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

Abstract 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 ( k S ) 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 k S 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 (Δ G hyd ) 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 Δ G hyd value due to an increased medium pH value.