Shear Relaxation of Imidazolium-Based Room-Temperature Ionic Liquids

The frequency-dependent shear viscosities of four representative imidazolium-based room-temperature ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([bmim]TFSA]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF6]), and 1-methyl-3-octylimidazolium hexafluorophosphate ([omim[PF6]), are measured from 5 to 205 MHz with shear impedance spectroscopy. A relaxation is observed in the measured frequency range in all cases. This is the first report on the shear relaxation of ionic liquids at room temperature, to our best knowledge. Comparing the spectra of the common cations, [bmim][TFSA] and [bmim][PF6], the normalized relaxation spectra, eta(nu)/eta(0), reduce to a single curve when plotted against eta(0)nu, where nu and eta(0) stand for the frequency and shear viscosity, respectively. The lower viscosity of the TFSA salt is thus elucidated by the shorter relaxation time. The lower viscosity at higher temperature is also attributed to the shorter relaxation time. On the other hand, the increase in the length of the alkyl chain of the cation leads to the lower-frequency shift of the relaxation frequency on the eta(0)nu scale. Therefore, the higher viscosity of the omim salt is the result of the compensation of the longer relaxation time for the smaller high-frequency shear modulus. In addition, the relaxation time distribution becomes broader with increasing chain length, which can be ascribed to the heterogeneity of the liquid structure.