Universality and corresponding state behaviour in the phase diagrams of alcohol solutions of ionic liquids with the BF 4− anion

Abstract

The phase diagrams of solutions of room temperature ionic liquids butyl- and hexyl-methylimidazolium tetrafluoroborate (C n mim BF 4) in various alcohols, reported in the literature, are analysed by means of a novel function, which presumes Ising behaviour and the applicability of the rectilinear diameter rule. The fit parameters obtained (critical temperature, critical composition, amplitude of the phase diagram, and slope of the diameter) allow for a good representation of the data. Estimates of the critical point data are in good agreement with that determined independently by the equal volume criteria. Two different types of corresponding state analysis are carried out. (a) The critical data are expressed in terms of the dimensionless variables of the model system of equal sized hard spheres in a dielectric continuum, termed RPM. The corresponding critical composition is above the figure predicted for the RPM. The reduced critical temperature increases linearly with the dielectric constant of the solvent. This correlation, which includes higher alcohols and water, indicates a continuous change from phase transitions determined by Coulomb interactions to those driven by solvophobic forces. (b) The thermodynamic corresponding states analysis, where the reduced variables are given by differences to the critical point, reduced by the critical data, reveals corresponding state behaviour for all solutions investigated. All phase diagrams map on one master curve.