title: Ultrasonic Relaxation Study of 1-Alkyl-3-methylimidazolium-Based Room-Temperature Ionic Liquids: Probing the Role of Alkyl Chain Length in the Cation.
abstract: 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.
authors.affiliation: Institute of Chemistry, University of Silesia , Szkolna 9, 40-006 Katowice, Poland.
authors.forename: Michał
authors.initials: M
authors.lastname: Zorębski
authors.affiliation: Institute of Chemistry, University of Silesia , Szkolna 9, 40-006 Katowice, Poland.
authors.forename: Edward
authors.initials: E
authors.lastname: Zorębski
authors.affiliation: Institute of Chemistry, University of Silesia , Szkolna 9, 40-006 Katowice, Poland.
authors.forename: Marzena
authors.initials: M
authors.lastname: Dzida
authors.affiliation: Institute of Chemistry, University of Silesia , Szkolna 9, 40-006 Katowice, Poland.
authors.forename: Justyna
authors.initials: J
authors.lastname: Skowronek
authors.affiliation: Institute of Chemistry, University of Silesia , Szkolna 9, 40-006 Katowice, Poland.
authors.forename: Sylwia
authors.initials: S
authors.lastname: Jężak
authors.affiliation: QUILL Research Centre, Queen's University of Belfast , BT95AG Belfast, Northern Ireland United Kingdom.
authors.forename: Peter
authors.initials: P
authors.lastname: Goodrich
authors.affiliation: QUILL Research Centre, Queen's University of Belfast , BT95AG Belfast, Northern Ireland United Kingdom.
authors.forename: Johan
authors.initials: J
authors.lastname: Jacquemin