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

Abstract

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 ( D H 2 O m ) were measured in the water swollen Nafion-117 membrane with Zn 2+, Ca 2+, Sr 2+, and Fe 2+ 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 D H 2 O m with polymer volume function V p/(1 − V p), where V p is polymer volume fraction, indicated that: (i) D H 2 O m in the membrane with multivalent counterions was lower than that reported for membrane with monovalent counterions at same V p, and (ii) the linear trends observed in variation of ln D H 2 O m with V p/(1 − V p) for multivalent and monovalent counterions were significantly different. The values of D H 2 O m in membrane normalized with D H 2 O m at V p = 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 D H 2 O m in the corresponding ionic forms of the membrane were analyzed to obtain the charge ( Z i ) 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 V p/(1 − V p) irrespective of charge on counterions. In order to study the influence of V p 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.