Heterogeneous Dynamics in Imidazolium-based Ionic Liquids

Abstract

We have investigated structure and relaxation phenomena for ionic liquids 1-octyl-3-methylimidazolium chloride (C8mimCl), hexafluorophosphate (C8mimPF₆) and bis (trifluoromethylsulfonyl) imide (C8mimTFSI) by means of neutron diffraction and neutron spin echo (NSE) techniques. The diffraction patterns show two distinct peaks appeared at scattering vectors Qs of 0.3 Å^-1 and 1.0 Å^-1. The former originates from the nanoscale structure characteristic to ionic liquids and the latter due to the inter-ionic correlations. Interestingly, the intensity of the low-Q peak drastically grows upon cooling. The NSE measurements have been performed at these two Q positions, to explore the time evolution of each correlation. The relaxation related to the ionic correlation (ionic diffusion) is of Arrhenius-type and exhibits non-exponential behavior. On the other hand, two kinds of relaxation processes, slower and faster ones, are found at the low-Q peak position. The most significant finding is that the fraction of the slower relaxation increases and that of the faster one decreases upon cooling. Combining the NSE data with the diffraction data, we conclude that there exist two parts in ILs; one with the ordered nanostructure exhibiting the slow relaxation, and the other with disordered structure showing faster relaxation.