5 research outputs found
An Ultrasonic Relaxation Study of 1-Alkyl-3-Methylmidazolium-Based Room Temperature Ionic Liquids: Probing the Role of Alkyl Chain Length in the Cation
Ultrasound absorption
spectra of four 1-alkyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imides were determined as a function of
the alkyl chain length on the cation from 1-propyl to 1-hexyl from
293.15 to 323.15 K at ambient pressure. Herein, the ultrasound absorption
measurements were carried out using a standard pulse technique within
a frequency range from 10 to 300 MHz. Additionally, the speed of sound,
density, and viscosity have been measured. The presence of strong
dissipative processes during the ultrasound wave propagation was found
experimentally, i.e., relaxation processes in the megahertz range
were observed for all compounds over the whole temperature range.
The relaxation spectra (both relaxation amplitude and relaxation frequency)
were shown to be dependent on the alkyl side chain length of the 1-alkyl-3-methylimidazolium
ring. In most cases, a single-Debye model described the absorption
spectra very well. However, a comparison of the determined spectra
with the spectra of a few other imidazolium-based ionic liquids reported
in the literature (in part recalculated in this work) shows that the
complexity of the spectra increases rapidly with the elongation of
the alkyl chain length on the cation. This complexity indicates that
both the volume viscosity and the shear viscosity are involved in
relaxation processes even in relatively low frequency ranges. As a
consequence, the sound velocity dispersion is present at relatively
low megahertz frequencies