Phase Transitions in Higher-Melting Ionic Liquids: Thermal Storage Materials or Liquid Crystals?

Abstract

To assess the application potential of a material in thermal energy storage, the knowledge of their thermophysical properties is of key importance. Specifically, an efficient material has to show, among others, large enthalpies of phase change and a sufficiently large thermal conductivity. In this work, imidazolium-based ionic liquids (ILs) with long alkyl chain substituents 1-hexadecyl-3-methylimidazolium chloride and 1-hexadecyl-3-methylimidazolium saccharinate were studied in view of their possible use as phase-change materials. Differential scanning calorimetry (DSC) and the heat-leak modulus methods were used to determine the temperatures and the enthalpies of phase transitions in the studied ILs, enabling us to study the influence of the heating rates on the measured properties. Enthalpies of fusion near to or larger than 100 J·g–1 were found in the studied ionic liquids, making them promising candidates for thermal energy storage. Furthermore, peaks corresponding to possible liquid crystalline phases in the DSC traces of 1-hexadecyl-3-methylimidazolium saccharinate were observed. The measured properties are not only essential characteristics of a thermal storage material, temperatures and enthalpies of melting are necessary in thermodynamic description and modelling of solid-liquid phase behavior and thus in the possible utilization of the material in separation and crystallization processes.