On the molecular mechanisms of α and β relaxations in ionic liquids

Using ²H NMR, we determine correlation times and motional mechanisms for the α and β relaxations of glass-forming imidazolium-based ionic liquids, explicitly, for the associated cation reorientation dynamics. It is shown that the α relaxation is faster, its nonexponentiality is stronger, and the fragility is higher for bis(trifluoromethylsulfonyl)imide anions than that for tetrafluoroborate anions. ²H NMR stimulated-echo studies reveal that the overall reorientation dynamics involved in the α relaxation is isotropic and composed of jumps about small angles, where the mean jump angles are smaller for larger cations. Moreover, we demonstrate that, in addition to a cation-specific β relaxation, all studied ionic liquids exhibit the genuine Johari-Goldstein β relaxation of glass-forming liquids. Various ²H NMR results consistently indicate that the associated rotational motion is spatially highly restricted. Altogether, our findings show that, despite strong electrostatic interaction and prominent microphase separation of ionic liquids, their glassy dynamics resemble that of their nonionic counterparts, including similar microscopic mechanisms for intrinsic α and β relaxations.