One of the characteristic and interesting features of ionic liquids (ILs) is nanometer-sized structure (nanostructure). For example, imidazolium-based ILs (ImILs) have nanostructures consisting of the polar-domains with imidazolium rings and anions and the non-polar domains with alkylchains of the cations. We have revealed that the nanostructure is essentially the same as that of a liquid-crystalline (LC) phase [1-3] and investigated the dynamics of ImILs by means of quasielastic neutron scattering (QENS) [1,2,4]. Recently, Yamada et al. found that some of alkylammonium-based salts, which are another popular ILs, have plastic-crystalline (PC) phases that are the counterparts to the LC phase; the LC phase has an orientationally-ordered positionally-disordered structure, while the PC phase has an orientationally-disordered positionally-ordered structure. We have measured the heat capacities and QENS data of methyldiethylisopropylammonium bis(trifluoromethylsulfonyl)imide (abbreviated as N1223’Tf2N) and the methylethylpropylisopropylammonium salt (N1233’Tf2N) in a temperature range between 4 and 400 K. They have crystalline (C), liquid (L) and two PC phases; one is an isotropic (normal) PC phase and the other is an anisotropic PC phase where the cations undergo uniaxial rotations. The QENS data were collected in a time range between 1 ps and 10 ns by means of HFBS and DCS spectrometers in NIST. Fig. 1 shows the result of the fixed window scan measured by HFBS. A clear decrease in intensity, corresponding to the acceleration of the relaxation, appeared at each transition. The details of the relaxations obtained from the QENS analysis will be compared with those of the ImILs in the meeting.

Thermal and neutron scattering studies on alkylammonium-based ionic liquids with plastic-crystalline phases

255th National Meeting (2018)

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