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Soft Matter. 2017 Oct 4;13(38):6947-6955. doi: 10.1039/c7sm01464c.
The nanosegregated bulk structure, and its evolution with the cation's alkyl length n, are studied by X-ray scattering for an unprecedentedly broad homologous series of a model room-temperature ionic liquid, [CMIM][NTf] (n = 4-22). A tri-periodic local structure is found, with the lateral periodicities, d and d independent of n, and a longitudinal one, d, linearly increasing with n. The results are consistent with a local structure comprising alternating layers of polar headgroups and apolar, interdigitated, partly overlapping, cations' alkyl tails, of an average macroscopic mass density close to that of liquid alkanes. A slope decrease in the linear d(n) suggests a change from a lower to a higher rate of increase with n of chain overlap for n ≥ 12. The order decay lengths of the layering, and of the lateral chain packing, increase with n, as expected from the increasing van der Waals interaction's domination of the structure. The headgroups' lateral packing decay length decreases with n, due to increasing frustration between the longer lateral periodicity preferred by the headgroups, and the shorter lateral periodicity preferred by the chains. A comparison of the bulk and surface structures highlights the surface's ordering effect, which, however, does not induce here a surface phase different from the bulk, as it does in liquid crystals and liquid alkanes.
采用 X 射线散射技术研究了纳米级大块结构及其随阳离子烷基长度 n 的演变，该技术用于研究一个空前广泛的模型室温离子液体[CMIM][NTf]（n = 4-22）同系物。发现了一种三周期局域结构，其横向周期性 d 和 d 与 n 无关，而纵向周期性 d 则随 n 线性增加。结果与局部结构一致，该结构由交替的极性头基层和非极性、互穿插、部分重叠的阳离子烷基尾层组成，宏观质量密度接近液体烷烃。线性 d(n)斜率下降表明，对于 n≥12，链重叠的增加速率从较低变为较高。分层和横向链堆积的有序衰减长度随 n 增加，这与范德华相互作用对结构的主导作用增加是一致的。头基的横向堆积衰减长度随 n 减小，这是由于头基更喜欢较长的横向周期性，而链更喜欢较短的横向周期性，从而导致头基之间的相互作用增加。大块和表面结构的比较突出了表面的有序化效应，但与液晶和液体烷烃不同，它在这里不会诱导与体相不同的表面相。
