Phase Behavior and Heat Capacities of Biocompatible Ionic Liquids and Low-Temperature Molten Salts

The perception of ionic liquids as green materials should not be overgeneralized due to the prevalence of severely toxic polyfluorinated or alkylated heterocyclic cations in current ionic liquids. The development of smart biocompatible solvents, potentially replacing the present synthetic ionic liquids, should also include thorough investigations of their phase behavior and thermophysical properties. The aims of this study are 2-fold: (i) turning the attention of the crystallographic community to the importance of both structural and thermal analysis in development of new solvents and (ii) turning the attention of the biocompatibleionic-liquids community to the importance of critically assessed and highly accurate thermophysical data. This study focuses on 15 biocompatible ionic liquids, derived mostly from selected proteinogenic amino acids and choline. Detailed calorimetric determinations of the phase behavior and heat capacities are reported and interpreted for these materials. Concurrently, X-ray diffraction experiments are used to decipher the crystal structures of the considered materials crystallizing under ambient conditions. Only in this concurrent application of structural and thermophysical methods can an exhaustive description of the phase behavior (polymorphism that is typically rich and vastly unexplored for ionic liquids) and lower-temperature stability bounds of the potential solvents be established. The data presented also enable the clarification of various ambiguities and contradictory conclusions contained in the scarce literature on these new classes of materials. In particular, the complex phase behavior of choline dihydrogen phosphate and choline nitrate are interpreted for the first time and the as yet unknown crystal structures for choline bistriflimide and choline dimethylphosphate are determined in this work.