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.