Incorporating a concentration-dependent dielectric constant into ePC-SAFT. An application to binary mixtures containing ionic liquids

Abstract

Primitive thermodynamic models for electrolyte solutions require the dielectric constant ε. This property strongly depends on the concentration of the electrolytes in the mixture. Neglecting this dependency might be reasonable for modeling solutions at low electrolyte concentrations. However, in solutions containing ionic liquids (ILs) and especially for the calculation of liquid-liquid equilibria (LLE) of systems with ILs, liquid phases often contain high IL concentrations. At such conditions, neglecting the influence of concentration on ε is an oversimplification. In this work, an approach to account for the concentration-dependent dielectric constant within the Debye-Hückel theory was implemented into electrolyte Perturbed-Chain Statistical Associating Fluid Theory (original ePC-SAFT). This new approach was then applied to model LLE of binary mixtures containing water and commonly used hydrophobic ILs. These common ILs are comprised of the IL-cations [Cnmim]+, [Cnpy]+, [Cnmpy]+, [Cnmpyr]+, [C4m4py]+ and the IL-anions [BF4]-, [NTf2]-, [PF6]-, [TFO]-. The LLE of binary mixtures water + IL were modeled at ambient pressure and different temperatures with the new ePC-SAFT and with the original ePC-SAFT [Ji et al. DOI: 10.1016/j.fluid.2012.05.029] without the concentration-dependent ε. Overall, the new approach within ePC-SAFT shows superior modeling as well as correlation capability compared to original ePC-SAFT, which was concluded by comparing both models with LLE data from literature.