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.

We measured microwave transmission and reflection spectra for typical room temperature ionic liquids, [C(4)min][TFSA], [C(4)min][PF(6)], [C(6)min][PF(6)], and [C(8)min][PF(6)], at frequencies between 40 MHz and 40 GHz in the temperature range up to 100 degrees C. The transmission spectra were analyzed using complex dielectric functions, and the static permittivity epsilon(S) was determined as a function of temperature. Applying the effective medium approximation to epsilon(S), we have estimated that the static permittivity of the polar domain is around 20, and that of the nonpolar domain around 2.5.