Preparation and characterization of N-methylene phosphonic and quaternized chitosan composite membranes for electrolyte separations.

Chitosan was functionalized either by introducing a phosphonic acid group or by quaternization of existing primary ammonium groups in order to make it a water-soluble material. Functionalized chitosans and poly(vinyl alcohol) (PVA)-based nanoporous charged membranes were prepared in aqueous media and gelated in methanol at 10 degrees C to tailor their pore structure. These membranes were extensively characterized for their physicochemical, electrochemical, and permeation characteristics using FTIR, TGA, DSC, water content, ion-exchange capacity, ionic transport properties, and membrane permeability studies. N-Methylene phosphonic chitosan (NMPC)/PVA-based membranes exhibited mild cation selectivity and quaternized chitosan (QC)/PVA composite membranes had mild anion selectivity, while a blend of NMPC-QC/PVA membranes exhibited weak cation selectivity because of formation of zwitterionic structure. Viscosity measurements and interaction studies for individual and mixed solutions of NMPC and QC were carried out for the prediction of charge interactions between -PO3H2 and -N+(CH3)3 groups and effect on molecular weight due to functionalization. Elaborate electrochemical and permeation experiments were conducted in order to predict suitability of these membranes for the separation of mono- and bivalent electrolytes based on their hydrated ionic radius, and it was found that among all the synthesized membranes, PC/QC-30 had the highest relative permeability, which may extend its suitability for electrolyte separations. Observations were correlated with equivalent pore radius of the different membranes as estimated by membrane permeability measurements.