Measurement and Correlation of Phase Equilibria in Aqueous Two-Phase Systems Containing Polyoxyethylene Lauryl Ether and Diammonium Hydrogen Phosphate or Dipotassium Hydrogen Phosphate at Different Temperatures

The phase diagrams for the systems containing polyoxyethylene lauryl ether (Brij35) and K2HPO4/(NH4)2HPO4 have been determined experimentally at three temperatures, (288.15, 298.15, and 308.15) K. Three experiential equations were used to correlate the binodal data. The liquid–liquid equilibrium (LLE) data of these systems obtained by the experimental method were fitted by using Othmer–Tobias and Bancroft along with simple Setschenow-type equations. Furthermore, the effect of temperature on the binodal curve for the disquisitive systems has been discussed, and it was found that two-phase region expands with the increase in temperature, which is corresponded to the value of the effective excluded volume (EEV) calculated according to the binodal model. In the investigated systems, the free energy (ΔG), enthalpy change (ΔH), and entropy change (ΔS) were calculated, and it indicated that the phase separation processes are endothermic and driven by the entropy increase. That the slope of tie-line increases with the rising temperature was mainly caused by the hydrophobicity of Brij35 which changes with the temperature being varied.