J. of Supercritical Fluids 30 (2004) 139–144 High-pressure phase behavior of systems with ionic liquids Part III. The binary system carbon dioxide + 1-hexyl-3-methylimidazolium hexafluorophosphate Alireza Shariati, Cor J. Peters * Laboratory of Physical Chemistry and Molecular Thermodynamics, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received 6 May 2003; accepted 3 September 2003 Abstract The phase behavior of a binary mixture consisting of a supercritical fluid and an imidazolium-based ionic liquid was studied experimentally. Carbon dioxide and 1-hexyl-3-methylimidazolium hexafluorophosphate were the selected supercrit- ical fluid and ionic liquid, respectively. A synthetic method was used to measure vapor-liquid boundaries. Results are re- ported for carbon dioxide concentrations ranging from 9.8 to 72.7 mol% and within temperature and pressure ranges of 298.31–363.58 K and 0.64–94.60 MPa, respectively. The experimental results obtained in this work were compared with phase behavior data of the binary system of carbon dioxide together with another member of the same homologous ionic liquid family (1-ethyl-3-methylimidazolium hexafluorophosphate). The CO 2 + [hmim][PF 6 ] binary system has a two-phase region liquid–vapor extending up to very high pressures. © 2003 Elsevier B.V. All rights reserved. Keywords: 1-Hexyl-3-methylimidazolium hexafluorophosphate; Carbon dioxide; Ionic liquid; Phase behavior; Experimental; Supercritical 1. Introduction Room-temperature ionic liquids (ILs) are organic salts, which due to their unique characteristics, are the subjects of many researches nowadays as potential re- placements for organic solvents. ILs have very good solvency power for both organic and inorganic mate- rials, polar and non-polar. ILs have insignificant vapor pressures, therefore, eliminating environmental pollu- * Corresponding author. Tel.: +31-15-278-2660; fax: +31-15-278-8668. E-mail address: c.j.peters@tnw.tudelft.nl (C.J. Peters). tion and highly reducing working exposure hazards in comparison with the conventional organic solvents currently being used in industries. Most ILs are in the liquid state at room temperature and will remain liq- uid up to very high temperatures, so processes such as heterogeneous reactions can instead be performed in a single liquid phase within a wide temperature range. ILs are thermally stable and non-flammable. Based on these outstanding characteristics, Brennecke and Mag- inn [1] gave an extensive perspective of potential in- dustrial applications of ILs in catalytic reactions, gas separations, liquid-liquid extractions, electrolyte/fuel cells, and as lubricants and heat transfer fluids. 0896-8446/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2003.09.001