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How is charge transport different in ionic liquids and electrolyte solutions?
Hemant K Kashyap1, Harsha V R Annapureddy, Fernando O Raineri
1Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States.
The Journal of Physical Chemistry. B
|October 26, 2011
Summary
The discrepancy between impedance and NMR conductivity in molten salts and ionic liquids stems from same-charge ion motion. In contrast, electrolyte solutions show opposite-charge ion motion contributing to this conductivity difference.
Area of Science:
- Physical Chemistry
- Materials Science
- Electrochemistry
Background:
- Conductivity measurements in electrolyte solutions and molten salts often show discrepancies between impedance and NMR diffusion experiments.
- Understanding the origins of these discrepancies is crucial for accurate material characterization and predicting ion transport behavior.
Purpose of the Study:
- To elucidate the different origins of conductivity deviations in electrolyte solutions versus molten salts/ionic liquids.
- To develop a theoretical framework for quantifying ion motional coupling in ionic liquids and molten salts using impedance and NMR data.
Main Methods:
- Analysis within a barycentric reference frame.
- Application of momentum conservation laws and linear response theory.
- Derivation of equations relating impedance and NMR measurements to diffusion coefficient matrices.
Main Results:
- In electrolyte solutions, solvent + ions satisfy momentum conservation, leading to correlated motion of oppositely charged ions.
- In molten salts/ionic liquids, only ions satisfy momentum conservation, with same-charge ion motion causing anticorrelation and reducing impedance conductivity.
- The cation-anion distinct diffusion coefficient in binary salts is negative definite, opposing contributions from same-charge ion diffusion.
Conclusions:
- The observed conductivity differences arise from distinct ion correlation mechanisms in different systems.
- The derived equations provide a novel method to experimentally access ion motional coupling in ionic liquids and molten salts.
- Anticorrelated motion of like-charged ions is identified as the primary cause for impedance conductivity reduction in ionic liquids and molten salts.
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