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Translation-rotation decoupling and nonexponentiality in room temperature ionic liquids

Philip J. Griffin1, Alexander L. Agapov2, and Alexei P. Sokolov1,2,3

  • 1Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996-1600, USA
  • 2Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, USA
  • 3Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830-6197, USA

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Phys. Rev. E 86, 021508 – Published 31 August, 2012

DOI: https://doi.org/10.1103/PhysRevE.86.021508

Abstract

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.

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Phys. Rev. E 86, 021508– Published 31 August, 2012
  • Received 5 July 2012
Article part of CHORUS

©2012 American Physical Society

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