Modification of properties of ion exchange membranes. VII. Relative transport number between various cations of cation exchange membrane having cationic polyelectrolyte layer and mechanism of selective permeation of particular cations

Abstract Electrodialytic behavior of cation exchange membrane having cationic polyelectrolyte layer on its surface (relative transport number between two cations, P , current efficiency, and electric resistance of the membrane during the electrodialysis) was measured using various cations, and the mechanism of selective permeation of lower‐valent cations than the higher and of larger hydrated cations than the smaller was investigated. The following conclusions were obtained: (1) the cationic polyelectrolyte layer narrowed the pathway for ions; (2) when the pathway was made narrower by other methods, the remarkable change of the permselectivity of the membrane did not occur; (3) the effectiveness of the cationic polyelectrolyte layer was depressed by the treatment with anionic polyelectrolytes to neutralize the cationic charge on the membrane surface; and (4) the cation with larger hydrated diameter (potassium ions) more easily permeated through the membrane having the polyelectrolyte layer than that with the smaller hydrated diameter (sodium ions). It is concluded that the change of P by the cationic charge layer on the membrane surface is based on the difference in the strength of the electrostatic repulsion between two cations against the cationic charge layer.