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Systematic Study on the Solubility of Gases in Alkyl-Imidazolium Based Ionic Liquids from a Molecular Modeling Approach

Jordi S. Andreu, Institut de Cičncia de Materials de Barcelona, Consejo Superior de Investigaciones Científicas (ICMAB-CSIC) and MatGas Research Center, Campus de la UAB, Bellaterra. Barcelona, 08193, Spain and Lourdes F. Vega, Institut de Ciencia de Materials de Barcelona. Consejo Superior de Investigaciones Cientificas (ICMAB-CSIC), MatGas Research Center and Air Products and Chemicals, Inc, Campus de la UAB., Bellaterra. Barcelona, 08193, Spain.

Much scientific and technological interest is currently focused on room temperature ionic liquids (ILs). This is due the “designer” properties of some ILs, particularly those based on the imidazolium cation where the physical properties of the liquid may be controlled by incorporation of appropriate functional groups, along with the environmentally friendly properties of ILs, most notably negligible vapour pressure, high thermal stability, good solvent ability toward a wide range of solutes, etc., making ILs attractive candidates as replacements for volatile organic solvents. A careful characterization of them is needed prior they are put into final use for a specific application; this includes knowledge of the relationship between the structure and the properties. This knowledge can be obtained by experimental techniques, simulations and/or theoretical approaches. Theoretical approaches offer the advantage of the speed in which these calculations are performed, provided they are accurate enough to reproduce the experimental data.

We will present results obtained by using the soft-SAFT EoS to describe the solubility of gases in ionic liquids. The particular systems are chosen because of their scientific and technological relevance. We focus here on imidazolium-based ILs with three different anions: BF4, PF6 and Tf2N. Before applying the equation to mixtures, a model for the pure components should be proposed, from which, by fitting to experimental data, the molecular parameters of the equation are obtained. We have observed that the molecular parameters of the imidazolium-based ILs correlate with the molecular weight as the chain length of the alkyl chain increases, making the equation a predictive tool for other compounds not included in the fitting procedure. The solubility of CO2 and other gases in these ionic liquids, at low and high pressures, is predicted with high accuracy. The molecular basis of the equation allow to a fundamental understanding of the solubility process, showing, that at least for these ILs, the solubility of CO2 in them is a physical process, with no specific interactions between CO2 and the ILs.

This work is funded by the Spanish government (CTQ2005-00296/PQ), the Catalan government (SGR2005-00288). Additional funding from MatGas and Air Products is gratefully acknowledged.