Deep eutectic behavior in binary mixtures of protic ionic liquids

The phase behavior of PILs mixture was disclosed and solid–liquid equilibrium data was correlated successfully by the two-suffix Margules equation. [Display omitted] •The deep eutectic behavior of protic ionic liquids was studied.•The solid–liquid equilibrium data was correlated by the two-suffix Margules equation.•The relationship of melting temperature depression and melting point difference.•Quantum calculation was conducted to calculate the exchange energy of pure components. The solid–liquid phase equilibrium of ionic liquids (ILs) is a kind of fundamental data, and the deep eutectic behavior of protic ionic liquids (PILs) is scarcely studied in literature. In this work, four PILs, 1,3-dimethyl-2-imidazolidinone bisulfate ([DMIH][HSO4]), pyridinium bisulfate ([Hpy][HSO4]), 2-methylpyridinium bisulfate ([2-mHpy][HSO4]), and 3-methylpyridinium bisulfate ([3-mHpy][HSO4]), were prepared and six binary mixture systems from these four PILs were used to evaluate the deep eutectic behavior. All six systems were found to have a deep eutectic point in the solid–liquid equilibrium curves, and the largest melting temperature depression is 68.8 K appearing in [DMIH][HSO4]-[Hpy][HSO4]. The eutectic compositions and temperatures were determined, and the solid–liquid equilibrium data of the six binary systems were calculated successfully using the two-suffix Margules equation. The molecular interactions were also investigated using quantum calculations to show the non-ideality in the eutectic systems. Since the deep eutectic behavior is in favor of broadening the liquid window and tuning the physical property of ionic liquids, more binary or ternary eutectic ionic liquids can be prepared as functional materials by just simple mixing. In this sense, the study in this paper helps to elucidate the deep eutectic principle for ILs material design.