Dynamic crossover in [VIO2+][Tf2N−]2 ionic liquid* *Project supported by the Science Foundation of Civil Aviation Flight University of China (Grant Nos. J2019-059 and JG2019-19).

Abstract

Ionic liquids usually behave as fragile liquids, and the temperature dependence of their dynamic properties obeys supper-Arrhenius law. In this work, a dynamic crossover is observed in ([VIO2+][Tf2N−]2) ionic liquid at the temperature of 240–800 K. The diffusion coefficient does not obey a single Arrhenius law or a Vogel–Fulcher–Tammann (VFT) relation, but can be well fitted by three Arrhenius laws or a combination of a VFT relation and an Arrhenius law. The origin of the dynamic crossover is analyzed from correlation, structure, and thermodynamics. Ion gets a stronger backward correlation at a lower temperature, as shown by the fractal dimension of the random walk. The temperature dependence function of fractal dimension, heterogeneity order parameter, and thermodynamic data can be separated into three regions similar to that observed in the diffusion coefficient. The two crossover temperatures observed in the three types of data are almost the same as that in diffusion coefficient fitted by three Arrhenius laws. The results indicate that the dynamic crossover of [VIO2+][Tf2N−]2 is attributed to the heterogeneous structure when it undergoes cooling.