Permeability of water in poly(perfluorosulfonic) acid membrane with different counterions

The change in physical structure of Li+ form of Nafion-117 membrane as a function of Li+ exchange with Cs+ ions was studied by measuring desorption rates profiles of tritium tagged water (HTO) from membrane (originally in Li+-form) at different equilibration time in Cs-salt solution. This study indicated that the water self-diffusion mobility did not change during and after complete exchange of Li+ with Cs+ ions in the membrane. The profile of HTO-desorption rates from this membrane sample matched with that reported for Nafion-117 membrane with Cs+-form only after 24 h equilibration in 0.25 mol/L CsNO3 solution, which is quite longer than the time required (300 s) for complete exchange of Li+ with Cs+ counterions in the membrane. This seems to indicate that the membrane retained its physical structure corresponding to its initial ionic form during exchange process, and slowly reorganized to attain its physical structure corresponding to Cs+ counterions. In order to correlate the self-diffusion coefficient of water (DH2Om) with permeability of water across the membrane, the radiotracer permeation experiments were carried out to measure the permeability coefficients (P) of tritium tagged water in H+, Na+, K+, Cs+, Cu2+, and Eu3+ ionic forms of the Nafion-117 membrane. DH2Om was measured in the present work for Cu2+, and DH2Om for other ionic forms of the membrane were taken from our earlier work. The values of P  (exp) obtained from the permeation experiments were compared with those obtained by the relation P (cal)=K(H2O)DH2Om, and partition coefficient of water (K(H2O)K(H2O)) equal to water volume fraction (VH) in different forms of the Nafion-117 membrane. This comparison indicated that values of P(exp) for Cs+, Cu2+, and Eu3+ ionic forms of the Nafion-117 membrane are close to P(cal). In H+, Na+ and K+ ionic forms of membrane, the values of P(exp) were considerably higher than P(cal).