Liquid-Liquid Equilibrium of Polymer-Inorganic Salt Aqueous Two-Phase Systems: Experimental Determination and Correlation

An aqueous two-phase system (ATPS) is a liquid-liquid extraction method which is commonly employed to separate and purify biomolecules because of their innate properties such as high water content of the phases and the low interfacial tension. Liquid-liquid equilibrium (LLE) data is a basic requirement for the design of any ATPS. In the present investigation, the experimental liquid-liquid equilibrium data for polyethylene glycol 600-inorganic salts (potassium phosphate, sodium and ammonium sulfate) ATPSs were analyzed through experimental determination and correlation at 27 °C.The binodal data for these three systems were first obtained by the titration method and correlated with various empirical equations. The best-fit correlation was chosen and the phase compositions were determined directly by solving a set of equations formed by combining it with the lever rule. The two phase region of the PEG 600-potassium phosphate system was wider due to the higher solubility of the phosphate salt. The tie-line length (TLL) of three ATPS were determined experimentally and correlated by Othmer-Tobias and Bancroft equations. The results suggested that the method of calculation and the corresponding tie-lines are reliable. In addition, the experimental volume ratios of the selected midpoints of all the three systems showed a better agreement with the predicted values.