Temperature Dependence of Viscosity and Specific Conductivity of Fluoroborate-Based Ionic Liquids in Light or the Fractional Walden Rule and Angell's Fragility Concept

Temperature-dependent conductivity and viscosity data of over ten new fluoroborate-based ionic liquids (ILS) were measured in a temperature range spanning about 100 K. Data are presented and evaluated according to the fractional Walden rule and Angell's fragility concept. All ILs show excellent linear relationships for their Walden plots with similar slopes in the range from about 0.90 to about 0.94. It was found empirically that the slopes of the Walden plots reflect the ratio of the corresponding Arrhenius activation energies for the ILS temperature-dependent viscosities and molar conductivities. Further analysis of viscosity data of ILs leads to the conclusion that all investigated ILs, including some more common ones, can be classified as highly fragile. very weak liquids, reaching even the limiting value estimated by Vilgis.