Measurements and thermodynamic modelling of liquid–liquid equilibrium of PEG 6000–phosphate affinity aqueous two-phase systems at various ligand concentrations and pH

Abstract

ABSTRACT The equilibrium behaviours of affinity two-phase liquid−liquid systems composed of polyethylene glycol (PEG) 6000 + phosphate salt + water were experimentally investigated at various pH values (4.5, 6, 7.5 and 8.4) and phosphate ligand mass fractions (PEG–PO4) (0%, 25%, 50%, 75%, 100%). The binodal curves were fitted to an empirical equation and the coefficients for the different pH and ligand concentration were estimated. The experimental tie-line compositions were estimated and correlated using the Othmer–Tobias and Bancroft equations, and the parameters were evaluated. The effects of the medium pH and the ligand amount on the phase behaviour were studied. It is found that the pH increasing and ligand concentration decreasing cause an expansion of the biphasic region. The liquid–liquid equilibrium experimental data obtained were well correlated to the activity coefficients of the non-random two-liquid (NRTL) model, and the mean deviations less than 0.68% between experimental and calculated mass fractions show that the NRTL activity model has a good ability to correlate the equilibrium compositions of the present system.