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

Abstract

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 1butylpyridinium cation by means of differential scanning calorimetry and variabletemperature 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][NTf 2 ] 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][PF 6 ] 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][PF 6 ] and [BPy][OTf] are resolved and may act as a solid foundation for future theoretical computations.

Long-Chain Bio-Based Nylon 514 Salt: Crystal Structure, Phase Transformation, and Polymerization

“…Moreover, the anhydrous forms and hydrates of a compound may sometimes differ greatly in terms of bulk density, hygroscopic properties, stability, and mechanical properties [ 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. Intermolecular interactions analysis, thermodynamic analysis, and quantitative calculation as promising and useful tools for polymorph exploration are commonly used to assess the stability and the structure–activity relationship of target crystalline forms, as well as to gain insight into the mechanism of transformation between them [ 36 , 37 , 38 , 39 , 40 ]. Meanwhile, the solid-state transformations (SST) [ 41 , 42 , 43 ] and solution-mediated polymorphic transformations (SMPT) [ 44 , 45 , 46 , 47 ] experiments were usually designed to investigate the transformation mechanism between the crystal forms in recent research.…”