(181ar) Application of the CPA Equation of State to Ionic Liquids and Their Mixtures

Ionic liquids (ILs) are a class of solvents with certain characteristics that make them very different than traditional molecular solvents. They are defined as ionic compounds with a melting temperature lower than 100 °C and they are typically constituted by bulky organic cations (like imidazolium, pyridinium, pyrrolidinium, phosphonium or ammonium) and organic or inorganic anions (for example, bis(trifluoromethylsulfonyl)imide, tetrafluoroborate or halides). Some of their attractive properties are very low vapour pressure, high chemical and thermal stability, selectivity and solvation capability, and easiness of recovery [1]. The typical cations that constitute ILs allow different substituents, like alkyl chains of different lengths or even with other functional groups. This leads to a huge number of different possible cations and anions, which can originate an enormous variety of ILs. Additionally, it is possible to adjust some of the properties of ILs (like melting point, density, viscosity or hydrophobicity) by making simple changes in the structure of its ions. For this reason, ILs are regarded as ‘designer solvents’ which can be specifically tailored for each application. Consequently, there has been a growing interest in ILs for applications in different fields such as catalysis, analytical chemistry, electrochemistry, separation processes or biotechnology.

With such a vast number of different ILs available, extensive experimental work becomes time-consuming and expensive. Therefore, it would be of great value to have predictive tools for the calculation of certain properties of ILs as well as their mixtures with other components. In this work, the Cubic Plus Association (CPA) equation of state [2] has been applied to model the properties of two ILs and their phase behavior in mixtures with CO₂ and water. The two ILs are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim][NTf₂]). Several sets of pure compound parameters were estimated for both ILs by fitting the model results to experimental vapor pressure and liquid density data. Associating schemes 2B and 4C were considered for the ILs. According to the results obtained, the model is able to accurately describe the properties of the pure ILs. After this, the sets of pure compound parameters were used to model the vapor-liquid equilibria (VLE) of the two ILs with CO₂. A good description of the experimental VLE data was achieved for both ILs using one temperature-independent binary interaction parameter. The average absolute deviations (AAD) obtained were around 8-13% for [C₂mim][NTf₂] and 12% for [C₄mim][NTf₂]. Similar results were obtained for the modeling of the VLE using both associating schemes considered for the ILs. Finally, the liquid-liquid equilibria (LLE) of the two ILs with water were also modeled. For this kind of phase equilibrium, the model does not provide equally satisfactory results. For the LLE with water and considering associating scheme 4C for the ILs, CPA cannot accurately describe the phase behavior. However, with the optimum sets of pure compound parameters for associating scheme 2B and a temperature-independent binary interaction parameter, good results are obtained, with AAD of 7.3 and 42.4% for the IL-rich phase (for [C₂mim][NTf₂] and [C₄mim][NTf₂], respectively) and 15.5 and 8.6% for the water-rich phase. Additional work is ongoing for the application of the CPA equation of state to other ILs from the same family as well as from other families.

References:

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

[2] Kontogeorgis, G.M., Voutsas, E.C., Yakoumis, I.V., Tassios, D.P. (1996) Ind. Eng. Chem. Res. 35, 4310-4318.