On Enhanced Translational Diffusion or the Fractional Stokes−Einstein Relation Observed in a Supercooled Ionic Liquid

From their experimental studies of the supercooled molecular ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), Ito and Richert [J. Phys. Chem. B 2006, in press.] found that the Stokes-Einstein and the Debye-Stokes-Einstein laws do not hold. Instead, enhanced translational diffusion or fractional Stokes-Einstein and fractional Debye-Stokes-Einstein relations are observed, just like in nonionic glass-forming liquids, including 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene, o-terphenyl, and sucrose benzoate. The comprehensive measurements made by Ito and Richert have determined the critical parameters that the coupling model needs to explain the observed fractional Stokes-Einstein and fractional Debye-Stokes-Einstein relations in the supercooled molecular ionic liquid.