Numerous reports have shown that the self-assembling properties of 12-s-12 bis(quaternary ammonium) gemini surfactants in aqueous solution are significantly influenced by s, the number of methylene groups in the covalent spacer. However, the role played by s on the phase behavior of the single compounds has not been investigated in a similarly systematic way. Here, we report on the thermotropic phase behavior of the anhydrous compounds with s = 2–6, 8, 10, and 12, resorting to differential scanning calorimetry (DSC), polarized light microscopy (PLM), and X-ray diffraction (XRD). All of the compounds show a stepwise melting behavior, decomposing at 200 °C. As the spacer length increases, nonmonotonic trends are observed for the thermodynamic parameters of the thermotropic phase transitions, mesophase formation, and solid-state d_00l spacings. In particular, the number and type of mesophases (ordered smectic phases and/or fluid smectic liquid crystals) depend critically on s. Further, upon heating molecules with s < 8 decompose before the liquid phase, while those with long spacers, s = 8–12, reach the isotropization (clearing) temperature, hence forming both ionic liquid crystals and ionic liquid phases. We demonstrate that the melting behavior and type of ionic mesophases formed by gemini molecules can be usefully manipulated by a simple structural parameter like the length of the covalent linker.

中文翻译：

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双子表面活性剂间隔物长度对离子液晶形成和热致行为的关键作用

许多报告显示，12-自组装性质小号-12双（季铵）的水溶液双子表面活性剂是由显著影响小号，亚甲基基团的共价间隔物的数量。但是，尚未以类似的系统方式研究s在单一化合物的相行为上所起的作用。在这里，我们与无水化合物的热致相行为报告小号= 2-6、8、10和12，采用差示扫描量热法（DSC），偏振光显微镜（PLM）和X射线衍射（XRD）。所有化合物均表现出逐步熔融行为，在200°C时分解。随着间隔物长度的增加，可观察到热致相变，中间相形成和固态d _00l间距的热力学参数的非单调趋势。特别地，中间相（有序近晶相和/或流体近晶液晶）的数量和类型主要取决于s。此外，在加热时，s <8的分子在液相之前分解，而间隔长的分子则在s之前分解。= 8–12，达到各向同性（清除）温度，因此形成离子液晶和离子液相。我们证明了双子分子形成的离子中间相的熔化行为和类型可以通过简单的结构参数（如共价接头的长度）有效地进行控制。