Fluid Phase Equilibria 296 (2010) 173–177 Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid Solute partitioning in polymer–salt ATPS: The Collander equation Sara C. Silvério a,b , Oscar Rodriguez a , José A. Teixeira b , Eugénia A. Macedo a,∗ a LSRE/LCM–Laboratory of Separation and Reaction Engineering, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b IBB–Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal article info Article history: Received 18 January 2010 Received in revised form 14 April 2010 Accepted 19 April 2010 Available online 24 April 2010 Keywords: Aqueous two-phase system Polymer–salt Collander equation Proteins abstract The partition coefficients for several solutes (five nitrophenylated-monosaccharides and four proteins) were experimentally determined, at 23 ◦ C, in three different tie-lines of two polymer–salt aqueous two- phase systems (ATPS): UCON-K 2 HPO 4 and UCON-NaH 2 PO 4 . These partition coefficients together with others obtained from the literature for five dinitrophenylated-amino acids were used to investigate the suitability of the Collander equation to correlate partition coefficients in polymer–salt ATPS. This equation was first proposed to describe the linear correlation between partition coefficients of solutes in differ- ent water–organic solvent systems. More recently, it was proved that partition coefficients for several biomolecules in polymer–polymer ATPS can also be correlated with this equation. In this work, several correlations were tested: partition coefficients obtained for different tie-lines within the same system and also partition coefficients obtained from different systems. In both cases, a linear relation was observed, despite a less satisfactory correlation was found when different ATPS were compared. Overall, it was demonstrated that the Collander equation can be used to satisfactorily correlate solute partitioning in the studied polymer–salt ATPS. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Aqueous two-phase systems (ATPS) are biphasic systems com- posed mainly by water. This kind of systems can be obtained combining aqueous solutions of different compounds (two poly- mers, a polymer and a salt, or other alternative components) that become partially immiscible above some critical conditions like concentration or temperature. ATPS have been successfully applied for the separation of different kinds of solutes, such as metal ions [1,2], proteins [3,4], DNA and RNA [5], cells [6] or viruses [3]. The Collander equation was first developed in 1947 for par- titioning in (water + organic solvent) systems [7]. In his original work, R. Collander observed that partition coefficients of several organic solutes in a given organic solvent/water binary system plotted against the partition coefficients of the same solutes in a different organic solvent/water system gave a straight line when using a logarithmic scale. Therefore, the following linear equation was obtained: ln K 1 = a · ln K 2 + b (1) where K 1 and K 2 represent the partition coefficients in the sys- tems 1 and 2, respectively, and a and b are two empirical constants. The Collander equation assumes a linear relation between the log- ∗ Corresponding author. Tel.: +351 22508 1653. E-mail address: eamacedo@fe.up.pt (E.A. Macedo). arithms of the partition coefficient of solutes in different biphasic systems. During the years this equation has been used in many different situations [8–10]. Recently, the partition coefficients for several biomolecules in different polymer–polymer ATPS were suc- cessfully correlated according to the Collander equation [11,12]. In this work the linear correlation described by the Colander equation was investigated in polymer–salt ATPS. For this purpose, the partition coefficients of several solutes (five nitrophenylated- monosaccharides, NP-monosaccharides, and four proteins) were experimentally determined in three different tie-lines of two biphasic systems: UCON-K 2 HPO 4 and UCON-NaH 2 PO 4 . These experimental data were completed with data from the literature for 5 dinitrophenylated-amino acids (DNP-amino acids) previously reported for the same ATPS. The polymer used, UCON, belongs to the group of the so-called thermo-separating polymers. When heated above the critical temperature (about 50 ◦ C) UCON separates from the aqueous solution, thus facilitating polymer recovery and reuti- lization [13,14]. 2. Experimental 2.1. Materials UCON 50-HB-5100, a random copolymer (average molecular weight Mr = 3900) of 50% ethylene oxide and 50% propylene oxide, was obtained from Union Carbide (NY, USA). Di-potassium hydro- gen phosphate (K 2 HPO 4 ) (anhydrous, 99.99 Suprapur) and sodium 0378-3812/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2010.04.009