Transport Properties of Anion-Exchange Membranes Having a Hydrophobic Layer on Their Surface in Electrodialysis

After a hydrophobic layer having cationic charge had been formed on a strongly basic anion-exchange membrane by coating cationic polysoap solutions on the membrane surface or by reacting tertiary amines having hydrophobic groups with chloromethyl groups of the cross-linked membrane, the transport numbers of various anions, sulfate, bromide, nitrate, fluoride, and iodide ions, relative to chloride ions in electrodialysis, were evaluated. The layer formed on the desalting side of the membrane enhanced the permeation of bromide, iodide, and nitrate ions and depressed that of sulfate and fluoride ions with increasing hydrophobicity of the layer without a remarkable increase in electrical resistance and any decrease in the current efficiency of the membrane. On the other hand, the polysoap layer on the concentrated side gave a similar change to the transport numbers of the anions, but caused the current efficiency to decrease. The anion-exchange membranes reacted with the hydrophobic tertiary amines with one surface of the membrane showed a similar change in the transport numbers of the anions relative to chloride ions without any decrease in the current efficiency. From an analysis of the ionic ratio in the membrane just after electrodialysis, the uptake of less hydrated anions was enhanced, and strongly hydrated anions were difficult to ion exchange with the membrane by the formation of the hydrophobic layer on the membrane surface. This is closely related to the hydration energy of anions in water.