Calorimetric and Crystallographic Phase-Behavior Study of Selected 1‑Butylpyridinium Ionic Liquids

Despite the growing popularity of ionic liquids (ILs) as attractive solvents with various interesting properties, the lack of low-uncertainty thermodynamic data persists for many of these compounds. Even for commonly available ILs, crucial parameters such as melting temperatures and fusion enthalpies are often unknown or remain burdened with significant uncertainties. Therefore, this study focuses on a thorough characterization of the phase behavior for eight ionic liquids containing the 1-butylpyridinium cation by means of differential scanning calorimetry and variable-temperature X-ray powder diffraction. In addition to phase behavior studies, the heat capacities of the ILs were determined using a Tian-Calvet-type calorimeter. For many of the compounds studied, the presented data are the first of their kind. Four of the targeted ILs exhibit polymorphic behavior, but its particular manifestations differ noticeably. The crystal-crystal phase transition in [BPy]­[NTf2] is irreversible and barely noticeable by both methods used, while the phase transition in [BPy]­[OTf] is associated with a phase change entropy of a magnitude similar to the melting of the same compound. [BPy]­[PF6] exhibits a textbook example of a lambda-shaped second-order phase transition between two crystalline phases, being a rare phenomenon that is more often observed with superfluids. Crystal structures of both polymorphs of [BPy]­[PF6] and [BPy]­[OTf] are resolved and may act as a solid foundation for future theoretical computations.