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

Phase behavior, crystal structures, andheat capacitiesare determined experimentally for selected 1-butylpyridinium ILs.First-principles models of the observed local disorder of individualcrystalline phases are presented. A combination of various techniquesleads to a comprehensive description of crystallographic and thermodynamicproperties of condensed phases for compounds with potential industrialuse. Despite the growing popularity of ionic liquids (ILs)as attractivesolvents with various interesting properties, the lack of low-uncertaintythermodynamic data persists for many of these compounds. Even forcommonly available ILs, crucial parameters such as melting temperaturesand fusion enthalpies are often unknown or remain burdened with significantuncertainties. Therefore, this study focuses on a thorough characterizationof the phase behavior for eight ionic liquids containing the 1-butylpyridiniumcation by means of differential scanning calorimetry and variable-temperatureX-ray powder diffraction. In addition to phase behavior studies, theheat capacities of the ILs were determined using a Tian-Calvet-typecalorimeter. For many of the compounds studied, the presented dataare the first of their kind. Four of the targeted ILs exhibit polymorphicbehavior, but its particular manifestations differ noticeably. Thecrystal-crystal phase transition in [BPy][NTf2] is irreversibleand barely noticeable by both methods used, while the phase transitionin [BPy][OTf] is associated with a phase change entropy of a magnitudesimilar to the melting of the same compound. [BPy][PF6]exhibits a textbook example of a lambda-shaped second-order phasetransition between two crystalline phases, being a rare phenomenonthat is more often observed with superfluids. Crystal structures ofboth polymorphs of [BPy][PF6] and [BPy][OTf] are resolvedand may act as a solid foundation for future theoretical computations.