Determination of Thermal Diffusivities, Thermal Conductivities, and Sound Speeds of Room-Temperature Ionic Liquids by the Transient Grating Technique

We report measurements of thermal diffusivity of several room-temperature ionic liquids (RTILs) using the transient grating method. Measurements are carried out using ionic liquids with small concentrations of an inert dye that is excited by the 532 nm output of a Nd:YAG laser in a grating with a fringe spacings of (92 and 104) μm. The experiments give thermal diffusivities from which thermal conductivities can be determined, sound speeds, and acoustic damping parameters for seven ionic liquids. In this study, we have used combinations of the cation 1-butyl-3-methylimidazolium ([BMIm]+) with the anions tetrafluoroborate ([BF4]), hexafluorophosphate ([PF6]), and bis(trifluoromethylsulfonyl)imide ([Tf2N]) and combinations of the anion [Tf2N] with the cations 1-ethyl3-methylimidazolium ([EMIm]+), 1-pentyl-3-methylimidazolium ([PMIm]+), 1-hexyl-3-methylimidazolium ([HMIm]+), and 1-octyl-3-methylimidazolium ([OMIm]+) to determine the effect of anion and cation on the thermophysical properties of the RTILs. Results obtained indicate that the anion exerts a strong influence not only on the sound speed but also on the thermal diffusivity and acoustic damping of the RTILs. For RTILs with the same cation [BMIm]+, changing the anion from [BF4] to either [PF6] or [Tf2N] leads to decreases in the sound speed, thermal diffusivity, and thermal conductivity. The size of the cation, however, does not significantly influence the sound speed or the thermal diffusivity of the RTILs.