Phase diagrams for liquid–liquid and liquid–solid equilibrium of the ternary polyethylene glycol + di-sodium hydrogen citrate + water system

Abstract

The complete phase diagram for the {polyethylene glycol (PEG)+di-sodium hydrogen citrate+water} with PEG of molar mass 4000gmol−1 was determined at T=298.15K. Composition of the liquid–liquid and liquid–solid equilibria were determined from calibration curves of the solution, and X-ray diffraction analyses were made on the solid. Liquid–liquid equilibria of {PEG4000+di-sodium hydrogen citrate+water} was also studied at T=(298.15, 308.15 and 318.15) K. Furthermore for this system the free energies, enthalpies and entropies of cloud points were calculated at the mentioned temperatures in order to investigate the driving force formation of this two-phase system. To investigate the effect of molar mass of the polymer on the tie-line, similar measurements were also made at T=298.15K on this two-phase system consisting of the PEG with molar masses of 600 and 2000gmol−1. Finally the effect of molar mass of polymer on the binodals of {PEG+di-sodium hydrogen citrate+water} was studied by measuring liquid–liquid equilibrium at T=298.15K for six different molar masses (600–20,000) gmol−1. For representing the experimental binodal data the Merchuk equation in the original form and with the temperature dependency and an empirical equation were used. A temperature dependent Setschenow and the segment-based local composition models (extended NRTL and the modified NRTL) were used to fit the tie-line data of these systems.