Deep eutectic behavior in binary mixtures of protic ionic liquids

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 eutec-tic 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. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This work investigated the deep eutectic behavior and solid-liquid phase equilibrium data of protic ionic liquids (PILs) through the preparation of four PILs and six binary mixture systems. The molecular interactions were studied using quantum calculations, and the solid-liquid equilibrium data were successfully calculated using the Margules equation. The findings provide guidance for expanding the application range of ionic liquids and tuning their physical properties.