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Published on: August 2, 2012
Dynamics in supercooled ionic organic liquids and mode coupling theory analysis.
Jie Li1, Irene Wang, Kendall Fruchey
1Department of Chemistry, Stanford University, Stanford, California 94305, USA.
The Journal of Physical Chemistry. A
|September 1, 2006
Summary
Orientational dynamics of supercooled ionic liquids were studied using optical Kerr effect. Mode coupling theory successfully modeled N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide, revealing similarities to van der Waals liquids.
Area of Science:
- Physical Chemistry
- Materials Science
- Condensed Matter Physics
Background:
- Supercooled ionic liquids exhibit complex orientational dynamics.
- Understanding these dynamics is crucial for their application in various fields.
- Previous studies focused on van der Waals liquids, leaving ionic liquids less explored.
Purpose of the Study:
- To investigate the orientational dynamics of supercooled ionic organic liquids: N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (PMPIm) and 1-ethyl-3-methylimidazolium tosylate (EMImTOS).
- To compare the observed dynamics with theoretical models, specifically Mode Coupling Theory (MCT).
- To identify similarities and differences between the dynamics of ionic liquids and conventional van der Waals liquids.
Main Methods:
- Optically heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy was employed.
- Experiments were conducted over a wide range of temperatures, including near the critical temperature (Tc) and glass transition temperature.
- Data analysis involved fitting to power law decays and exponential relaxation, alongside MCT modeling.
Main Results:
- The orientational dynamics of PMPIm showed complex relaxation patterns, including power law decays and a temperature-dependent exponential decay, consistent with van der Waals liquids.
- Mode Coupling Theory (MCT), specifically the Sjögren model, successfully reproduced PMPIm data across a wide time range (ps to ns) and temperatures above Tc.
- EMImTOS exhibited dynamics on the ps timescale, including evidence of the boson peak near its glass transition temperature, a novel finding for ionic liquids in the time domain.
Conclusions:
- The study demonstrates that the orientational dynamics of supercooled ionic liquids share similarities with those of organic van der Waals liquids.
- Mode Coupling Theory provides a good framework for understanding these dynamics, although deviations arise near the glass transition temperature.
- The findings contribute to a deeper understanding of the fundamental dynamics governing ionic liquids, paving the way for their tailored applications.
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