Physicochemical interactions of organic acids influencing microstructure and permselectivity of anion exchange membrane

Permselectivity and water uptake behaviors of anion exchange membrane (AEM) were investigated with organic acids. Contribution of the number of carboxyl group (acetic acid: AA, malic acid: MA, citric acid: CA) and hydrocarbon chain lengths (formic acid: FA, AA, propionic acid: PA and n-butyric acid: BA) in water uptake and permselectivity values were separately investigated with five different concentrations 0.025, 0.05, 0.075, 0.1 and 0.125 mol.L-1 of each acid at two different pH 2.0 and 7.5 conditions. Microstructural changes occurring due to interaction of carboxylate anion and NR4+ groups of AEM resulted in permselectivity behavior. Permselectivity trend: CA < MA < AA could be explained by counter-ion condensation resulting out of strong counter-ion interactions with AEM fixed charges. Co-ion mobility values could explain variation in permselectivity at pH 2.0 and 7.5. While, water uptake and ionic size were dominant factors to explain the permselectivity trend: FA > AA > PA > BA. Complex nature of interactions due to properties (size, charge, ionic charge density, mobility, diffusivity, stokes radius etc.) of carboxylate anions were explained using diffusivity ratio (counter/co-ion, D-2/D-1), adsorption equilibrium and fraction of dissociated species.