Self-diffusion coefficients of water in Nafion-117 membrane with multivalent counterions

The tritium tagged water (HTO) exchange rates between water swollen Nafion-117 membrane and equilibrating water were measured to obtain self-diffusion coefficients of water (DH2Om) in the membrane samples with several multivalent counterions, viz. Ba2+, Fe3+, Y3+, La3+, Eu3+ and Th4+. The values of DH2Om were obtained by analyzing the isotopic exchange rates profiles using an analytical solution of Fick's second law. The values of DH2Om in these ionic forms of Nafion-117 membrane were found to be in the range of 1.8 × 10−6–2.2 × 10−6 cm2/s. The water uptake capacity in the membrane samples with multivalent counterions was also not found to vary significantly. Since diffusion of water occurs through water channels in the membrane, the narrow spread of DH2Om values in membrane samples with multivalent counterions was attributed to nearly constant polymer volume fraction (Vp = 0.75 ± 0.02) in the membrane. The comparison of DH2Om values obtained in the present work with that reported for Nafion-117 membrane having same Vp (0.75) with Na+ counterions (2.85 × 10−6 cm2/s) indicated that the self-diffusivity of water is higher in monovalent forms of membrane. DH2Om represents tortuosity factor in membrane and hence it is dependent on Vp and microstructure of the membrane. Therefore, the slight difference in DH2Om values in monovalent and multivalent forms of membrane with same Vp could be attributed to minor difference in transport channels of membrane in these forms of Nafion-117 membrane.