Modification of properties of ion exchange membranes. II. Transport properties of cation exchange membranes in the presence of water-soluble polymers

Abstract

Electrodialytic transport properties of commercial cation exchange membranes in the presence of various water-soluble polymers were observed. The transport properties measured in this report were the relative transport number of calcium ion to sodium ion ( P Na Ca), the current efficiency of cations and the electric resistance of the membrane during the electrodialysis. Water-soluble polymers used were polyethyleneimine, poly- N,N-dimethylethyleneammonium sulfate, poly- N-vinylpyrrolidone and polyvinylalcohol. When electrodialysis was carried out using the cation exchange membrane in the presence of cationic polyelectrolytes, P Na Ca of most membranes decreased remarkably while the electric resistance of the membrane during the electrodialysis scarcely increased and the current efficiency increased or decreased slightly. For example, P Na Ca of the cation exchange membrane, Neosepta CL-25T, was 0.18 when the membrane had been exposed to the mixed salt solution containing 27.9 ppm of poly- N,N-dimethylethyleneammonium sulfate for 24 hr while P Na Ca of Neosepta CL-25T in the absence of it was 2.52. However, P Na Ca of the same membrane increased in the presence of poly- N-vinylpyrrolidone or polyvinylalcohol. In the case of polyethyleneimine, it was confirmed that monolayer type adsorption or ion exchange occurred on the surface of the membrane, about 90% of imino groups of polyethyleneimine which adsorbed on or ion exchanged with the membrane did not ion exchange with sulfonic acid groups of the membrane and P Na Ca decreased much more when pH of the mixed salt solution used in electrodialysis was low. Moreover, P Na Ca decreased remarkably in the presence of strongly basic polyelectrolyte more than weakly basic polyelectrolyte. The membrane property of monovalent cation permselectivity by adsorption of or ion exchange with polyelectrolyte was not easily lost. These membranes are preferentially permeable to sodium ion because the electrostatic repulsion force between calcium ion and the cationic surface charge of the membrane is larger than that between sodium ion and the cationic surface charge of the membrane. However, the increase of P Na Ca in the presence of poly- N-vinylpyrrolidone and polyvinylalcohol was not sufficiently explained in this report.