Dynamic Heterogeneity and Flexibility of the Alkyl Chain in Pyridinium-Based Ionic Liquids.

Changing the number of carbon atoms in the substituents of ionic liquids (ILs) is a way to shift the balance between Coulomb and van der Waals forces and, thus, to tune physicochemical properties. Here we address this topic on the microscopic level by employing quasielastic neutron scattering (QENS) and provide information about the stochastic ionic motions in the N-alkylpyridinium based ILs in a relatively expanded time range, from short time (subpicosecond) particle rattling to long time diffusive regime (hundreds of picoseconds). We have systematically investigated the effect of the alkyl chain length on the picosecond dynamics by employing partial deuteration of the samples and varying the number of carbon atoms in the alkyl substituent. The localized dynamics of the side groups have appeared to be enhanced for bulkier cations, which is opposite to the trend observed for the translational motion. This result highlights the role of the conformational flexibility of the alkyl group on the dynamical properties of ILs.