-117 ion-exchange membrane

The self-diffusion coefficients of monovalent (Na+, K+, Cs', and Ag+) and d ivalent (Ca2+, Sr2+, Ba2+, and Zn2+) cations in Nafion-117 have been measur ed using the '"nonstationary radiotracer diffusion method" for the first ti me. This method is based on the analysis of absorption/desorption of radiot racer ions in the membrane sample as a function of time using an analytical solution of Fick's second law. It has been found to be simple and faster t han that of the commonly used "radiotracer permeation method". An attempt h as been made to interpret the self-diffusion data based on the free-volume theory. The self-diffusion coefficients for monovalent and divalent ions, n ormalized with respect to their aqueous diffusion coefficients, correlate w ell with the polymer volume fractions. However, separate trends are observe d for monovalent and divalent ions. The factors that lead to higher self-di ffusion coefficients of the counterions in the membrane appear to lower the ir selectivity coefficients.