Study on physical and electrostatic interactions of counterions in poly(perfluorosulfonic) acid matrix: Characterization of diffusion properties of membrane using radiotracers

The self-diffusion coefficients of water and ions were used to study the physical (tortuosity) and electrostatic interactions of counterions in poly(perfluorosulfonic) acid membrane (Nafion-117) matrix. The self-diffusion coefficients of water (DH2Om) were measured in the water swollen Nafion-117 membrane with Zn2+, Ca2+, Sr2+, and Fe2+ counterions by analyzing the experimental exchange rates between tritium tagged water (HTO) in membrane and equilibrating water. In order to study the effects of equilibrating solution, the HTO-desorption rate profiles between the membrane samples in H+ or Cs+ forms and equilibrating solution containing CsCl or HCl (0.25 mol/L) were measured. It was observed that the HTO-exchange rate profile was slower in case of membrane sample in Cs+-from equilibrated with salt/acid solution than that equilibrated with deionized water in same ionic form. However, HTO-exchange rate profile did not alter in case of H+-form of membrane on equilibration with salt or acid solution. The variation of ln DH2Om with polymer volume function Vp/(1 − Vp), where Vp is polymer volume fraction, indicated that: (i) DH2Om in the membrane with multivalent counterions was lower than that reported for membrane with monovalent counterions at same Vp, and (ii) the linear trends observed in variation of ln DH2Om with Vp/(1 − Vp) for multivalent and monovalent counterions were significantly different. The values of DH2Om in membrane normalized with DH2Om at Vp = 0 were taken as an estimate of the tortuosity factor for self-diffusion of ions in the membrane matrix. The self-diffusion coefficients of ions reported in the literature along with tortuosity factor obtained from DH2Om in the corresponding ionic forms of the membrane were analyzed to obtain the charge (Zi) independent electrostatic interaction parameter g(φ) of monovalent and divalent ions in the membrane. This analysis indicated that g(φ) also vary exponentially as a function of Vp/(1 − Vp) irrespective of charge on counterions. In order to study the influence of Vp on diffusional transport rates of Na+ and Cs+ ions in membrane, a permeation experiment was carried out using H+-form of membrane having high water volume fraction. The diffusional transport rates of Cs+ and Na+ in H+-form of membrane were found to be similar indicating that the water volume fraction in membrane has strong influence on the parameters that govern the diffusion across the Nafion-117 membrane.