Fluid Phase Equilibria 294 (2010) 15–30 Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid Modeling ionic liquids and the solubility of gases in them: Recent advances and perspectives Lourdes F. Vega a,b,c,∗ , Oriol Vilaseca a,b , Fèlix Llovell b , Jordi S. Andreu a,b a MATGAS Research Center, Campus de la UAB, 08193 Bellaterra, Barcelona, Spain b Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones Científicas (ICMAB-CSIC), Campus de la UAB, 08193 Bellaterra, Barcelona, Spain c Carburos Metálicos – Air Products Group, C/Aragón, 300 Barcelona, Spain article info Article history: Received 19 November 2009 Received in revised form 7 February 2010 Accepted 9 February 2010 Available online 13 February 2010 Keywords: Ionic liquids CO2 solubility Weak complexation Molecular-based models Classical equations Lattice models Soft-SAFT tPC-PSAFT SWCF abstract The fascinating properties of ionic liquids, their versatility for different applications and their highly non-ideal behavior have promoted the study of these systems from different perspectives. This article provides an overview of the different approaches that have been applied to describe the thermodynamic behavior of ionic liquids and the solubility of selected compounds in them, including carbon dioxide, hydrogen, water, BF 3 and other compounds. The paper deals with some of the most recent and refined approaches involving physical models developed to characterize the ionic liquids. Emphasis is put on the models based on statistical mechanics, highlighting the advantages of these models versus classical ones. New modeling results involving the chemical association of BF 3 in ionic liquids and interfacial properties of selected ionic liquids within the framework of soft-SAFT are also presented. It is seen that the great advance in the refined modeling tools allows not only quantitative agreement with known experimental data, but also a guide to some of the physics governing the behavior of these systems, a step forward into developing ad hoc ionic liquids for specific applications. © 2010 Published by Elsevier B.V. 1. Introduction Ionic liquids, also known as liquid electrolytes, ionic melts, ionic fluids, liquid salts, or ionic glasses, is a term generally used to refer to salts that form stable liquids. Nowadays it is considered that any organic salt that is liquid below 100 ◦ C falls into this category. They are usually formed by a large organic cation like quaternary ammo- nium, imidazolium or pyridinium ions combined with an anion of smaller size and more symmetrical shape such as [Cl] − , [Br] − , [I] − , Abbreviations: AMQs, additive molar quantities; COSMO-RS, COnductor like Screening MOdel for Realistic Solvents; EoS(s), equation(s) of state; GC, group contri- bution method; IFP, Institut Franc ¸ ais du Pétrole; IL, ionic liquid; LJ, Lennard–Jones; LLE, liquid–liquid equilibrium; LLV, liquid–liquid–vapor; NRTL, nonrandom two- liquid model; NRTL-SAC, nonrandom two-liquid segment activity coefficient model; PC, perturbed chain; PCM, polarizable continuum model; PR, Peng–Robinson; pVT, pressure–volume–temperature; RK, Redlich–Kwong; RST, Regular Solution Theory; SAFT, Statistical Associating Fluid Theory; SLE, solid–liquid equilibrium; SWCFs, square-well for chain fluids equation; tPC-PSAFT, truncated Perturbed Chain Polar Statistical Associating Fluid Theory; UNIFAC, universal functional activity coefficient model; UNIQUAC, universal quasi-chemical approach; vdW, van der Waals; VLE, vapor–liquid equilibrium. ∗ Corresponding author at: MATGAS Research Center, Campus de la UAB, 08193 Bellaterra, Barcelona, Spain. Tel.: +34 935 929 950; fax: +34 935 929 951. E-mail addresses: vegal@matgas.com, lvega@icmab.es (L.F. Vega). [BF 4 ] − , [PF 6 ] − , [Tf 2 N] − , etc., although some symmetric cations are also combined with asymmetric anions to form ionic liquids. In spite of their strong charges, their asymmetry frustrates them from being solid below 100 ◦ C and this is why they remain liquid at these low temperatures. It is believed that the first synthesized ionic liquid reported in the literature is ethanolammonium nitrate, published by Gabriel [1]. However, one of the earlier known truly room-temperature ionic liquids was [EtNH 3 ] + [NO 3 ] − , the synthesis of which was pub- lished in 1914 [2,3]. Much later, different ionic liquids based on mixtures of 1,3-dialkylimidazolium or 1-alkylpyridinium halides and trihalogenoaluminates, initially developed for their use as elec- trolytes, were to follow [4,5]. Ionic liquids remained unused for years, mostly because of their moisture sensitivity and their acid- ity/basicity (the latter can sometimes be used as an advantage). However, when in 1992, Wilkes and Zawarotko [6] reported the preparation of ionic liquids with a new set of alternative, ‘neutral’, weakly coordinating anions such as hexafluorophosphate ([PF 6 ] − ) and tetrafluoroborate ([BF 4 ] − ), a much wider range of applications for ionic liquids were envisioned, and this has been a field of con- tinuous growth since then. There are some key properties of these compounds that make them particularly attractive for different applications: in fact, their extremely low volatility has become one of their most important 0378-3812/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.fluid.2010.02.006