Corresponding states analysis of the critical points in binary solutions of room temperature ionic liquids

A systematic study of liquid-liquid phase transitions in mixtures of room temperature ionic liquids (RTIL) with nonionic solvents of different dielectric constant is presented. We report phase diagrams and the location of the critical points in the binary systems. The salts are 1-alkyl-3-methylimidazolium tetrafluoroborates and hexafluorophosphates with butyl-, hexyl-, octyl- side chains. The solvents are water, n-alcohols (n = 1 - 16), branched alcohols, and dialcohols. A corresponding state analysis of the data is carried out in terms of the variables of the restricted primitive model (RPM), which considers ionic fluids as equal-sized charged hard spheres in a dielectric continuum. The reduced critical temperatures are found to depend almost linearly on the dielectric constant of the solvent. The data of all investigated systems are represented by a master plot, which indicates a continuous crossover from a Coulomb driven phase transition, reasonably described by the RPM, towards a phase transition determined by hydrophobic interactions.