Transport Properties of Various Ionic Liquids During Electrodialysis

Abstract

We have measured changes in ionic liquid (IL) concentrations as functions of time for seven different imidazolium based ILs undergoing electrodialysis (ED). We also measured IL concentrations in the corresponding aqueous solutions, and subsequently estimated IL diffusion coefficients in each medium. All ILs were successfully transported from the aqueous solutions in the ED process; nevertheless, their transport behavior in the membranes differed significantly. Diffusion coefficients in the membranes were several hundred times smaller than those in aqueous solution; moreover, diffusion coefficients in the membranes showed stronger ion size dependence than that predicted by the Stokes–Einstein equation. This dependence was more prominent when the cation size was increased compared to that when the anion size was increased. This behavior can be attributed to the geometry of pores in the membranes. The average pore size in the membranes was estimated to be ~4 A, which is comparable to the size of IL ions (2–4 A) used here and therefore slows down the ion migration of ILs, particularly the ions with larger ion size. Moreover, it is suggested that the cation exchange membranes have longer permeation paths for ions than the anion exchange membranes, which explains the slower diffusivity of the cation in the ED process.