Ionic Liquids (ILs) have unique physicochemical properties that make them attractive for use in electrochemistry and energy storage applications. Their complex phase change behaviors over a wide temperature range are important to understand under the context of these applications. We use differential scanning calorimetry coupled with variable temperature infrared spectroscopy to study phase transitions and structural changes of imidazolium IL mixtures with water and acetonitrile as a function of temperature. Our results show how anion polarity and hydrogen bonding affect the system’s behavior, particularly for ILs 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][OTf]). As these liquids are diluted in water and acetonitrile solvents, our results show significant changes in phase transition enthalpies and shifts in phase transition temperatures, as well as new phase transitions not observed in neat IL or neat solvent. Comparison of IL and solvent vibrational modes (peak shift, broadening, and relative intensity) before and after phase transitions gave insight into the intra- and intermolecular interactions within each phase state and dilution. Results suggest the formation of ‘solvate’ structures under certain dilution conditions with the greatest shifts in behavior occurring between IL mole fractions of 0.42 < x < 0.20 (IL:solvent). We also identify several eutectic mixtures. Additional phase transitions, shifts in transition temperatures, the energy required to pass from or through them, and the resulting physical forms of these liquids will significantly affect solution properties thus impacting their application performance.

$DSC thermograms for a series of [BMIM][PF6] mixtures with acetonitrile ranging from neat [BMIM][PF6] (top) to neat acetonitrile (bottom). Thermograms are offset relative to the mole fraction of [BMIM][PF6] in the mixture (shown on right). All thermograms were collected at a rate of 5 °C/min.$

DSC thermograms for a series of [BMIM][PF₆] mixtures with acetonitrile ranging from neat [BMIM][PF₆] (top) to neat acetonitrile (bottom). Thermograms are offset relative to the mole fraction of [BMIM][PF₆] in the mixture (shown on right). All thermograms were collected at a rate of 5 °C/min.

Brian Bellott

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