Highly conductive and permselective anion exchange membranes for electrodialysis desalination with series-connected dications appending flexible hydrophobic tails

A series of series-connected dication ionic ionomers with flexible hydrophobic tails (QPTs) designed for electrodialysis desalination (ED) anion exchange membranes have been synthesized successfully from a monoionic quaternization agent N-(2-(dimethylamino)ethyl)-N,N-dimethylhexan-1-aminium bromide and brominated poly(2,6-dimethyl-1,4-phenylene oxide). The properties of QPTs membranes were evaluated in terms of ion exchange capacity (IEC), water uptake (WU), swelling ratio (SR), thermal stability, membrane area resistance (R-m), permselectivity (P) and ED experiments. The results confirm that the coaction of the series-connected dications linking by a short chain of two methylene groups and the middle length aliphatic tails offers QPTs excellent conductivity and permselectivity simultaneously. Especially, QPT-4 membrane has an excellent permselectivity of 94.0% and the lowest R-m of 1.60 Omega cm(2) at the highest IEC of 2.33 mmol g(-1). Moreover, QPT-4 membrane shows the best ED performances with current efficiency of 89.14%, salt flux of 75.57 mg m(-2) s(-1) and energy consumption of 2.16 kWh kg(-1) in the mostly reported NaCl ED system, which are much higher than those of commercial TWEDA1 (the corresponding values of 83.96%, 71.19 mg m(-2) s(-1) and 2.67 kWh kg(-1)). The excellent ED stability in the repeated tests further qualifies QPT-4 to be a very promising candidate for ED application.