Fractional Walden rule for aprotic ionic liquids: Experimental verification over a wide range of temperatures and pressures

The Walden rule, revealing a universal conductivity-viscosity relationship, is considered as a useful tool to investigate the transport mechanism in ionic liquids (ILs). In this work, we constructed the Walden plots covering 10 orders of magnitude for two ILs: 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BMMIm][NTf2] and 1-butyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide [BMIm][BETI] using experimental viscosity data at pressures up to 950 MPa supplemented by high-pressure conductivity and density data. Moreover, the high-pressure viscosity data of studied ILs revealed an inflection point denoting a transition from slower to faster than exponential dependence of viscosity on pressure. Finally, we found that the experimental viscosity data conform to the thermodynamic density scaling idea for each IL, with the scaling parameter equal to the ratio of scaling exponent obtained for conductivity data to the slope of the Walden plot. Consequently, our results show a promising method to generate high-pressure viscosity data especially for highly viscous aprotic ionic liquids, which are technically impossible to measure.