Predicting the ionicity of ionic liquids in binary mixtures based on solubility data

Abstract

Ionic liquids (ILs) are ionic compounds which are liquid at room temperature and pressure, and can be applied in many fields, such as industrial synthesis, catalysis, electrochemistry, and liquid-liquid separations. To enhance the description of mixtures containing ILs and, consequently, to allow a more rigorous evaluation of the feasibility of chemical processes, it is important to study properties such as the ionicity (α) of ionic liquids, i.e., the fraction of dissociated molecules. However, experimental determinations of ionicity (also known as dissociation degree or dissociation extent) are time-consuming and require the handling of expensive equipment, so the prediction of this property as a function of the system's composition using thermodynamic modelling is of great relevance. For that purpose, the UNIversal QUAsi-Chemical (UNIQUAC) model was combined with the Pitzer-Debye-Hückel (PDH) equation, which is often referred as PDH+UNIQUAC, and it was used to predict the ionicity of ILs in binary mixtures based on liquid-liquid equilibria (LLE) data. The results were compared with the available experimental data from literature and low deviations were observed, validating thereafter the developed approach. In total, the IL ionicity was predicted, as function of composition, for fourteen ionic liquid / solvent (water or 1-butanol) binary systems. Further, a sensitivity analysis was performed to evaluate the influence of the LLE uncertainties on ionicity (α), and it was concluded that α was not significantly affected by small deviations, for which the methodology proposed in this work was considered robust. The prediction of IL ionicity with composition using PDH+UNIQUAC thermodynamic modelling was considered a breakthrough, since very accurate results were achieved without the need of undergoing complex experiments.