Liquid–liquid phase separation in solutions of ionic liquids: phase diagrams, correspondingstate analysis and comparison with simulations of the primitive model

Abstract

Phase diagrams of ionic solutions of the ionic liquidC18mim+NTF2− (1-n-octadecyl-3-methyl imidazolium bistrifluormethylsulfonylimide) indecalin, cyclohexane and methylcyclohexane are reported and comparedwith that of solutions of other imidazolium ionic liquids with the anionsNTF2−,Cl− andBF4− inarenes, CCl4, alcohols and water. The phase diagrams are analysed presuming Ising criticality andtaking into account the asymmetry of the phase diagrams. The resulting parameters arecompared with simulation results for equal-sized charged hard spheres in a dielectriccontinuum, the restricted primitive model (RPM) and the primitive model (PM)that allows for ions of different size. In the RPM temperature scale the criticaltemperatures vary almost linearly with the dielectric permittivity of the solvent. TheRPM critical temperatures of the solutions in non-polar solvents are very similar,somewhat below the RPM value. Correlations with the boiling temperatures of thesolvents and a dependence on the length of the side chain of the imidazoliumcations show that dispersion interactions modify the phase transition, which ismainly determined by Coulomb forces. Critical concentrations, widths of thephase diagrams and the slopes of the diameter are different for the solutions inprotic and aprotic solvents. The phase diagrams of the solutions in alcohols andwater get a lower critical solution point when represented in RPM variables.