J. of Supercritical Fluids 88 (2014) 46–55 Contents lists available at ScienceDirect The Journal of Supercritical Fluids j o ur na l ho me page: www.elsevier.com/locate/supflu High pressure density and solubility for the CO 2 + 1-ethyl-3-methylimidazolium ethylsulfate system Pedro J. Carvalho a , Teresa Regueira b , Josefa Fernández b , Luis Lugo b,c , Javid Safarov d , Egon Hassel d , João A.P. Coutinho a,∗ a Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal b Laboratorio de Propiedades Termofísicas, Departamento de Física Aplicada, Universidade de Santiago de Compostela, E1782 Santiago de Compostela, Spain c Departamento de Física Aplicada, Facultade de Ciencias, Universidade de Vigo, 36310 Vigo, Spain d Institute of Technical Thermodynamics, University of Rostock, Albert-Einstein-Str. 2, D-18059 Rostock, Germany a r t i c l e i n f o Article history: Received 7 January 2014 Received in revised form 24 January 2014 Accepted 25 January 2014 Keywords: Ionic liquids Carbon dioxide Solubility Density High pressure ILs a b s t r a c t The solubility and density of the CO 2 + 1-ethyl-3-methylimidazolium ethylsulfate system were investi- gated. The carbon dioxide solubility in the IL was measured in the temperature range 273–413 K, for pressure up to 5 MPa and CO 2 mole fractions ranging from 0.02 to 0.5 using the isochoric method, while the system density was carried out at temperatures ranging from 278.15 K to 398.15 K, pressures from 10 MPa to 120 MPa and 0.2, 0.4, 0.7 and 0.8 CO 2 mole fractions. Similar to what was previously observed for phosphonate-based ILs, the ionic liquid high polarity leads to positive deviations from ideality resulting from unfavorable interactions with the CO 2 . The results from the density and solubility derived properties show that the system presents important negative excess molar volumes, over the whole range of compositions and temperatures, and a negative entropy of solvation that suggests an increase in ordering of the solvent molecules surrounding the solute. The observed negative excess molar volumes result from the large difference between the molecular volumes of the species involved, with the small carbon dioxide molecules occupying the empty spaces between the larger IL ions, supporting the notion that the carbon dioxide, upon dissolution, occupies essentially the bulk free volume since the IL does not significantly expand upon gas absorption. These results portray ionic liquids as a porous media, like a soft sponge, with a huge free volume in which large amounts of carbon dioxide are able to accommodate during the dissolution process. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Outstanding properties, allied to the possibility of fine-tuning them to target a specific application or overcome current process and/or legal limitations, have made ionic liquids (ILs) one of the most interesting, relevant and, consequently, studied solvents of the decade. The replacement of volatile organic compounds on gas sepa- ration processes rose as one of the most promising applications for ILs, with a great deal of work being carried on solubilities and thermophysical properties of ILs + carbon dioxide (CO 2 ) binary systems. Despite the large amount of work reported in literature, the mechanism of solvation is still not widely accepted. Several authors have complemented experimental measurements of thermophysical and thermodynamic properties with spectroscopy measurements [1–8] to ultimately understand the mechanism of solvation and the interactions that control the CO 2 solubility ∗ Corresponding author. E-mail address: jcoutinho@ua.pt (J.A.P. Coutinho). in ILs. Even though the interactions observed by spectroscopic techniques are of high relevance for an understanding of the solvation of CO 2 , they are not enough to fully explain its solubility or the deviations to the ideal behavior observed on these systems [9–11]. In fact, based on the solubility and spectroscopic evidence for the interactions of CO 2 with BF 4 and PF 6 anions, Kazarian et al. [12] were the first to recognize that “the strength of the interactions cannot be solely responsible for the solubility of CO 2 in ionic liquids”. Later, Seki et al. [8] reported that, although the interactions of CO 2 with BF 4 and PF 6 anion-based ILs are stronger than those with the NTf 2 , the solubility of CO 2 on these ILs is larger than in the former, and thus the interactions alone are not enough to provide an explanation for the CO 2 sorption. Furthermore, these authors recognize that the strong Lewis acid–base interactions observed have no promotional effect on the solubility of CO 2 [8]. Shi and Maginn [13] have shown, through atomistic simulation, that the [C 6 mim][NTf 2 ] underlying structure is not disturbed upon the CO 2 dissolution. Moreover, the authors have shown that the CO 2 absorbs mainly in regions of free volume and that CO 2 associates strongly with the ions, “collapsing” the liquid and reducing the overall volume expansion. Since the IL does not present cavities 0896-8446/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2014.01.011