Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto , R. Campo Alegre 687, P-4169-007 Porto, Portugal.
J Phys Chem B. 2013 Sep 19;117(37):10889-97. doi: 10.1021/jp406374a. Epub 2013 Sep 4.
Aiming at evaluating the impact of the cation symmetry on the nanostructuration of ionic liquids (ILs), in this work, densities and viscosities as a function of temperature and small-wide angle X-ray scattering (SWAXS) patterns at ambient conditions were determined and analyzed for 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (asymmetric) and 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide (symmetric) series of ionic liquids. The symmetric IL series, [CN/2CN/2im][NTf2], presents lower viscosities than the asymmetric [CN-1C1im][NTf2] counterparts. For ionic liquids from [C1C1im][NTf2] to [C6C6im][NTf2], an odd-even effect in the viscosity along the cation alkyl side chain length was observed, in contrast with a linear increase found for the ones ranging between [C6C6im][NTf2] and [C10C10im][NTf2]. The analysis of the viscosity data along the alkyl side chain length reveals a trend shift that occurs at [C6C1im][NTf2] for the asymmetric series and at [C6C6im][NTf2] for the symmetric series. These results are further supported by SWAXS measurements at ambient conditions. The gathered data indicate that both asymmetric and symmetric members are characterized by the occurrence of a distinct degree of mesoscopic structural organization above a given threshold in the side alkyl chain length, regardless the cation symmetry. The data also highlight a difference in the alkyl chain dependence of the mesoscopic cluster sizes for symmetric and asymmetric cations, reflecting a different degree of interdigitation of the aliphatic tails in the two families. The trend shift found in this work is related to the structural segregation in the liquid after a critical alkyl length size (CALS) is attained and has particular relevance in the cation structural isomerism with higher symmetry.
为了评估阳离子对称性对离子液体（ILs）纳米结构化的影响，在这项工作中，我们测定并分析了在环境条件下，1-烷基-3-甲基咪唑𬭩双（三氟甲基磺酰基）亚胺（不对称）和 1,3-二烷基咪唑𬭩双（三氟甲基磺酰基）亚胺（对称）系列离子液体的温度和小角度广角 X 射线散射（SWAXS）图谱作为温度的函数的密度和粘度。与非对称[CN-1C1im][NTf2]相比，对称的 IL 系列[CN/2CN/2im][NTf2]具有较低的粘度。对于从[C1C1im][NTf2]到[C6C6im][NTf2]的离子液体，在阳离子烷基侧链长度上观察到粘度的奇偶效应，而在[C6C6im][NTf2]和[C10C10im][NTf2]之间发现的粘度线性增加则相反。沿烷基侧链长度分析粘度数据揭示了一个趋势转变，在非对称系列中发生在[C6C1im][NTf2]，在对称系列中发生在[C6C6im][NTf2]。这些结果得到了环境条件下 SWAXS 测量的进一步支持。所收集的数据表明，无论是对称还是非对称成员，都具有在特定的侧链烷基长度阈值以上发生明显程度的介观结构组织的特征，而与阳离子的对称性无关。数据还突出了对称和非对称阳离子的介观簇尺寸对烷基链的依赖关系的差异，反映了两种族中脂肪尾部的不同程度的互贯。在这项工作中发现的趋势转变与在达到临界烷基长度（CALS）后液体中的结构分离有关，对于具有更高对称性的阳离子结构异构性具有特殊意义。
