Direct Correlation between Ionic Liquid Transport Properties and Ion Pair Lifetimes: A Molecular Dynamics Study.

Self-diffusivities as a function of temperature were computed for 29 different ionic liquids (ILs) covering a wide variety of cation and anion classes. Ideal ionic conductivities (σNE) were estimated from the self-diffusivities via the Nernst-Einstein relation. The ion pair (IP) lifetimes (τIP) and ion cage (IC) lifetimes (τIC) of each IL were also computed. A linear relationship between the calculated self-diffusivities and the inverse of IP or IC lifetimes was observed. A similar inverse linear relationship was also observed for ideal ionic conductivity. These relationships were found to be independent of temperature and the nature of the IL. These observations connect macroscopic dynamic properties with local atomic-level motions and strongly suggest that the dynamics of ILs are governed by a universal IP or IC forming and breaking mechanism. Thus, in order to design an ionic liquid with enhanced dynamics, one should consider how to minimize IP or IC lifetimes.