Diffusional Transport of Ions in Plasticized Anion-Exchange Membranes

Diffusional transport properties of hydrophobic anionexchange membranes were studied using the polymer inclusion membrane (PIM). This class of membranes is extensively used in the chemical sensor and membrane based separation processes. The samples of PIM were prepared by physical containment of the trioctylmethylammonium chloride (Aliquat-336) in the plasticized matrix of cellulose triacetate (CTA). The plasticizers 2-nitrophenyl octyl ether, dioctyl phthalate, and tris(2-ethylhexyl)phosphate having different dielectric constant and viscosity were used to vary local environment of the membrane matrix. The morphological structure of the PIM was obtained by atomic force microscopy and transmission electron microscopy (TEM). For TEM, platinum nanoparticles (Pt nps) were formed in the PIM sample. The formation of Pt nps involved in situ reduction of PtCl62- ions with BH4- ions in the membrane matrix. Since both the species are anions, Pt nps thus formed can provide information on spatial distribution of anion-exchanging molecules (Aliquat-336) in the membrane. The glass transitions in the membrane samples were measured to study the effects of plasticizer on physical structure of the membrane. The self-diffusion coefficients (D) of the I- ions and water in these membranes were obtained by analyzing the experimentally measured exchange rate profiles of I-131(-) with I-nat(-) and tritiated water with H2O, respectively, between the membrane and equilibrating solution using an analytical solution of Fick's second law. The values of D(I-) in membrane samples with a fixed proportion of CTA, plasticizer, and Aliquat-336 were found to vary significantly depending upon the nature of the plasticizer used. The comparison of values of D with properties of the plasticizers indicated that both dielectric constant and viscosity of the plasticizer affect the self-diffusion mobility of I- ions in the membrane. The value of D(I-) in the PIM samples did not vary significantly with concentration of Aliquat-336 up to 0.5 mequiv g(-1), and thereafter D(I-) increased linearly with Aliquat-336 concentration in the membrane. The self-diffusion coefficients of water D(H2O) in PIM samples were found to be 1 order of magnitude higher than the value of D(I-) and varied slightly depending upon the plasticizer present in the membrane. It was observed in electrochemical impedance spectroscopic studies of the PIM samples that diffusion mobility of NO3- ions was 1.66 times higher than that of I- ions, and diffusion mobility of SO42- ions was half of that for I- ions. The theoretical interpretation of experimental counterions exchange rate profiles in terms of the Nernst Planck equation for interdiffusion also showed higher diffusion mobility of NO3- ions in the PIM than Cl-, I-, and ClO4- ions, which have comparable diffusion mobility.