Liquid–Liquid Equilibria of an Affinity Aqueous\nTwo-Phase System Containing Polyethylene Glycol and Sodium Sulfate\nSalt at Various Ligand Concentrations and pH: Experimental Determination,\nCorrelation, and Thermodynamic Modeling

The behavior of liquid–liquid\nequilibrium (LLE) of aqueous\nbiphasic systems including polyethylene glycol 6000 + sodium sulfate\nsalt + water was investigated experimentally at various pH values\n(4.5, 6, 7.5, and 9) and phosphate ligand mass fractions (PEG–PO<sub>4</sub>) (0%, 25%, 50%, 75%, and 100%) at 30 °C. The results\nshowed that increasing the pH as well as decreasing the ligand concentration\ncauses the expansion of the biphasic region. Binodal curves were also\nfitted using an experimental equation, and the coefficients of these\nequations were estimated for different pH values and ligand concentrations.\nIn addition, the experimental tie-line compositions were estimated\nand correlated using the Othmer–Tobias and Bancroft relations,\nand the equation constants were calculated. Also, the new modified\nnonrandom two-liquid (NRTL) activity coefficient model as a function\nof the ligand concentration and pH was used for the correlation of\nthe experimental tie-line data, and new interaction parameters were\nestimated. The total root-mean-square deviation between the experimental\nand predicted LLE compositions was 0.272%, showing the excellent ability\nof the NRTL model to predict the affinity of the aqueous two-phase\nsystems studied.