(620b) Thermodynamic Study of Ionic Liquids and Their Mixtures for Separation and Extraction Processes Using the Soft-SAFT EoS

Room Temperature Ionic Liquids (RTILs) are receiving much attention in recent years from both, academia and industry. They are defined as salts that remain at the liquid state at ambient temperature. This is caused by the asymmetry between the anion (generally a small molecule such as [Cl], [BF4], [PF6], etc.) and the cation that usually includes a cyclic compound and a alkyl chain. The difference in size does not allow the structures to order in order to reach the solid state at room temperature. The most interesting property of ionic liquids is their negligible vapor pressure. In addition, their high solubility rates with CO2 and other gases make them particularly suitable to substitute volatile organic solvents commonly used in separation and extraction processes. High thermal and electronic stability, high ionic conductivity, a wide liquid temperature and good solubility characteristics complete the list of advantages versus other traditional compounds for different applications [1]. Finally, it is noteworthy to mention their tunability, which means that their properties can be modified by changing the anion or the cation.

As a result, the interest for ionic liquids has increased significantly, and a considerable amount of groups are currently working in the synthesis of ionic liquids for particular applications. From a theoretical perspective, ionic liquids are not as easy to model, due to the relative complexity of the molecule, and several efforts have been done in the last 5 years in order to find appropriate models for their accurate description [2].

In this contribution, we present a complete thermodynamic characterization of the three main families of current imidazolium ionic liquids, those with [BF4], [PF6] and [Tf2N] as anions, by means of the statistical mechanics based soft-SAFT equation of state [3]. This characterization includes a comparison between a relatively simple model [4]-[5], where the cation and the anion are considered as a chain, and a contribution-group method approach, where the cation and the anion are separated and have a specific model. The associating interactions within the molecules are explicitly considered. Single phase and two-phase equilibrium, enthalpy of vaporization, interfacial tension and derivative properties are provided in a wide range of temperatures and pressures in good agreement with experimental data. Then, the solubility of CO2, Xenon, Hydrogen and low hydrocarbons in these families is predicted. Finally, a study the dissociation of ionic liquids in water is also performed.

This tool helps in getting additional insights into the underlying mechanisms governing the behavior of these systems, which are the basic knowledge needed for a rational design previous to their use.

Bibliography:

[1] T. Welton, Chem. Rev. 99 (1999) 2071-2084.

[2] L. F. Vega, O. Vilaseca, F. Llovell, J. S. Andreu, Fluid Phase Equilib. (2010) doi.org/10.1016/j.fluid.2010.02.006

[3] F. J. Blas, L. F. Vega, Mol. Phys. 1997, 92, 135-150.

[4] J. S. Andreu, L. F. Vega, J. Phys. Chem C (2007), 111, 16028.

[5] J. S. Andreu, L. F. Vega, J. Phys. Chem B (2008), 112, 15398.