Comparison of Shear and Conductivity Relaxation Times for Concentrated Lithium Chloride Solutions

Shear viscosity measurements over the temperature interval $\text{23 –− 124\hspace{0.17em}°}\mathrm{C}{\text{\hspace{0.17em}(5×10}}^{\text{−2}}\mathrm{–}{10}^8\hspace{0.17em}\mathrm{P})$ and shear impedance measurements at 88 MHz over the interval −75–−130°C have been performed for concentrated aqueous lithium chloride solutions in the concentration range $\text{R=4.49}\mathrm{–}\text{5.77}$ where R is the mole ratio of water to salt. Average shear relaxation times, ${\mathrm{〈\; \tau }}_s〉$ , have been calculated from these results and compared with average conductivity relaxation times, ${\mathrm{〈\; \tau }}_{\sigma }〉$ , reported previously by Moynihan, Bressel, and Angell. ${\mathrm{〈\; \tau }}_s〉$ and ${\mathrm{〈\; \tau }}_{\sigma }〉$ are within 30% of one another at the low viscosity end ${\text{( ∼10}}^4\hspace{0.17em}\mathrm{P})$ of the range of comparison, but at higher viscosities the ${\mathrm{〈\tau }}_s{\mathrm{〉/〈\tau }}_{\sigma }〉$ ratio becomes significantly larger than unity (2–3 at the highest viscosities for which data are available). Estimated ${\mathrm{〈\tau }}_s{\mathrm{〉/〈\tau }}_{\sigma }〉$ ratios for other ionic liquids suggest that the occurrence of ${\mathrm{〈\tau }}_s{\mathrm{〉/〈\tau }}_{\sigma }〉$ ratios greater than unity at high visosities may be a common phenomenon. Description of the shear relaxation process for concentrated LiCl—H₂O solutions is found to require a fairly broad, asymmetric (Cole—Davidson) distribution of relaxation times.