Phase diagrams and liquid-liquid equilibrium (LLE) data of the poly(ethylene glycol) (PEG) (Mn: 6000) + sodium hydroxide system have been determined at (298.15, 308.15, and 318.15) K. As temperature is increased, this aqueous two-phase system exhibits a phase inversion. Specifically, the PEG-rich phases which at low temperatures is the less dense top phasesbecomes the lower phase at elevated temperatures. The effect depends on the concentration of both PEG and sodium hydroxide. At the temperatures 308.15 K and 318.15 K, the system shows inversion; and at these temperatures inversion occurs at lower concentrations of either phase-forming constituent. Also, it was found that as temperature is increased the inversion-phase region expands. The osmotic virial equation with temperature-dependent parameters was used to correlate the phase behavior of this system.
