Studies on the Electrochemical Characterization of Cellulose Acetate and Dowex-50 Membranes for Uni-Univalent Electrolytes in Aqueous Solutions

Development of urethane acrylate composite ion-exchange membranes and their electrochemical characterization

With the objective of introducing antifouling characteristics into interpolymer types of cation and anion exchange membranes, the surface of these membranes was coated with a 12-microm-thick urethane acrylate layer and was cured by UV radiation of wavelengths 308 and 172 nm under a complete inert atmosphere. Different urethane acrylate composite ion exchange membranes developed were characterized in NaCl solution by measuring their ion-exchange capacity, volume fraction of water, contact angle with water, membrane conductance, and membrane potential. It was found that the electrochemical transport properties of urethane acrylate composite cation-exchange membranes were increased due to resonance stabilization of the urethane group, which acts as a weak acid and dissociates as a negatively charged urethane ion and a positively charged proton. This contributes toward the net charge density of the membrane matrix responsible for enhanced selectivity and conductivity, while for urethane acrylate composite anion-exchange membranes reduction in net charge density was responsible for reduction in electrochemical transport properties. Counterion transport number, permselectivity, and counterion diffusion coefficient values for these membranes were also estimated. Experiments were also carried out in higher homologs of sodium carboxylate solutions in order to observe the fouling tendencies of these membranes. It was concluded that it is possible to obtain antifouling characteristics of ion-exchange membranes by coating and curing thin hydrophilic layers of urethane acrylate on their surfaces without sacrificing their electrochemical transport properties.

Electrochemical studies on surfactant-modified cellulose acetate membrane

The possibility of electrochemical modification of cellulose acetate membrane upon immobilization of the anionic surfactant (SDS) has been explored on the basis of membrane potential studies. Surface tension measurements with and without cellulose acetate membrane were carried out to ascertain the extent of immobilization of the surfactant. Cellulose acetate membrane practically does not exhibit any ion selectivity. However, modified membrane exhibits cation selectivity which varies with concentration of the surfactant till its critical micelle concentration is reached. An attempt has also been made to demonstrate correspondence between the immobilized surfactant and the permselectivity of the membrane.

Studies on the electrochemical and permeation characteristics of asymmetric charged porous membranes

Asymmetric charged porous membranes were prepared by imbedding 10% (W/W) ion-exchange resin in cellulose acetate binder. Membrane potential and conductance measurements have been carried out in sodium chloride solutions at different concentrations to investigate the relationship between concentration of fixed charges and electrochemical properties of developed nonselective cation- and anion-exchange membranes. Counterion transport number and permselectivity of these membranes were found to vary due to the presence of ion-exchange resin. The hydrodynamic and electroosmotic permeability of sodium chloride solutions has been studied in order to compute equivalent pore radius. For cation- and anion-exchange membranes good agreement was observed between pore radius values estimated from hydrodynamic and electroosmotic permeability coefficient separately, while for nonselective membranes no correlation was found. Membrane conductance data, along with values of concentration of fixed charges, were used for the estimation of the tortuosity factor, salt permeability coefficient, and frictional coefficient between solute and membrane matrix employing an interpretation by nonequilibrium thermodynamic principles based on frictional forces. Moreover, surface morphological studies of these membranes also have been carried out and the membranes were found to be reasonably homogeneous.