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

Abstract

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-ethyl-3-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.

Ultrasonic Relaxation Study of 1-Alkyl-3-methylimidazolium-Based Room-Temperature Ionic Liquids: Probing the Role of Alkyl Chain Length in the Cation

“…However, contrary to many 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 , only one isothermal value is available to date. 10 Generally, it appears that the agreement between the values determined in this study and the values reported in the literature is acceptable ( Figure 6). With the exception of the isothermal data of Demizu et al 17 and Fukuda et al 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 method show very...…”

“…Using a transient grating (TG) method, 14 additional experimental results have been reported by Frez et al 10 (for seven RTILs) and Kozlov et al 11 (for four RTILs). Unfortunately, in both cases measurements have been carried out only at single frequency at 296.85 K or 301 K, respectively.…”

“…Apart from abovementioned studies using acoustic technique, [1][2][3][4] only two papers were published using an optic method, to date. 10,11 ILs are currently defined as molten salts with a melting temperature below 373 K. Simultaneously, salts that are liquid at room temperature form a subclass of ILs called room temperature ionic liquids (RTILs). 12,13 To the best of our knowledge, the available literature data on the ultrasound absorption are limited to only RTILs in practice.…”

Measurements of ionic liquids thermal conductivity and thermal diffusivity

“…However, there is scant literature that can be found for thermal conductivity measurement of ILs. Frez et al utilized the transient grating method for the thermal conductivity measurement of 7 ILs [35]. In the work of Ge et al, the transient hot-wire method was utilized to measure the thermal conductivity of 11 pure ILs [36].…”

“…Frez et al [35] are the only researchers who experimentally measured the thermal diffusivity of pure ILs according to our own knowledge. In the 3 work of Frez et al, transient grating technique was utilized for measurement of pure ILs thermal diffusivity [35]. We found the thermal diffusivity data of ILs are rarely known in the current literatures.…”