Modification of properties of ion exchange membranes. VI. Electrodialytic transport properties of cation exchange membranes with a electrodeposition layer of cationic polyelectrolytes

When a cation exchange membrane is immersed in a cationic polyelectrolyte solution to form a thin layer on the membrane surface, the membrane properties are changed: permselectivity between cations with different electric charges (a relative transport number of the calcium ions to sodium ions, P ), current efficiency, and electric resistance of the membrane. Here the more compact the cationic polyelectrolyte layer, the more outstanding the change in permselectivity. To make a more compact layer, an electrodeposition method was adopted and a change in the permselectivity of the resultant cation exchange membrane was investigated. By using the electrodeposition method a strongly basic polyelectrolyte with a larger molecular weight effectively changed the permselectivity of the cation exchange membrane: the P value dropped to about 0.3 from about 2.5 of the P of the untreated membrane during electrodialysis of the sodium chloride—calcium chloride system, and an increase in the electric resistance of the membrane (i.e., organic fouling) due to a cationic surface‐active agent could be prevented. It is noteworthy that by using the strongly basic polyelectrolyte with a larger molecular weight the electrodeposition method was effective, whereas the immersion method was ineffective. Furthermore, even with the electrodeposition method the cationic polyelectrolyte which had a relatively smaller molecular weight resulted in a more remarkable change in the P value than did that with a larger molecular weight. In the electrodeposition method the amount of polyelectrolyte cohered onto the membrane surface in creased with an increase in the concentration of the polyelectrolyte, and weakly basic polyelectrolyte, and weakly basic polyelectroyte (polyethyleneimine) was also available independent of its molecular weight.