Preparation of a Monovalent-Selective Anion-Exchange Membrane: Effect of Alkyl Chain Length and Crystallinity

Crosslinked anion-exchange membranes (AEMs) with reasonable water uptake are used for the separation of monovalent anions from their mixture with bivalent anions by electrodialysis (ED). Herein, we report the preparation of a series of monovalent-selective crosslinked AEMs containing imidazole rings from the polyacrylonitrile-co-poly(vinylimidazole) (PAN-co-PVIm) copolymer (AN/VIm ratio = 75:25 mol/mol), followed by N-alkylation with alkyl halide of different chain lengths (carbon number: 1,6,10,18), followed by reaction with 1,6-diamino hexane. The difference in the obtained permselectivity (PSO42–Cl–) was compared with those of un-crosslinked and commercial Ionsep membranes. The effects of crosslinking, the chain length of the alkyl spacer attached to imidazolium nitrogen of the membrane matrix, and the partial crystallinity occurring due to the arrangement of long alkyl chains on PSO42–Cl– were investigated. A representative crosslinked membrane (MQ18) with an octadecyl alkyl chain length (C18) attached to the imidazole moiety exhibited moderate water uptake (22%), high fixed charge density (0.037 meq/cm3), and high PSO42–Cl– (6.4) during the separation of a mixture of 0.01 M sodium chloride and 0.01 M sodium sulfate (2000 mg/L) by the ED process. The PSO42–Cl– value is further increased from 6.4 to 16.7 by increasing the electrolyte concentration to 0.05 M sodium chloride and 0.05 M sodium sulfate. The rigid imidazolium structure, locally organized higher chain length of the alkylating group, and the crosslinked network structure provide steric hindrance for the passage of more hydrophilic sulfate anions, which facilitates higher selectivity.