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Protic ionic liquids based on decahydroisoquinoline: lost superfragility and ionicity-fragility correlation
Kazuhide Ueno1, Zuofeng Zhao, Masayoshi Watanabe
1Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States.
The Journal of Physical Chemistry. B
|December 1, 2011
Summary
Protic ionic liquids (PILs) derived from decahydroisoquinoline (DHiQ) show tunable ionicity and suppressed fragility, with some exhibiting superionic behavior and record-low fragility (m=45). This study correlates ionicity with m-fragility in DHiQ-based PILs.
Area of Science:
- Materials Science
- Physical Chemistry
- Condensed Matter Physics
Background:
- Protic ionic liquids (PILs) are salts that are liquid at ambient temperatures and formed by proton transfer.
- The fragility of ionic liquids (ILs) describes how their viscosity changes with temperature, impacting glass formation.
- Decahydroisoquinoline (DHiQ) is an extremely fragile molecular liquid, and its transformation into PILs is of interest.
Purpose of the Study:
- To investigate the physicochemical properties, ionicity, and fragility of protic ionic liquids (PILs) based on DHiQ.
- To explore the relationship between ionicity and fragility in DHiQ-derived PILs.
- To assess the potential for these PILs to exhibit superionic behavior and influence glass formation.
Main Methods:
- Synthesized DHiQ-based PILs by protonating DHiQ with various Brønsted acids.
- Evaluated ionicity using the Walden plot diagnostic.
- Quantitatively measured m-fragility using variable scan rate differential scanning calorimetry (DSC) based on the Moynihan-Wang-Velikov method.
Main Results:
- DHiQ-derived PILs span the full range of IL ionicities, from poor to superionic, depending on the Brønsted acid used.
- The initial high fragility (m=128) of DHiQ is significantly suppressed in PILs (m=45-91), falling into the intermediate to fragile range.
- The superionic DHiQ-based PIL with a hydrogensulfate anion ([HSO(4)(-)]) exhibits the lowest reported m value (m=45) for any IL, attributed to an extended hydrogen bond network.
Conclusions:
- A strong correlation exists between ionicity and m-fragility in DHiQ-based PIL systems.
- Good DHiQ PILs show fragility comparable to typical aprotic ILs, while poor PILs retain some parent fragility.
- The observed range of noncrystallizing and non-glassforming behaviors suggests these PILs could serve as a model system for studying ideal glassformers.
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