Translation-rotation decoupling and nonexponentiality in room temperature ionic liquids

Journal Article · Sun Jan 01 00:00:00 EST 2012 · Physical Review E

DOI:https://doi.org/10.1103/PhysRevE.86.021508· OSTI ID:1096339

Griffin, Phillip ^[1]; Agapov, Alexander L ^[2]; Sokolov, Alexei P ^[2]

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1. University of Tennessee (UTK) and Oak Ridge National Laboratory (ORNL)
2. ORNL

Using a combination of light scattering techniques and broadband dielectric spectroscopy, we have measured the temperature dependence of structural relaxation time and self diffusion in three imidazolium-based room temperature ionic liquids: [bmim][NTf2], [bmim][PF6], and [bmim][TFA]. A detailed analysis of the results demonstrates that self diffusion decouples from structural relaxation in these systems as the temperature is decreased toward Tg. The degree to which the dynamics are decoupled, however, is shown to be surprisingly weak when compared to other supercooled liquids of similar fragility. In addition to the weak decoupling, we demonstrate that the temperature dependence of the structural relaxation time in all three liquids can be well described by a single Vogel-Fulcher-Tamann function over 13 decades in time from 10 11 s up to 102 s. Furthermore, the stretching of the structural relaxation is shown to be temperature independent over the same range of time scales, i.e., time temperature superposition is valid for these ionic liquids from far above the melting point down to the glass transition temperature.We suggest that these phenomena are interconnected and all result from the same underlying mechanism strong and directional intermolecular interactions.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: DE-AC05-00OR22725
• OSTI ID: 1096339
• Journal Information: Physical Review E, Vol. 86, Issue 2; ISSN 1539--3755
• Country of Publication: United States
• Language: English