Communication: Are hydrodynamic models suited for describing the reorientational dynamics of ions in ionic liquids? A case study of methylimidazolium tetra(hexafluoroisopropoxy)aluminates

We report on dielectric relaxation spectra of six homologous ionic liquids (ILs) with tetra(hexafluoroisopropoxy)aluminate ([Al(hfip)(4)](-)) as a common anion. The dominating mode on the time scale of several 100 ps mainly results from cation reorientation. Because the viscosities are low and cation modification does not substantially change the viscosity, these ILs are interesting candidates for testing hydrodynamic models of rotational dynamics. The calculated hydrodynamic volumes are extraordinarily low, and roughly agree with values calculated from literature data for ILs with the same cations, but different anions. Comparison with magnetic relaxation data shows that the peculiarities are founded in the rotational dynamics and are not special to dielectric relaxation. Collectively, the observations make a strong case against the applicability of hydrodynamic approaches to the orientational dynamics of ions. (c) 2010 American Institute of Physics. [doi:10.1063/1.3478221]