Calorimetric study of polymorphism in 1-butyl-3-methylimidazolium hexafluorophosphate

• Thermodynamic properties for [C4mim]PF6 polymorphs were determined.
• Structures of the crystal phases were assigned using literature data.
• The complete pattern of phase transformations is described.

Stable and metastable crystal phases of 1-butyl-3-methylimidazolium hexafluorophosphate were obtained in an adiabatic calorimeter providing precise temperature control during crystallization. Heat capacities, as well as temperatures and enthalpies of phase transitions, including fusion, were determined for three polymorphic sequences (I, II, and III). Structures of the crystal phases were assigned using crystallographic studies from the literature. The standard entropy of the liquid phase at T = 298.15 K determined using the calorimetric data for each sequence was the same within the uncertainty of the measurements. This, in combination with the crystallographic results, implies that the lowest-temperature crystal form is ordered for all sequences. The complete pattern of phase transformations in this ionic liquid at ambient pressure is described, and results show that the relative stability of Sequences II and III invert at T ≈ 281 K.

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