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Tuning the low-temperature phase behavior of aqueous ionic liquids
Johannes Bachler1, Isabella Daidone2, Laura Zanetti-Polzi3
1Institute of Physical Chemistry, University of Innsbruck, Innrain 52c, Innsbruck A-6020, Austria. thomas.loerting@uibk.ac.at.
Physical Chemistry Chemical Physics : PCCP
|March 12, 2024
Summary
Aqueous ionic liquid solutions exhibit exothermic transitions due to anion hydrophobicity, not a liquid-liquid phase transition. This behavior, similar to Krafft temperature effects, offers insights into designing solutions with tunable liquid fragility.
Area of Science:
- Physical Chemistry
- Materials Science
- Chemical Physics
Background:
- Water's anomalous properties are often explained by a two-liquid model involving high-density liquid (HDL) and low-density liquid (LDL) states, potentially separated by a liquid-liquid phase transition (LLPT).
- Aqueous solutions of ionic liquids, such as hydrazinium trifluoroacetate, were proposed to exhibit an LLPT, offering an observable alternative to pure water due to suppressed ice crystallization.
- The occurrence of an LLPT versus complex phase separation phenomena in these ionic liquid solutions remains a subject of debate.
Purpose of the Study:
- To investigate the low-temperature phase behavior of aqueous solutions containing hydrazinium trifluoroacetate and related ionic liquids.
- To determine the nature of the observed exothermic transitions and structural changes upon cooling.
- To propose a model explaining the observed phenomena and discuss implications for designing ionic liquid solutions.
Main Methods:
- Detailed low-temperature calorimetry experiments.
- X-ray diffraction studies on aqueous ionic liquid solutions.
- Molecular dynamics (MD) simulations.
- Development of a model inspired by micelle formation in surfactant solutions.
Main Results:
- Aqueous solutions of hydrazinium trifluoroacetate, ammonium trifluoroacetate, ethylammonium trifluoroacetate, and hydrazinium pentafluoropropionate (at x=0.175) showed exothermic transitions unrelated to crystallization.
- Remarkable structural changes were observed upon cooling into the glassy state for these solutions.
- The observed first-order transitions were attributed to phase separations below the Krafft temperature, driven by the hydrophobic nature of fluorinated anions, rather than an LLPT.
Conclusions:
- The exothermic transitions in these aqueous ionic liquid solutions are not indicative of a liquid-liquid phase transition (LLPT) but rather phase separations akin to those below the Krafft temperature.
- The hydrophobic character of fluorinated anions drives aggregation, which reverses upon cooling, leading to the observed exothermic transition.
- Differences in liquid fragility were observed, decreasing with anion hydrophobicity, suggesting potential for systematic tuning of ionic liquids for specific solution properties.
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