The thermal conductivity of ionic liquids. Experiment and molecular interpretation

Ionic liquids (ILs) are salts in the liquid state at room temperature with some relevant properties, namely their low vapor pressure over a wide temperature range and higher thermal conductivity than molecular fluids and molten salts. They are considered sustainable novel solvents in chemical technology for many applications, making them possible candidates for heat transfer and storage. Among all known ILs, imidazolium-based cations are the most studied, given their safety, low cost, low viscosity, and for some of them, low toxicity and biodegradability. Recent efforts to recommend reference values for the thermophysical properties of an IUPAC standard ionic liquid, [C(6)mim][(CF3SO2)(2)N], showed that there are still problems with the agreement of experimental data. To fully understand their properties at a molecular/ionic level and thermal conduction in these ionic liquids, accurate experimental measurements and theoretical analysis are necessary. This work analyses recent work on pure ionic liquids thermal conductivity, reporting new data for two of them, [C(4)mim][(N(CN)(2)] and [C(2)mim][SCN], and analyzing the effect of the cation and anion on the macroscopic thermal conductivity. Thermal conductivity measured with a transient hot-wire probe in the temperature range 303.15 K < T < 348.15 K has an estimated expanded uncertainty U-r (lambda) = 0.02. Regularities of molecular mass and scaled volumes dependence on the effect of anion and cation on the thermal conductivity of the ionic liquids studied are discussed, as limitations to the existing approximate theoretical methods for the calculation of thermal conductivity, namely the Bridgman model.