Supramolecular structure fluctuations of an imidazolium-based protic ionic liquid

At 20, 25, 30, and 40 °C, the ultrasonic absorption spectra of the protic ionic liquid 3-(butoxymethyl)-1H-imidazol-3-ium salicylate have been measured between 0.6 and 900 MHz. Below 250 MHz, the absorption coefficient decreases with temperature, potentially indicating a major effect of the viscosity and/or a relaxation time. Essentially the broad spectra can be favorably represented by two relaxation terms in addition to an asymptotic high-frequency contribution. One term reflects an asymmetric relaxation time distribution. It is described by a model of noncritical fluctuations in the structure and thermodynamic parameters of the liquid in order to yield the fluctuation correlation length and the mutual diffusion coefficient. Applying the Stokes-Einstein-Kawasaki-Ferrell relation, these quantities can be used to show that the effective shear viscosity controlling the fluctuations is substantially smaller than the steady-state shear viscosity. This result is consistent with dispersion in the shear viscosity as revealed by viscosity measurements at 25, 55, and 81 MHz. The other term can be well described by a Debye-type relaxation function. It has been tentatively assigned to a structural isomerization of the butoxymethyl chain of the imidazole molecule. However, it cannot be completely excluded that this term reflects, at least in parts, a Brønstedt acid-base equilibrium or a specific association process.