Thermal and Thermodynamic Properties of Ionic Liquids and Molten Salts with High Thermal Stability

Ionic liquids have garnered significant interest in research due to their unique beneficial properties: vanishing vapor pressures, low flammability, chemical inertness, large liquid ranges, and wide versatility. Coupled with their tenability from being organic salts, they have promising applications to replace current volatile solvents that used in reactions, separations, bioprocessing, etc. Furthermore, since one of their properties is high thermal stability, using ionic liquids for thermal energy storage, heat transfer and other high-temperature situations are other viable applications. However, this has been recently shown to be inaccurate. Previously considered stable ionic liquids have been shown to decompose at temperatures of 200-300°C when under long-term thermal stress. Therefore, ionic liquids for heat transfer fluids, high temperature solvents, or lubricants is limited.

To this end, we have synthesized several peraryl sulfonium and phosphonium cations coupled with various anions to be thermally stable at temperatures of at least 300°C with low to negligible mass loss. Along with these, we have recently studied the thermal stability of various anions previously not considered in literature coupled with a PNP⁺ cation. In this work, we present the melting points for these salts along with their solid and liquid heat capacities and compare these to traditional organic heat transfer fluids. Additionally, we examine the relative effects of the enthalpy and entropy of fusion on the melting points of homologous series of these salts along with mass loss data from long-term thermal stability tests. In the perspective of heat transfer fluids, their heat capacities were determined to compare them with common heat transfer fluids.

Preliminary experiments with two perarylphosphonium salts combined with excess aromatic molecules creates an aromatic-ionic liquid biphasic system. The phase equilibria of these molten salts with five different aromatics is explored by constructing liquid-liquid phase diagrams. These initial studies along with synthesis of novel thermally stable salts can aid future work related to thermal energy storage and high-temperature processes.