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Translation-rotation decoupling and nonexponentiality in room temperature ionic liquids.
Philip J Griffin1, Alexander L Agapov, Alexei P Sokolov
1Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996-1600, USA.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|September 26, 2012
Summary
Structural relaxation and self-diffusion in ionic liquids show weak decoupling near the glass transition temperature. This behavior, along with temperature-dependent relaxation, suggests strong intermolecular interactions govern dynamics.
Area of Science:
- Physical Chemistry
- Materials Science
- Condensed Matter Physics
Background:
- Ionic liquids (ILs) are salts with low melting points, exhibiting unique properties.
- Understanding their dynamics, especially near the glass transition temperature (Tg), is crucial for applications.
- Supercooled liquids often show decoupling of structural relaxation and diffusion.
Purpose of the Study:
- To investigate the temperature dependence of structural relaxation time and self-diffusion in three imidazolium-based room temperature ionic liquids.
- To determine the extent of decoupling between self-diffusion and structural relaxation.
- To analyze the temperature dependence of relaxation times and time-temperature superposition.
Main Methods:
- Light scattering techniques
- Broadband dielectric spectroscopy
- Analysis of temperature-dependent dynamics
Main Results:
- Self-diffusion decouples from structural relaxation as temperature decreases towards Tg.
- The observed decoupling is weaker than in other supercooled liquids of similar fragility.
- Structural relaxation times follow a Vogel-Fulcher-Tamann function over 13 decades.
- Time-temperature superposition is valid for structural relaxation from above the melting point to Tg.
Conclusions:
- Strong and directional intermolecular interactions are likely responsible for the observed dynamics.
- Weak decoupling and valid time-temperature superposition suggest a unified mechanism in these ILs.
- These findings offer insights into the fundamental behavior of ionic liquids.
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