Relationship of permselectivity between two anions to water content of anion exchange membranes with pyridinium groups

Abstract

Anion exchange membranes having various pyridinium groups as anion exchange groups were prepared, and the transport properties of the membranes, especially the change in transport numbers of various anions relative to chloride ions, was examined in connection with the water content of the membranes, and the species and position of a substituent on the pyridinium groups. In general, when the water content of the membranes decreased, less hydrated anions compared to chloride ions, nitrate ions and bromide ions, permeated selectively through the membranes. On the other hand, the permeation of strongly hydrated anions compared to chloride ions, fluoride ions and sulfate ions, was not necessarily affected by the change in the water content of the membranes. This might be due to the strong hydrophobicity of the backbone polymer used in the membranes. The transport numbers of anions relative to chloride ions were also affected by the species of the substituent and its position in the pyridinium groups. For example, when hydrophilic groups, such as -OH, exist at the 2-position on the pyridinium groups, permeation of bromide ions decreased, and the existence of ethyl groups at the 2-position enhanced the permeation of bromide ions.