High-frequency shear viscosity and ionic mobility of solutions of polyethylene glycol in ionic liquids

Abstract

The frequency-dependent complex shear viscosity of the solutions of polyethylene glycol (PEG) in various ionic liquids is measured in the frequency range of 5–205 MHz, and the ionic mobility of the corresponding solutions was also determined. The plateau viscosity in the MHz regime increases on the addition of PEG, and the relaxation in the 100 MHz regime is retarded simultaneously. The amount of the increase in the plateau viscosity correlates with that in the relaxation time, suggesting that the retardation of the relaxation is a reason for the increase in the plateau viscosity. Although the increase in the plateau viscosity is far smaller than that in the zero-frequency viscosity, these two values are correlated with each other, which can be explained as the consequence of the strength of the same kind of the solute-solvent attractive interaction. The decrease in the molar ionic conductivity also correlates with the increase in the plateau viscosity, suggesting that the plateau viscosity can be regarded as the local viscosity of ionic liquids related to the ion conduction. Based on the correlation with the Kamlet-Taft α parameter, the hydrogen bond between the ionic liquid and the oxygen atoms of PEG is suggested to be a possible origin of the increase in the plateau viscosity.