Viscoelastic properties of room temperature ionic liquids

The sound absorption coefficient α and sound velocity vS 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 α and available viscosity data, [Cn mim] PF₆ 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 γ of a dashpot to the experimental frequency dependence of α, one finds that G is insensitive to n, while the relaxation time τ (=γG), which is on the order of nanoseconds, does depend on n.