(489a) Thermally Robust Molten Salts & Ionic Liquids: Thermodynamics Properties and Phase Behavior

Ionic liquids have enjoyed much interest in the past 20 years because of their unique and tunable properties. The vanishingly low vapor pressures have lead them to be considered as environmentally benign replacement solvents for reactions and separations as they have very low risk for fugitive emissions. Additionally, as they are often comprised of organic ions, their physical and chemical properties can be tuned at the molecular level using the full palette of organic chemistry.

In addition to being solvents, there has been a great deal of interest in using ionic compounds as heat transfer fluid due to the large range of temperatures where they remain liquids. Unlike traditional molten salt heat transfer fluids, ionic liquids (which formally have melting points below 100°C) can be liquids at room temperature, and even far below 0°C. Because of this versatility, our group has begun studying ionic liquids and molten salts (those with melting points between 100 and 200°C), that are also stable at temperatures up to and above 350°C in air for weeks. Such thermal stability is achieved by limiting the functional groups to arene and substituted arenes. In doing so, the structural diversity is highly constrained and molecular design choices are limited. Previously, we have described these challenges in terms of how they limit structure/property analysis, in that it is difficult to develop homologous series which are consistently thermally stable.

In this work, we examine a new and expanded class of phosphonium- and sulfonium-based ionic compounds, present their synthesis, thermophysical properties (melting points, enthalpy and entropies of fusion, heat capacity) and the liquid-liquid equilibrium of several of these compounds with molecular aromatics. Additionally, kinetic thermal gravitational analysis of the compounds will be presented to understand the thermal stability as a function of temperature and time. These compounds are of potential interest in a number of areas related to industrial use, including use as heat transfer fluids, lubricants and separations media.