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Cation Dynamics in Supercooled and Solid Alkyl Methylimidazolium Bromide Ionic Liquids.

The molecular dynamics of alkyl methylimidazolium bromide ionic liquids with different side groups of the cation are studied over a wide range of temperatures, covering the supercooled and crystalline states. Nuclear magnetic resonance relaxation dispersion (NMRD) at different magnetic field strengths was combined with NMR pulsed field gradient (PFG) diffusion measurements in order to obtain a description of the temperature dependence of the cationic mobility. While an Arrhenius dependence of the correlation times was found at high temperatures, a deviation is observed below a critical temperature of Tdyn ∼ 275 K which corresponds to about 1.25 Tg for two of the substances. The macroscopic diffusion coefficient, on the other hand, is best described by a VFT dependence down to a similar temperature, and a much weaker temperature dependence below. Measurements carried out in the crystalline state of 1-butyl-3-methylimidazolium bromide (Bmim Br) exhibit a dramatically increased self-diffusion coefficient in agreement with earlier reports of strong dynamic heterogeneity in the presence of minute amounts of water.