Thermodynamic Modeling of Imidazolium-Based Ionic Liquids with the [PF₆]⁻ Anion for Separation Purposes

Abstract

There is an increased interest in developing accurate tools to relate the physicochemical properties of ionic liquids (ILs) to their microscopic structure as this information is needed to speed up the design of new ionic liquids for chemical and industrial processes. Molecular models can be used for this purpose. We explore here the extended capabilities of a model previously developed in the context of soft-SAFT, by Andreu and Vega in 2007 to reproduce the thermodynamic behavior of imidazolium hexafluorophosphate-based ([C_nmim][PF₆]) ionic liquids. The molecular parameters optimized in the previous work have been used here in a transferable manner; some new members of the [C_nmim][PF₆] family have also been added, as new recent experimental data has been published. The interfacial tensions have been calculated using a Density Gradient Approach and the results have been compared with available experimental data. The solubility of carbon monoxide and hydrogen in those ILs has been studied in the range of temperatures and pressures of application for separation processes. Binary mixtures with other imidazolium ionic liquids with different anions have been calculated, in a predictive manner. Finally, calculations of mixtures of ionic liquids with water also show very good agreement with experimental data. This work highlights the importance of using a simple but robust thermodynamic model, including the right level of interactions, to accurately describe the properties of these highly non-ideal systems.

Keywords:

• carbon monoxide
• equilibrium properties
• hexafluorophospate
• hydrogen
• imidazolium
• ionic liquid
• soft-SAFT equation of state
• water

ACKNOWLEDGEMENTS

F. Llovell acknowledges a JAE-Doctor fellowship from the Spanish Government. Helpful discussions with Edoardo Valente and Rosa M. Marcos are fully appreciated. This work has been partially financed by the Spanish government, Ministerio de Ciencia e Innovación, under projects CTQ2008-05370/PPQ and CENIT SOST-CO2 CEN2008-01027, a CENIT project financed by CDTI, belonging to the Ingenio 2010 program. Additional support from Carburos Metálicos, Air Products Group, and from the Catalan government, under project 2009SGR-666, is also acknowledged.