Fluid Phase Equilibria 344 (2013) 6–12 Contents lists available at SciVerse ScienceDirect Fluid Phase Equilibria journa l h o me page: www.elsevier.com/locate/fluid “Green Meets Green” – Sustainable solutions of imidazolium and phosphonium ionic liquids with poly(ethylene glycol): Solubility and phase behavior M.S. Calado a , G.R. Ivanis b , J.M. Vuksanovic b , M.Lj. Kijevcanin b , S.P. Serbanovic b , Z.P. Visak a,∗ a Centro Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal b Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia a r t i c l e i n f o Article history: Received 19 September 2012 Received in revised form 11 January 2013 Accepted 16 January 2013 Available online 4 February 2013 Keywords: Liquid–liquid equilibrium Solid–liquid equilibrium Sustainable solvents Ionic liquids Poly(ethylene glycol) a b s t r a c t Liquid–liquid and solid–liquid phase behaviors of the binary solutions of imidazolium [C 2 mim] + or trihexyltetradecyl phosphonium [P 6,6,6,14 ] + ionic liquids having triflate [OTf] - , bistriflamide [NTf 2 ] - or ethyl-sulfate [EtSO 4 ] - anions with poly(ethylene glycol) (PEG) of average molecular mass (Mw) 200, 400 and 2050, were studied. The respective temperature–composition phase diagrams were constructed thus mapping the homogeneous and heterogeneous regions of the studied solutions. The impact of cation, anion and of PEG polymer chain length on the studied phase behavior was investigated and discussed in the terms of the important interactions in the solutions – hydrogen bonds and ion-dipole interactions. Imidazolium cation showed significant advantage over the phosphonium in promoting both liquid–liquid and solid–liquid solubility. On the other hand, the anion effect was found to be dependent on the outcome of the interplay between cation–anion and anion–PEG interactions. As expected, longer PEG chain led to worse solubility with the studied ionic liquids. This study is a first step to explore the possibilities to use the studied (PEG + ionic liquid) solutions as potential sustainable hybrid materials or combined (mixed) solvents of tunable thermophysical proper- ties and/or solvent power. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The search for new hybrid materials and/or sustainable media for (co)polymerization as well as fundamental issues (interactions and solvation mechanism) brought efforts to link ionic liquids and polymers [1]. These attempts were strongly backed by favorite fea- tures of ionic liquids: sustainability (practical non-volatility) [2–5]; ionic nature (polarity) and good ability to form hydrogen bonds (fine hydrogen bond basicity and acidity) [6–14]; diversity and versatility in solvent power – being ionic (polar) and due to the presence of non-polar (aliphatic) domains they dissolve both polar and non-polar compounds [5]. The most straightforward way to join these two large classes of compounds is established through the direct solubilization of (co)polymers by ionic liquids [1]. In this respect several studies [15–20] presented phase diagrams that mapped homogeneous and heterogeneous regions of (ionic liquids + polymer) solutions. These diagrams showed both upper critical and lower critical solution temperature (UCST and LCST, respectively) liquid phase behav- ior. The latter phenomenon – phase demixing as temperature is increased – is generally not a very common phenomenon. However, ∗ Corresponding author. Tel.: +351 21 841 9229; fax: +351 21 846 4455. E-mail addresses: zoran.visak@ist.utl.pt, zoranvisak@gmail.com (Z.P. Visak). it was frequently found in the aforementioned studies thus imply- ing the specific phase behavior of ionic liquid–polymer solutions – the fine interplay between enthalpic and entropic contributions, where the latter have a particular role [16]. Poly(ethylene glycol) (PEG) is considered environmentally acceptable, non-toxic, practically eatable polymer, according to the detailed and complex toxicity studies [21]. Liquid PEG, is virtually nonvolatile and represents a suitable substitution for clas- sical (toxic) solvents in catalysis [21] and organic synthesis [22]. Recently we have pointed out the diverse abilities of PEG as a solvent toward polar (pyridine and nicotine) [23] and non-polar solutes (arenes) [24]. Namely, PEG is highly polar [25,26] and both good proton donor and proton acceptor [27]. However, it can some- times change its polar conformation depending on the polarity of the other constituent – it remains polar in polar solvents while turns into nonpolar in nonpolar ones [28]. This work is a continuation of our previous research on PEG as a sustainable solvent [23,24] and follows the slogan “Green Meets Green” – investigating the mutual solubility of sustainable compounds such as PEG and ionic liquids and the respected phase behavior. In particular, it is focused on the solutions of imidazolium [C 2 mim] + and trihexyltetradecyl phosphonium [P 6,6,6,14 ] + ionic liquids having trifluoromethanesulfonate (tri- flate) [OTf] - , bis{(trifluoromethyl)sulfonyl}amide (bistriflamide) [NTf 2 ] - or ethyl-sulfate [EtSO 4 ] - anions with PEG of average 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.01.019