Phase equilibria and thermophysical properties of aqueous solutions of two bis(fluorosulfonyl)imide-based ionic liquids

Abstract

Ionic liquids comprising bis(fluorosulfonyl)imide (FSI) anion have been recognized to be well suited for technological applications as solvent and electrolyte media. In this work, we thoroughly investigated the thermodynamic and transport behavior of binary aqueous systems of two hydrophobic FSI-based ionic liquids, namely [EMIM][FSI] and [BMPYR][FSI]. Systematic measurements of phase equilibria, density, viscosity, and electric conductivity were conducted, our aim being to provide truly reliable information that would substantially upgrade that existing in the literature. Employing DSC measurements on the neat ILs and dynamic measurements of freezing or cloud point temperatures as well as static solubility measurements for aqueous ILs, phase diagrams for both these systems over a 100 K window were established. Rather large solubilities of water in these ILs allow for measurements of various other properties in the homogeneous IL-rich region. Extensive data thus obtained on water activity in this region were correlated for each system simultaneously with the mutual liquid-liquid solubilities, using an extended SSF (Sum of Symmetrical Functions) equation. This global excess Gibbs energy model not only accurately captures all the VLE, LLE, and SLE data we determined experimentally, but also discloses an enthalpy-driven system energetics with hydrophobic hydration effects that are more pronounced in the case of [BMPYR][FSI]. The measurements of density, viscosity, and conductivity covering the systems from the neat ILs to those almost saturated with water gave positive excess volumes and negative viscosity deviations that rise and diminish, respectively, as T increases. The conductivity increases monotonously with increasing water content and temperature. Accurate functional representations were also established for the volumetric, viscosity, and conductivity data, capturing simultaneously their composition and temperature dependence. The linkage between conductivity and viscosity was further analyzed by means of the fractional Walden rule which provided excellent fits for both isopleths and isotherms and indicated comparatively weaker coupling for the [BMPYR][FSI] system and the direction of the isothermal composition changes. Finally, comparing the FSI ILs with their TFSI analogs, the former were found to be clearly less hydrophobic.