Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids
Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids
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DOI：
10.1063/1.5037217
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发表时间：
2018-08-07
影响因子：
4.4
通讯作者：
Yamamuro, Osamu
中科院分区：
文献类型：
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作者：
Nemoto, Fumiya;Kofu, Maiko;Yamamuro, Osamu
烷基甲基咪唑离子液体CnmimX（n：烷基碳数，X：阴离子）具有短程层状结构，由离子和中性（烷基链）区域组成。为了研究层间、非离子基团和烷基链间相关性的温度依赖性，我们测量了C16 mimPF（6）、C95 mimPF（6）和C8 mimPF（6）在4 K到470 K温度范围内的中子衍射（ND）。还测量了C16 mimPF（6）的准弹性中子散射（QENS），以研究每个相关性的动力学。C16 mimPF（6）在T-c = 394 K时表现出液相（L）和液晶（LC）之间的一级相变。C8 mimPF（6）的玻璃化转变温度为200 K。C9.5mimPF（6）是C8 mimPF（6）和C10 mimPF 6之间的1：3混合物，在T-c = 225 K和T-g = 203 K处具有两个转变。在ND实验中，所有样品都表现出对应于上述相关性的三个峰。层间峰的宽度在约。0.2埃（-1）在L-LC转变处急剧变化，而电子峰在约100埃处。1埃（-1）在T-c处表现出小的跳跃。三个峰的峰位和峰面积在过渡处变化不大。T-g时结构变化最小。QENS实验表明，层间运动的弛豫时间在T-c处增加了10倍，而其他运动的弛豫时间在整个温度范围内是单调的。上述结构和动力学变化是咪唑类离子液体中L-LC转变的特征。出版社：AIP Publishing
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.