Phase separation in aqueous solutions of polypropylene glycol and sodium citrate: Effects of temperature and pH

The phase diagrams for the {polypropylene glycol (PPG) 400 + tri-sodium citrate} aqueous biphasic systems (ABS) were determined experimentally at T = (293.15, 298.15, 308.15, and 318.15) K. Various medium pH values (3.00, 4.00, 5.00, 6.00 and 7.00) were assayed at T = 298.15 K. The phase boundary data were satisfactorily fitted to three empirical nonlinear equations. Furthermore, the phase behavior of {PPG + tri-sodium citrate} ABS was described by the modified version of the segment-based local composition NRTL model. In this work, factors affecting the phase-forming capability in the aqueous solutions containing PPG and a salt, such as temperature and pH were also evaluated. In the investigated system, the effects of temperature and the aqueous medium pH on the phase-forming capability were assessed based on the salting-out coefficient (kS) obtained from fitting the tie-line data to a Setschenow-type equation. With increasing the two-phase area due to an increased temperature or pH, the values of kS increase. The effect of pH on the ABS-promoting capability of citrate anions in the aqueous PPG solutions was also explained based on the structural hydration Gibbs energy (ΔGhyd) of the citrate anions at different pH. The mutual immiscibility for the PPG and sodium citrate in water was observed to promote when the citrate ions have more negative ΔGhyd value due to an increased medium pH value.