Sorption phenomena in nafion membranes

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Sorption phenomena of water and aqueous salt solutions by a perfluorinated polymer containing sulfonic acid groups (Nafion) were investigated. The temperature and concentration dependencies of the sorption by the membranes in the acid and salt forms were studied. The apparent activation energies for the diffusion of water in the H-form membrane and in the K-salt form were obtained as 4.9 and ca. 13.0 kcal/mole, respectively. The sorption kinetics during the neutralization of the membranes were observed in several aqueous solutions. A maximum in the sorption curve during the neutralization process was found and explained as resulting from the differences in the diffusion coefficients of water and of the cations and from the different number of water molecules absorbed by a SOH+ (acid) site and a neutralized site. The diffusion coefficients D of several cations (K+, Cs+, Ba2+, and Ca2+) were determined and found to be considerably smaller than that of water. For the various cations, log D was related linearly to q/a, where q is the cation charge and a is the separation between centers of charge of the cation and anion. The dependence of water sorption upon the degree of neutralization of the membrane was also studied at room temperature. It was observed that for membranes of a low degree of neutralization a secondary sorption process existed, while no such secondary sorption could be found for the pure acid or the highly neutralized membranes. This secondary sorption was attributed to a structural rearrangement in the polymer. The apparent diffusion coefficient of water and the number of water molecules absorbed at equilibrium by an ionic site, ns, were obtained as a function of the degree of neutralization. The diffusion coefficient of water was dependent strongly on both the degree of neutralization and type of the salt, but no quantitative relation could be established. For all the salts studied in this paper, ns was linearly related to the degree of neutralization, x, supporting the assumption that the value of ns could be divided into those water molecules absorbed by an SOH+ (acid) site, nh, and those absorbed by a neutralized site, nm. It was found that the value of q × nm had a strong correlation with a characteristic constant of the cations since a plot of q × ns versus log (q/r) yielded a straight line (r being the radius of the cation).