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Microviscosity Offered by Ionic Liquids and Ionic Liquid-Glycol Mixtures Is Probe Dependent
Anu Kadyan1, Siddharth Pandey1
1Department of Chemistry, Indian Institute of Technology Delhi , Hauz Khas, New Delhi 110016, India.
The Journal of Physical Chemistry. B
|January 21, 2017
Summary
Spectroscopic probes reveal ionic liquid (IL) and IL-TEG mixture fluidity, but microviscosity trends do not correlate with bulk viscosity. Probe responses depend on the specific probe and solvent interactions.
Area of Science:
- Physical Chemistry
- Materials Science
- Supramolecular Chemistry
Background:
- Ionic liquids (ILs) are versatile solvents whose properties are governed by complex interactions.
- Understanding the local environment (cybotactic region) around solutes is crucial for predicting chemical process outcomes.
- Spectroscopic probes offer insights into microviscosity and solute-solvent interactions within IL-based media.
Purpose of the Study:
- To investigate the fluidity of 11 ionic liquids (ILs) and their equimolar mixtures with tetraethylene glycol (TEG) using various spectroscopic microviscosity probes.
- To assess the correlation between probe-reported microviscosity and bulk dynamic viscosity.
- To elucidate the role of IL cation, anion, and C2-H functionalities in hydrogen bonding and local environment dynamics.
Main Methods:
- Utilized five fluorescence probes from three classes: intramolecular excimer formation, steady-state fluorescence anisotropy, and fluorescence intensity.
- Studied 11 ILs and their equimolar mixtures with TEG under ambient conditions.
- Compared probe responses with bulk dynamic viscosity measurements of the IL and IL-TEG systems.
Main Results:
- No consistent trend was observed between probe-derived microviscosity and the bulk dynamic viscosities of the ILs and IL-TEG mixtures.
- The cybotactic region viscosity was probe-dependent, varying relative to neat IL and TEG viscosities.
- No evidence was found for the C2-H group on imidazolium cations significantly influencing IL or IL-TEG hydrogen bonding.
Conclusions:
- Microviscosity measurements using spectroscopic probes provide a nuanced view of IL and IL-TEG mixture fluidity, distinct from bulk viscosity.
- The local environment viscosity is highly sensitive to the specific probe and solvent composition.
- The C2-H functionality on the imidazolium cation does not appear to play a critical role in the hydrogen bonding interactions studied.
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