Glass Transitions and Low-energy Excitations of Ionic Liquids

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We have reviewed our recent heat capacity and neutron scattering works on various ionic liquids with 1-butyl-3-methylimidazolium (bmim) and its related cations. The heat capacity data revealed that above ionic liquids exhibit glass transitions with large Cp jumps below room temperatures. The temperature dependence of the mean square displacements of the ionic liquids, which were determined from the neutron scattering data, indicated that fast (THz) relaxations take place above their glass transition temperatures. The relation between the glass transition and fusion temperatures of the ionic liquids is consistent with the empirical law (Tg/Tfus=2/3) for molecular liquids. The relation between Tg and anion size suggested that the configurational motions of cations, surrounded by a relatively-rigid anion framework, is dominant for the glass transitions of ionic liquids. The temperature dependence of the configurational entropy and the size of cooperatively rearranging region (CRR) of the ionic liquids are similar to those of molecular liquids. Both heat capacity and inelastic neutron scattering data showed that the ionic glasses exhibit so-called boson peaks around 2meV. By summarizing the above results, it is concluded that the ionic liquids with bmim and its related cations are similar to molecular liquids.
日本熱測定学会の論文