Characterization of Aqueous 1-Ethyl-3-Methylimidazolium Ionic Liquids for Calculation of Ion Dissociation.

Abstract

We employ precise measuring techniques to determine the densities, viscosities, and ionic conductivities of three aqueous 1-ethyl-3-methylimidazolium [emim]+ ionic liquid (IL) systems with minimal experimental uncertainty. We simultaneously present a novel method for estimating ion dissociation relying only on these three measurements and the estimated Stokes radii of the ions based on the Stokes-Einstein and Nernst-Einstein equations. Ion dissociation values are estimated across a range of IL concentrations, emphasizing dilute IL regions, using ionic radii calculated from widely used UNIQUAC and UNIFAC values. With these approximations and assuming the presence of only ion pairs, the ion dissociation of all three ILs reaches a minimum value at a water mole fraction of about 0.98. Upon further dilution with water, the ion dissociation increases as the system approaches infinite dilution of the IL. We postulate that the apparent minimum in the ion dissociation is caused by the Stokes radii of the cation and anion increasing as the concentration becomes more dilute, due to the formation of ion triplets.