Modeling liquid-liquid equilibrium of ionic liquid systems with NRTL, electrolyte-NRTL, and UNIQUAC

Characterization of liquid-liquid equilibrium (LLE) in systems that contain ionic liquids (ILs) is important in evaluating ILs as candidates for replacing traditional extraction and separation solvents. Although an increasing amount of experimental LLE data is becoming available, comprehensive coverage of ternary liquid-phase behavior via experimental observation is impossible. Therefore, it is important to model the LLE of mixtures that contain ILs. Experimental binary and ternary LLE data that involve ILs can be correlated using standard excess Gibbs energy models. However, the predictive capability of these models in this context has not been widely studied. In this paper, we study the effectiveness with which excess Gibbs energy models can be used to predict ternary LLE solely from binary measurements. This is a stringent test of the suitability of various models for describing LLE in systems that contain ILs. Three different excess Gibbs free energy models are evaluated: the non-random two-liquid (NRTL) model, the universal quasi-chemical (UNIQUAC) model, and the electrolyte-NRTL (eNRTL) model. In the case of eNRTL, a new formulation of the model is used, based on a symmetric reference state. To our knowledge, this is the first time that an electrolyte excess Gibbs energy model has been formulated for and applied to the modeling of multicomponent LLE for mixtures that involve ILs. Ternary systems (IL, solvent, co-solvent) that exhibit experimental phase diagrams of various types have been chosen from the literature for comparison with the predictions. Comparisons of experimental and predicted octanol-water partition coefficients are also used to evaluate the models studied.