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(6), C95mimPF(6), and C8mimPF(6) in the temperature region from 4 K to 470 K. 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-c = 394 K. C8mimPF(6) exhibits a glass transition at T-g = 200 K. C9.5mimPF(6), which is a 1:3 mixture between C8mimPF(6) and C10mimPF6, has both transitions at T-c = 225 K and T-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 angstrom(-1) changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 angstrom(-1) exhibited a small jump at T-c. The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at T-g. The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at T-c, 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. Published by AIP Publishing.