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

Alkyl-methyl-imidazolium ionic liquids CnmimX (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 C16mimPF, C9.5mimPF, and C8mimPF in the temperature region from 4 K to 470 K. The quasielastic neutron scattering (QENS) of C16mimPF was also measured to study the dynamics of each correlation. C16mimPF shows a first-order transition between the liquid (L) and liquid crystalline (LC) phases at T = 394 K. C8mimPF exhibits a glass transition at T = 200 K. C9.5mimPF, which is a 1:3 mixture between C8mimPF and C10mimPF, has both transitions at T = 225 K and T = 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 Å changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 Å exhibited a small jump at T. The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at T. The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at T, while those of other motions were monotonous in the whole temperature region. The structural and dynamical changes mentioned above are characteristic of the L-LC transition in imidazolium-based ionic liquids.