(193ba) Thermally Stable Peraryl Phosphonium Ionic Liquids and Molten Salts: Thermodynamic and Thermophysical Properties

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 tunability 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, previously considered stable ionic liquids have been shown to decompose at temperatures of 200-300°C when under long-term thermal stress.

Because of this, we have synthesized several phosphonium cations coupled with bistriflimide anion to be thermally stable at temperatures of at least 300°C with low mass losses. In this work, we present the melting points for these salts along with their 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. Additionally, 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 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