(462e) Thermodynamics & Thermophysical Properties of Thermally Robust Ionic Liquids and Their Mixtures

Although ionic liquids are often described as having high thermal stability, evidence shows that many traditional ionic liquids begin decomposing at temperatures much lower than those reported from TGA decomposition experiments, often losing a few percent mass in just a few hours at temperatures 50-100C lower their reported decomposition temperatures. Functional groups appended to the ions to impart a particular physical or chemical property often dramatically decrease the thermal stability of compound. Aryl groups provide a great deal of thermal stability; however, they typically increase the melting point of the compound significantly. Thus, there is an inherent tradeoff between thermal stability, properties of interest (high heat capacity, solubility, etc.) and low melting point, and the vast structural diversity available in designing ionic liquids is highly constrained when thermal stability is a desired characteristic.

For the past few years, our groups have studied and characterized a number of classes of thermally robust ionic compounds. In this work, we study several classes of thermally robust ionic liquids, including per-aryl phosphonium and sulfonium salts, from a cation perspective, as well as benzesulfonate salts from an anion perspective. In addition to pure component thermodynamic and thermophysical properties (melting points, heat capacities, phase equilibria, etc.) we demonstrate that desired properties for these fluids (low melting point, high heat capacity) can be achieved through mixtures of these salts with each other and with molecular aromatics.

Traditionally, ionic liquids are distinguished from molten salts by having melting points lower than 100C, however, the term “ionic liquid” is evolving to encompass ionic compounds that are liquids in the temperature range in which they used. Here we embrace the expanded term as the classes of compounds that we are examining have structurally similar representatives some of which melt above 100C and some below, highlighting the arbitrary nature of the distinction.