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.

Effects of Polarity and Hydrogen Bonding on Physical Properties and Ion Dissociation in 1-Ethyl-3-methylimidazolium Ionic Liquid + Non-aqueous Solvent Systems

“…In this section, we report ion dissociation values calculated using the framework described above and in our previous work. 35 The ion dissociation values at 298.15 and 313.15 K can be found in Table S9 of the Supporting Information. It is important to acknowledge that these values are calculated using fixed Stokes radii calculated using the group contribution method and do not account for changes in temperature or ion concentration.…”

“…In this section, we present a condensed description of our framework used to calculate ion dissociation; a more detailed description of this approach is available in our previous work. , …”

“…In this section, we present a condensed description of our framework used to calculate ion dissociation; a more detailed description of this approach is available in our previous work. 24,35 Ion dissociation (ξ) is calculated using the ratio of the molar conductivity (generally determined by impedance spectroscopy) (Λ) to the molar conductivity described by the Nernst− Einstein equation (Λ NE )…”

“…In the present work, we supplant the use of PG-NMR with a simpler estimation technique introduced previously for the same set of ILs in aqueous systems. 23,35 This approach models ion diffusion using the Stokes−Einstein equation approximating the ions as hard spheres…”

pH of Single and Double Salt SO3H-Functionalized Brønsted Acidic Ionic Liquids

“…Nordness et al 26 studied the dissociation of some ILs in aqueous solution. They found that the dissociation fraction ξ has a minimum value at a water mole fraction of 0.98.…”