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

摘要

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{sub}(H{sub}2O))){sup}m were measured in the water swollen Nafion-117 membrane with Zn{sup}(2+), Ca{sup}(2+), Sr{sup}(2+), and Fe{sup}(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{sup}+ or Cs{sup}+ 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{sup}+-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{sup}+-form of membrane on equilibration with salt or acid solution. The variation of ln ((D{sub}(H{sub}2O))){sup}m with polymer volume function V{sub}p/(1 - V{sub}p), where V{sub}p is polymer volume fraction, indicated that: (i) ((D{sub}(H{sub}2O))){sup}m in the membrane with multivalent counterions was lower than that reported for membrane with monovalent counterions at same V{sub}p, and (ii) the linear trends observed in variation of ln ((D{sub}(H{sub}2O))){sup}m with V{sub}p/(1 - V{sub}p) for multivalent and monovalent counterions were significantly different. The values of ((D{sub}(H{sub}2O))){sup}m membrane normalized with ((D{sub}(H{sub}2O))){sup}m at V{sub}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{sub}(H{sub}2O))){sup}m in the corresponding ionic forms of the membrane were analyzed to obtain the charge (Z{sub}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{sub}p/(1 - V{sub}p) irrespective of charge on counterions. In order to study the influence of V{sub}p on diffusional transport rates of Na{sup}+ and Cs{sup}+ ions in membrane, a permeation experiment was carried out using H{sup}+-form of membrane having high water volume fraction. The diffusional transport rates of Cs{sup}+ and Na{sup}+ in H{sup}+-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.