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

Abstract

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 C16mimPF$_6$, C9.5mimPF$_6$, and C8mimPF6 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 Å−1 changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 Å$^{-1}$ 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.