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On enhanced translational diffusion or the fractional Stokes-Einstein relation observed in a supercooled ionic liquid
1Naval Research Laboratory, Washington, DC 20375-5320 and Dipartimento di Fisica and INFM (UdR Pisa), Università di Pisa, Largo B. Pontecorvo 3, I-56127, Pisa, Italy. ngai@estd.nrl.navy.mil
The Journal of Physical Chemistry. B
|December 22, 2006
Summary
The Stokes-Einstein laws do not apply to the supercooled ionic liquid BMIM-HFP. Instead, fractional relations and enhanced translational diffusion are observed, similar to nonionic liquids.
Area of Science:
- Physical Chemistry
- Materials Science
- Condensed Matter Physics
Background:
- Supercooled molecular ionic liquids exhibit complex dynamics.
- Traditional Stokes-Einstein and Debye-Stokes-Einstein relations often fail to describe diffusion in these systems.
Purpose of the Study:
- To investigate the validity of Stokes-Einstein and Debye-Stokes-Einstein laws in a supercooled molecular ionic liquid.
- To explore the relationship between translational diffusion and molecular dynamics in ionic liquids.
Main Methods:
- Experimental studies on 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP).
- Analysis of translational diffusion and comparison with established theoretical models.
Main Results:
- The Stokes-Einstein and Debye-Stokes-Einstein laws were found to be inapplicable.
- Enhanced translational diffusion and fractional Stokes-Einstein/Debye-Stokes-Einstein relations were observed.
- Similar behavior was noted in nonionic glass-forming liquids.
Conclusions:
- Fractional dynamics are a common feature in supercooled glass-forming liquids, including ionic liquids.
- The coupling model provides a framework to explain these fractional relations.
- Experimental data determined critical parameters for the coupling model in BMIM-HFP.
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