Journal of Chromatography A, 1250 (2012) 54–62 Contents lists available at SciVerse ScienceDirect Journal of Chromatography A j our na l ho me p ag e: www.elsevier.com/locate/chroma Generalized linear solvation energy model applied to solute partition coefficients in ionic liquid–supercritical carbon dioxide systems Josef Planeta, Pavel Karásek, Barbora Hohnová, Lenka ˇ St’avíková, Michal Roth ∗ Institute of Analytical Chemistry of the ASCR, v. v. i., Veveˇ rí 97, 60200 Brno, Czech Republic a r t i c l e i n f o Article history: Available online 13 April 2012 Keywords: Supercritical fluid chromatography Ionic liquid Carbon dioxide Organic solute Partition coefficient a b s t r a c t Biphasic solvent systems composed of an ionic liquid (IL) and supercritical carbon dioxide (scCO 2 ) have become frequented in synthesis, extractions and electrochemistry. In the design of related applications, information on interphase partitioning of the target organics is essential, and the infinite-dilution parti- tion coefficients of the organic solutes in IL–scCO 2 systems can conveniently be obtained by supercritical fluid chromatography. The data base of experimental partition coefficients obtained previously in this laboratory has been employed to test a generalized predictive model for the solute partition coefficients. The model is an amended version of that described before by Hiraga et al. (J. Supercrit. Fluids, in press). Because of difficulty of the problem to be modeled, the model involves several different concepts – linear solvation energy relationships, density-dependent solvent power of scCO 2 , regular solution theory, and the Flory–Huggins theory of athermal solutions. The model shows a moderate success in correlating the infinite-dilution solute partition coefficients (K-factors) in individual IL–scCO 2 systems at varying tem- perature and pressure. However, larger K-factor data sets involving multiple IL–scCO 2 systems appear to be beyond reach of the model, especially when the ILs involved pertain to different cation classes. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Organic salts with melting points below 100 ◦ C, or ionic liquids (ILs), have attracted an ever increasing attention from diverse fields of science and technology [1]. A crucial point seems to have been the introduction of air- and moisture-stable ILs in 1992 [2], and the surge of interest in ILs has partly been driven by their extremely low vapor pressures [3–5] as contrasted to those of molecular (organic) solvents. Applications of ILs have become frequented, e.g., in chemical [6,7] and enzymatic [8] synthesis, extractions [9], and electrochemistry [10]. An important part of IL applications have combined the use of ILs and supercritical fluids, notably supercritical carbon dioxide (scCO 2 ). It appears that most ILs can dissolve large amounts of CO 2 while, in turn, ILs themselves are nearly insoluble in scCO 2 . These features are highly useful for delicate extraction of thermolabile organic compounds from IL media with scCO 2 [11,12], and also for application of biphasic IL–scCO 2 systems in phase transfer cataly- sis [13,14]. Several reviews of these and other aspects of IL–scCO 2 systems are available [15–18]. A qualified design of applications combining the use of an IL and scCO 2 certainly requires detailed information on the ∗ Corresponding author. Tel.: +420 532290171; fax: +420 541212113. E-mail address: roth@iach.cz (M. Roth). underlying phase behavior. Naturally, the applications often involve finite concentrations of all components in the IL-rich phase. Consequently, there are numerous studies of the phase behavior in the organic–IL–CO 2 systems at finite concentrations [19–23], and some of them indicate that the phase behavior can be highly com- plex [24]. In the development of thermodynamic models for the organic–IL–CO 2 systems, a complementary experimental informa- tion on infinite dilution partition coefficients of the organic solutes in the IL–scCO 2 systems can be very useful, and the partitioning data can conveniently be obtained from retention measurements by supercritical fluid chromatography (SFC) with the IL serving as the stationary liquid and scCO 2 as the carrier fluid. To determine the partitioning data, SFC retention measurements have been carried out with both wall-coated open tubular capillary columns [25–31] and packed columns [32–34]. In the past few years, predictive modeling of infinite-dilution partition coefficients has been developed along two differ- ent routes. In one of these, Machida et al. [32,33] used the Sanchez–Lacombe mean-field lattice gas equation of state [35,36] to model the equilibrium distribution of a trace amount of organic solute between both phases in an IL–scCO 2 system. The other route employed linear solvation energy relationships (LSERs) that have often been applied to other IL-containing systems as well [37–41]. Since LSERs have originally been developed for incompressible media, their applications to systems with supercrit- ical fluids are complicated because of density-dependent solvent 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.04.016