Phase behaviour at different temperatures of ionic liquid based aqueous two-phase systems containing {[Bmim]BF4 + salt sulfate (Zn2+ or Ni2+) + water}

Abstract

Phase diagrams and liquid+liquid equilibrium (LLE) data of aqueous two-phase systems (ATPSs) composed of the {1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4)+zinc sulfate (ZnSO4) or nickel sulfate (NiSO4)+water} systems have been determined experimentally at T=(283.15, 298.15, and 313.15)K. The effect of the temperature, composition, cation and ion exchange in the formation this ATPS were available. The temperature had a remarkable effect on the position of phase diagrams. The decrease in temperature promoted phase segregation indicating the exothermic character of formation of these ATPSs and there was phase inversion at a temperature of 283.15K for both. A scale was established to evaluate the ability of different cations of sulfate salts to induce the formation of ATPSs in mixtures involving [Bmim]BF4. Thermodynamic parameters of hydration were used together with experimental results of saturation solubility to make various adjustments that seem to show that the molar entropy of hydration is the driving force for the separation process. Thermodynamic parameters of transfer of components (cations, anions and water) between the phases were also calculated from the experimental values and indicated that the material transfer of the bottom phase to the top is not spontaneous and tends to be less spontaneous as the TLL value increases. It was experimentally observed that no significant exchange of ion pairs occurs in the phase separation process. Additionally, the binodal curves were fitted to an empirical non-linear expression and the salting out effect was explored using the type-Setschenow equation. Data consistencies were tested using the Othmer-Tobias and Bancroft equations.