Quantitative Analysis of Conductivity and Viscosity of Ionic Liquids in Terms of Their Relaxation Times

The frequency-dependent viscosity and conductivity of various ionic liquids were measured experimentally, and their mean relaxation times were determined. The relaxation times of the viscosity and conductivity were approximately correlated with their respective zero-frequency limiting values. The Walden products, however, appeared to have no correlation with the ratio of the relaxation time of viscosity to that of conductivity in general. When the alkyl chain of the cation is as short as butyl, more viscous ionic liquids tend to show larger difference between two relaxation times and larger Walden products. Lengthening the alkyl chain of the cation decreases the Walden product while slightly increasing the relaxation time ratio, which was elucidated in terms of the decrease in the high-frequency shear modulus. In addition, the contribution of the mesoscopic structure to viscosity was suggested in the case of the ionic liquid with the longest alkyl chain studied in this work, 1-dodecyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­amide

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