Interpretation of the Variation of the Walden Product of Ionic Liquids with Different Alkyl Chain Lengths in Terms of Relaxation Spectra

The shear relaxation spectra and the alternating-current (AC) conductivity of 1-alkyl-3-methylimidazolium hexafluorophosphate were measured in the MHz region, with the chain lengths varied from butyl to octyl. The relaxation times of both the conductivity and shear viscosity increased with increasing chain length approximately in proportion to the variation of the reciprocal molar conductivity. On the other hand, the increase in the shear viscosity was smaller than that of the relaxation time, which indicates that the high-frequency shear modulus decreases with the chain length. The decrease in the the Walden product with the chain length is thus ascribed to that of the high-frequency shear modulus.