Electrodialytic transport properties of anion-exchange membranes prepared from poly(vinyl alcohol), poly(N-ethyl 4-vinylpyridinium salt) and β-cyclodextrin

Anion-exchange membranes were prepared from hydrophilic components, poly(vinyl alcohol), β-cyclodextrin and poly(N-ethyl 4-vinylpyridinium bromide) by the casting method, following the cross-linking reaction with formaldehyde. Water content of the membranes increased with increasing content of β-cyclodextrin in the membranes, which means that the membranes became more hydrophilic. The prepared anion-exchange membranes were used in the electrodialysis of various mixed salt solutions to evaluate transport numbers of various anions such as sulfate, bromide, fluoride and nitrate ions, relative to chloride ions. The transport numbers of less hydrated anions, bromide and nitrate ions, relative to chloride ions, decreased and those of strongly hydrated anions, sulfate and fluoride ions, increased with increasing content of β-cyclodextrin in the membranes. Measurements of ion-exchange equilibrium constants of sulfate or nitrate ions to chloride ions and the ratio of mobility of sulfate or nitrate ions to that of chloride ions revealed that the change in the transport numbers between anions was based on the synergistic effect of both change in the ion-exchange equilibrium constant of anions to chloride ions and that in the mobility ratio between them, which was different from behavior of other anion-exchange membranes composed of mainly aromatic or heterocyclic groups. The β-cyclodextrin played a role in giving the membranes a strongly hydrophilic atmosphere and in changing the mobility ratio between anions in the anion-exchange membranes.