Transport Properties of Multivalent Cations in Nafion-117 Membrane with Mixed Ionic Composition

The transport characteristics of multivalent cations like Ba²⁺ and Eu³⁺ have been studied in bi-ionic form of the Nafion-117 membrane. The membranes have been prepared by loading different proportions of H⁺-Ba²⁺/Mg²⁺-Ba²⁺/Ba²⁺-Eu³⁺/H⁺-Eu³⁺/Na⁺-Eu³⁺. The cationic compositions of the membranes have been determined from the measured ion exchange isotherms. Results show that the self-diffusion coefficient of Ba²⁺ (D_Ba) in H-Ba/Mg-Ba systems as well as the self-diffusion coefficient of Eu³⁺ (D_Eu) in H-Eu/Na-Eu systems are strongly dependent on the membrane ionic compositions and decreased continuously with increasing concentration of the highly hydrated ions (H⁺/Na⁺/Mg²⁺) in the membrane. Increase in the proportion of H⁺/Na⁺/Mg²⁺ ions in the membrane increases the effective charge on the membrane matrix. This causes stronger electrostatic interaction of the less hydrated multivalent ions (Ba²⁺/Eu³⁺) with the membrane matrix charges, which ultimately results in their slower self-diffusion coefficients. The higher the valence, the stronger the electrostatic interaction is with the fixed ionic charges; hence, in general, D_Eu is affected more as compared to D_Ba. On the basis of the free-volume theory for polymers, the effective interaction potential (φ) of the Ba²⁺ with the fixed ionic sites in the membrane has been calculated and found to be on the order of approximately millivolts. The higher the proportion of hydrated ion in the membrane, the higher the φ is and the stronger the ion pair formation is with the fixed ionic sites in the membrane. However, in the Ba-Eu system, as the electrostatic interactions of the two ions with the membrane matrix are close, D_Ba and D_Eu are independent of the membrane ionic composition. The ionic composition dependence of D_Ba in the H-Ba system is reflected in the transport rate of Ba²⁺, showing the importance of such measurements in understanding the transport characteristics of the membrane.