An expanding field of research involves a class of tunable solvents known as room temperature ionic liquids (ILs). ILs are being investigated for a wide variety of reaction, separation and extraction processes involving liquid-liquid phase behavior. For instance, certain ILs have been shown to selectively extract alcohols from fermentation broths and recover amino acids from aqueous media [1,2]. Since the number of possible systems involving ILs is enormous, comprehensive coverage of ternary liquid-phase behavior via experimental observation is impossible. Therefore, it is important to model the liquid-phase behavior of mixtures containing ILs.

Experimental binary and ternary LLE data involving ILs can be correlated using conventional excess Gibbs energy models. However, the predictive capability of these models in this context is problematic. In previous work, we studied the effectiveness with which NRTL, UNIQUAC and electrolyte-NRTL (eNRTL) [3] could be used to predict ternary LLE for systems containing ILs from binary measurements and pure component data [4,5]. While there were some successes, there were also many cases in which the predictions were qualitatively incorrect. Subsequently, for systems containing water, we proposed a novel asymmetric framework in which the ionic liquid was assumed to be completely dissociated in the aqueous phase and completely paired (molecular) in the organic/IL-rich phase. Thus, equations for the Gibbs energy and equipotential conditions were developed [6] in which an electrolye model (eNRTL) was used to represent the nonideality in the aqueous phase and a conventional non-electrolyte gE model was used to represent the organic/IL-rich phase. This asymmetric framework provided qualitative improvements in ternary IL/solvent/water diagram predictions over using the gE models alone in a standard symmetric framework [7].

In this presentation, a new general thermodynamic framework that accepts different degrees of partial dissociation in different phases is presented. ILs, like all electrolytes, ionize to varying degrees in different phases, and an adequate physical description of their ionic state is important to modeling their LLE behavior. We also use a novel approach for calculating the degree of IL dissociation, based on using only conductivity and viscosity measurements. These degrees of ionization are incorporated into the new asymmetric partial dissociation model by using well known electrolyte activity coefficient models [3,8], which depend on mixed-solvent dielectric constants and IL concentrations. The resulting model is expressed in terms of equations for the total Gibbs energy and for the equipotential conditions. We present new strategies to use in connection with these equations to estimate model parameters using only binary and pure component data. In this work, we show that, when the degrees of partial dissociation are accounted for in this way, it is possible to obtain ternary LLE predictions with these binary parameters that are more accurate than those obtained from previous models.

[1] Fadeev, A. G.; Meagher, M. M., "Opportunities for Ionic Liquids in Recovery of Biofuels." Chem. Commun. 2001, 3, 295-296.

[2] Wang, J. J.; Pei, Y. C.; Zhao, Y.; Hu, Z. G., "Recovery of amino acids by imidazolium based ionic liquids from aqueous media." Green Chem. 2005, 7, (4), 196-202.

[3] Chen, C.-C.; Song, Y., "Extension of Nonrandom Two-Liquid Segment Activity Coefficient Model for Electrolytes." Ind. Eng. Chem. Res. 2005, 44, 8909-8921.

[4] Simoni, L. D.; Lin, Y.; Brennecke, J. F.; Stadtherr, M. A., "Reliable Computation of Binary Parameters in Activity Coefficient Models for Liquid-Liquid Equibrium." Fluid Phase Equilib. 2007, 255, 138-146.

[5] Simoni, L. D.; Lin, Y.; Brennecke, J. F.; Stadtherr, M. A., "Modeling Liquid-Liquid Equilibrium of Ionic Liquid Systems with NRTL, Electrolyte-NRTL, and UNIQUAC." Ind. Eng. Chem. Res. 2008, 47, 256-272.

[6] Simoni, L. D.; Brennecke, J. F.; Stadtherr, M. A., "Asymmetric framework for predicting liquid-liquid equilibria of ionic liquid-mixed solvent systems: I. Theory and parameter estimation." in final preparations 2008.

[7] Simoni, L. D.; Brennecke, J. F.; Stadtherr, M. A., "Asymmetric framework for predicting liquid-liquid equilibria of ionic liquid-mixed solvent systems: II. Ternary system predictions." in final preparations 2008.

[8] Sander, B.; Fredenslund, A.; Rasmussen, P., "Calculation of vapour-liquid equilibria in mixed solvent/salt systems using an extended UNIQUAC equation." Chem. Eng. Sci. 1986, 41, (5), 1171-1183.