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Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids

Nemoto, Fumiya*; Kofu, Maiko  ; Nagao, Michihiro*; Oishi, Kazuki*; Takata, Shinichi ; Suzuki, Junichi*; Yamada, Takeshi*; Shibata, Kaoru; Ueki, Takeshi*; Kitazawa, Yuzo*; Watanabe, Masayoshi*; Yamamuro, Osamu*

Alkyl-methyl-imidazolium ionic liquids C$$n$$mimX ($$n$$: alkyl-carbon number, X: anion) have short-range layer structures consisting of ionic and neutral (alkylchain) domains. To investigate the temperature dependences of the interlayer, interionic group, and inter-alkylchain correlations, we have measured the neutron diffraction (ND) of C$$n$$mimPF$$_6$$ ($$n$$ = 16, 9.5, 8). The quasielastic neutron scattering (QENS) of C16mimPF$$_6$$ was also measured to study the dynamics of each correlation. C16mimPF$$_6$$ shows a first-order transition between the liquid (L) and liquid crystalline (LC) phases at $$T_{rm c}$$ = 394 K. C8mimPF$$_6$$ exhibits a glass transition at $$T_{rm g}$$ = 200 K. C9.5mimPF$$_6$$ has both transitions at $$T_{rm c}$$ = 225 K and $$T_{rm g}$$ = 203 K. In the ND experiments, all samples exhibit three peaks corresponding to the correlations mentioned above. The widths of the interlayer peak at ca. 0.2 $AA$^{-1}$$ changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 $AA$^{-1}$$ exhibited a small jump at $$T_{rm c}$$. The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at $$T_{rm g}$$. The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at $$T_{rm c}$$, while those of other motions were monotonous in the whole temperature region. The structural and dynamical changes are characteristic of the L-LC transition in imidazolium-based ionic liquids.

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