Viscoelastic properties of room temperature ionic liquids

Abstract

The sound absorption coefficient alpha and sound velocity nu(S) have been measured for 1-alkyl-3-methylimidazolium hexafluorophosphate [Cn mim]PF(6), with n = 8,6,4 and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C4 mim]TFSI, at the frequencies of 11.3, 34.9, and 57.7 MHz in the temperature range from 293 to 393 K. From the obtained alpha and available viscosity data, [Cn mim]PF(6) with large n proves to be no longer Newtonian liquids at room temperatures. Applying a Maxwell viscoelastic model with the elastic modulus G of a spring and the shear coefficient gamma of a dashpot to the experimental frequency dependence of alpha, one finds that G is insensitive to n, while the relaxation time tau(= gamma/G), which is on the order of nanoseconds, does depend on n.

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

“…[6][7][8] Unfortunately, this crucial condition can sometimes be overlooked. In other words, it should be keep in mind that for all aprotic RTILs studied to date, the determined f rel decrease with the decreasing temperature 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 30 and increasing side alkyl chain length in 3-methylimidazolium cation, 1 3,58 In other words, the lower the temperature, the higher the increase of the speed of sound with the decreasing temperature (concave up curve) is observed, and it is typical behavior in dispersion regions. 54,55 In the case of Because of the relatively large ultrasonic absorption in many RTILs, ultrasonic velocity is subject to a considerable dispersion.…”

“…However, it should be noted that various techniques have been used, which are sometimes not specially suitable for the accurate speed of sound measurements. 3,10,18,17 Simultaneously, the ILs are in general difficult objects for speed of sound measurements. First of all, their purity, hygroscopic nature, high viscosity and high ultrasound absorption create problems, which are unfortunately most often ignored.…”

“…Apart from our previously reported results for 1-ethyl-3-methylimidazolium and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides 1 only two other research groups reported data obtained by means of a direct method based on the first travel pulse with a variable path length. 3,4 For example, Makino et al 3 reported data for four imidazolium-based RTILs in the temperature range of (293-393) K but at three similar frequencies (e.g. 11.3, 34.9, and 57.7…”

“…9 Unfortunately, there is a lack of ultrasound absorption in ILs data available in the literature. 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).…”

“…[1][2][3][4] However, the acoustic technique is also an interesting method because the ultrasound absorption is related to the energy dissipation caused by various irreversible processes taking place in the liquids. This energy dissipation results in the characteristic spectra and a further analysis of such spectra, especially in a wide frequency range, can be an integral and significant source of valuable information regarding the molecular structure of the liquids, as well as, the various physical and chemical processes occurring in the liquids at a nearly ideal thermal equilibrium.…”