Fluid Phase Equilibria 316 (2012) 74–84 Contents lists available at SciVerse ScienceDirect Fluid Phase Equilibria j o ur nal homep age: www.elsevier.com/locate/fluid Liquid–liquid and solid–liquid equilibria in the solutions of poly(ethylene glycol) with several organic solvents G.R. Ivanis a , J.M. Vuksanovic a , M.S. Calado b , M.Lj. Kijevcanin a , S.P. Serbanovic a , Z.P. Visak b,∗ a Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia b Centro Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal a r t i c l e i n f o Article history: Received 3 October 2011 Received in revised form 5 December 2011 Accepted 8 December 2011 Available online 17 December 2011 Keywords: Liquid–liquid equilibrium Solid–liquid equilibrium Molar excess volumes Sustainable solvents Poly(ethylene glycol) a b s t r a c t In this work, liquid–liquid and solid–liquid equilibria (LLE and SLE, respectively), at atmospheric pressure, of the solutions of poly(ethylene glycol) (PEG) with several organic solvents were studied. The studied solvents include benzene, toluene, o-xylene, p-xylene, tetrahydrofuran (THF), pyridine, nicotine, aniline, 1-hexanol, 1-octanol and 1-decanol. Previous and present solubility tests showed that pyridine, nicotine, THF and aniline are completely soluble in liquid PEG200 and PEG400. This study confirmed and extended the previous discoveries that PEG can adjust its polarity and may be soluble with both polar and non- polar compounds as well as that the addition of a methyl group dramatically reduces the solubility of PEG with arenes. These remarkable features were thoroughly reviewed and discussed. Further on, they were used to explain the new results brought by this work – LLE of the solutions of (liquid) PEG200/or PEG400 with xylene isomers and SLE of the solutions of (solid) PEG2050 with all the aforementioned compounds. Molar excess volumes (V E ) of the solutions (PEG200/or PEG400 + benzene/or toluene) were measured at 298.15 K – these results were used to additionally discuss and explain the LLE behavior of these solutions. Finally, the results of this study showed the possibilities for sustainable applications (i) of liquid PEG200/or PEG400 for the separation of of o-xylene from p-xylene and (ii) of (solid) PEG2050 for benzene, toluene, pyridine and aniline treatment. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Poly(ethylene glycol) (PEG) polymer is generally considered to be very environmental-friendly (nonvolatile and biodegradable) as well as low-toxic towards humans and animals, in most of the tox- icity aspects [1]. The latter allowed it to even be established as a food-additive [2,3]. Moreover, PEG and/or its aqueous solutions are nowadays introduced in a variety of sustainable liquid-phase extraction technologies [4], and in biotechnical and biomedical applications [5]. Liquid PEG, which is (practically) nonvolatile, is used as alternative solvent in catalysis [1] and organic synthesis [6], replacing the (toxic) standard organic solvents. From the sol- vent power point of view, liquid PEG has some excellent properties: it has high polarity [7,8] and, being both a good proton donor and acceptor, a good capability to form hydrogen bonds [9]. However, PEG in general has an even more specific ability to change its polar conformation (in polar solvents) to a nonpolar one (in nonpolar solvents) [10] – this issue makes liquid PEGs (particularly those liquid at ambient conditions) somewhat versa- tile solvents that dissolve very well some non-polar compounds ∗ Corresponding author. Tel.: +351 21 841 9229; fax: +351 21 846 4455. E-mail address: zoran.visak@ist.utl.pt (Z.P. Visak). as well (e.g. benzene or toluene). Thus, it was determined pre- viously [11] that benzene is completely miscible in PEG having average molecule weight (Mw) 200, 400 and 600 – PEG200, PEG400 and PEG600, respectively. In the case of toluene this is true only for PE400 and PEG600 since PEG200 exhibits lim- ited miscibility. The latter was found to exist as well in the solutions of both PEG200 and PEG400 in the higher alkyl deriva- tives, such as ethyl- and propyl-benzene. These liquid–liquid equilibria (LLE) follow a common UCST (upper-critical-solution- temperature) behavior – liquid phase separation is provoked as temperature decreases. Following the aforementioned study we have focused our atten- tion to PEG as a “green” (sustainable) solvent. In our recent work [12] we have found that pyridine and nicotine are completely solu- ble in liquid PEG200 and PEG400 and we performed spectroscopic and volumetric studies of these solutions. First part of the present paper is related to these liquid PEG and their solubility with other, practically very important compounds (solvents). These are aro- matic: o- and p-xylene, aniline, cyclic: tetrahydrofuran (THF) and aliphatic: 1-hexanol, 1-octanol, 1-decanol. All the studied compounds, except the two xylene iso- mers, exhibited complete miscibility with both PEG200 and PEG400, within the temperature range of the experiments (293.15–373.15 K). Thus, LLE temperature-composition phase 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.12.013