CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 34 (2010) 222–231 Contents lists available at ScienceDirect CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry journal homepage: www.elsevier.com/locate/calphad Phase diagrams for liquid–liquid and liquid–solid equilibrium of the ternary poly ethylene glycol di-methyl ether 2000 + tri-sodium phosphate + water system at different temperatures and ambient pressure Mohammed Taghi Zafarani-Moattar ∗ , Saeed Nasiri Physical Chemistry Department, University of Tabriz, Tabriz 51664, Iran article info Article history: Received 24 November 2009 Received in revised form 15 March 2010 Accepted 19 March 2010 Available online 13 April 2010 Keywords: Liquid–liquid equilibrium Poly ethylene glycol di-methyl ether Tri-sodium phosphate Local composition models Osmotic virial equation abstract Complete phase diagrams for the poly ethylene glycol di-methyl ether 2000 (PEGDME 2000 ) + Na 3 PO 4 + H 2 O system at T = (298.15, 308.15 and 318.15) K were determined. Liquid–liquid equilibria (LLE) for the aqueous PEGDME 2000 + Na 3 PO 4 system were determined experimentally at T = (298.15, 308.15, 313.15 and 318.15) K. The effects of temperature on the binodals and tie-lines of the investigated aqueous two-phase system (ATPS) were also studied. Furthermore, the modified local composition segment-based NRTL and Wilson models and also osmotic virial equation were used for the correlation and prediction of the liquid–liquid phase behavior of the studied system. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Separation processes are applied in various types of industries including chemical, pharmaceutical, and food. Classical methods involving liquid–liquid extraction use an organic solvent and an aqueous solution as the two immiscible phases for the fractionation and purification of molecules. However, these systems are very toxic and present risks to human health. Aqueous two-phase systems (ATPSs) have been of increasing importance as a method for separation and purification of biomaterials, such as proteins and enzymes, with several advantages compared to conventional methods, such as cost reduction, minimization of the separation stages, high purity products, and high recovery of the materials. An aqueous two-phase system is traditionally formed when two different polymers or a polymer and a salt are mixed with water. These ternary systems split in two phases, both of them water-rich, above a certain critical concentration of polymer or salt [1,2]. In laboratory-scale separations, the most commonly used systems are comprised of the poly ethylene glycol (PEG) and dextran, while for large-scale enzyme extraction, PEG–salt systems are used. The latter systems are more attractive because of their greater selectivity, lower viscosity, lower cost, rapid phase ∗ Corresponding author. Fax: +98 411 3340191. E-mail addresses: zafarani47@yahoo.com, zafarani@tabrizu.ac.ir (M.T. Zafarani-Moattar). disengagement, and availability of commercial separators, which allow a faster and continuous protein separation. Poly ethylene glycol di-methyl ether 2000 (PEGDME 2000 ) is a polymer that has a similar structure to the PEG, thus it can be used to form ATPSs with cosmotropic (i.e. water-structuring) salts. As far as we know there is a few experimental LLE data for ATPSs with this polymer [3,4]. In this regard, the current work is devoted to study the phase behavior of the polymer-based ATPS produced by the addition of the strong cosmotropic salt Na 3 PO 4 , to aqueous solutions of PEGDME 2000 including determination of the complete phase diagram and also how it is to be affected by the temperature. The local composition activity coefficient models such as nonrandom two-liquid (NRTL) and Wilson models have been gaining more attention in the correlation of the experimental vapor–liquid equilibrium (VLE) and LLE data due to their unique characters, since their advent. These models are comprehensive molecular thermodynamic models for systems with molecular and ionic species, molecules and ions of various size, and hydrophobic, hydrophilic and amphiphilic species. At first these models were only used to represent the excess Gibbs energy of aqueous electrolyte [5,6] or polymer solutions [7,8]. For electrolyte solutions the excess Gibbs energy is represented by the sum of the contributions of the long range and the short range interaction terms [5,6]; however, for polymer solutions the combinatorial and the short range interaction terms are used [7,8]. Recently, these models were used to represent the Gibbs energy of aqueous 0364-5916/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.calphad.2010.03.004