Thermophysical Characterization of [C ₆ mim][SbF ₆ ] Ionic Liquid as a Heat Transfer Fluid: Heat Capacity, Thermal Conductivity, and Viscosity

The ionic liquid 1-hexyl-3-methylimidazolium hexafluoroantimonate ([C₆mim]-[SbF₆]) was synthesized and characterized. The isobaric molar heat capacity was precisely measured from 79 to 393 K via automated adiabatic calorimetry, enabling the calculation of derived thermodynamic functions and phase transition parameters. By integration of previously reported density and surface tension data, the isochoric molar heat capacity, speed of sound, compressibility, and thermal storage density were computed. The thermal conductivity was experimentally determined from 293.15 to 343.15 K using the transient plane source method. This study was augmented by the investigation of its transport properties, including the dynamic viscosity and Prandtl number. The results demonstrate that [C₆mim]-[SbF₆] exhibits a competitive thermal conductivity (∼0.190 W·m^-1·K^-1 at 313.15 K), a moderately low viscosity, and a Prandtl number profile akin to high-performance commercial heat transfer fluids like Marlotherm SH and Therminol 66. The combination of a favorable thermal storage density, high thermal conductivity, low temperature dependence of the thermal conductivity, and desirable Prandtl number values underscores its significant potential as an efficient and sustainable heat transfer fluid.