Zhuang Wenchang, Zhao Chunhua, Pan Yue, Li Qintang
School of Materials and Chemical Engineering, Xuzhou University of Technology, Xuzhou 221018, P. R. China.
Soft Matter. 2021 Mar 28;17(12):3494-3502. doi: 10.1039/d1sm00039j. Epub 2021 Mar 3.
As tailorable solvents, the physiochemical properties of ionic liquids can be tuned by the structure of ions. Herein, we investigate the structural effects of ILs on the self-assembly of surfactants. It has been confirmed that the cationic surfactant 1-hexadecyl-3-methylimidazolium bromide (CmimBr) can self-assemble into micellar and lamellar lyotropic liquid crystal phases in the aprotic ionic liquid (AIL) 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF). In this work, we explore the aggregation behaviours in AILs with different alkyl chains on the imidazolium group, i.e., 1-propyl-3-methylimidazolium tetrafluoroborate ([Pmim]BF), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF), 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim]BF) and 1-octyl-3-methylimidazolium tetrafluoroborate ([Omim]BF). With the increase of the cation chain length, AILs have better solubility of the solvophobic part of the surfactants and hence a weaker driving force for self-assembly. Therefore, the critical micellization concentration of CmimBr in AILs increases as confirmed by the surface tension and small angle X-ray scattering characterizations. More interesting things happen to the phase behaviours. Besides the micellar and lamellar lyotropic liquid crystal phases, a hexagonal lyotropic liquid crystal phase is formed in [Pmim]BF while hexagonal and bicontinuous cubic lyotropic liquid crystal phases are formed in [Bmim]BF, [Hmim]BF and [Omim]BF. It is surprising to observe richer phase behaviours in solvents of lower cohesive energy. The detailed structural information of various aggregates has been obtained by small-angle X-ray scattering. It is demonstrated that AILs work as not only solvents but also co-surfactants.
作为可定制的溶剂，离子液体的物理化学性质可通过离子结构进行调节。在此，我们研究了离子液体对表面活性剂自组装的结构影响。已证实，阳离子表面活性剂1-十六烷基-3-甲基咪唑溴化物（CmimBr）在非质子离子液体（AIL）1-乙基-3-甲基咪唑四氟硼酸盐（[Emim]BF₄）中可自组装成胶束和层状溶致液晶相。在这项工作中，我们探索了咪唑基团上具有不同烷基链的AILs中的聚集行为，即1-丙基-3-甲基咪唑四氟硼酸盐（[Pmim]BF₄）、1-丁基-3-甲基咪唑四氟硼酸盐（[Bmim]BF₄）、1-己基-3-甲基咪唑四氟硼酸盐（[Hmim]BF₄）和1-辛基-3-甲基咪唑四氟硼酸盐（[Omim]BF₄）。随着阳离子链长度的增加，AILs对表面活性剂的疏溶剂部分具有更好的溶解性，因此自组装驱动力较弱。因此，通过表面张力和小角X射线散射表征证实，AILs中CmimBr的临界胶束浓度增加。更有趣的是相行为。除了胶束和层状溶致液晶相外，[Pmim]BF₄中形成了六方溶致液晶相，而[Bmim]BF₄、[Hmim]BF₄和[Omim]BF₄中形成了六方和双连续立方溶致液晶相。在较低内聚能的溶剂中观察到更丰富的相行为令人惊讶。通过小角X射线散射获得了各种聚集体的详细结构信息。结果表明，AILs不仅作为溶剂起作用，还作为共表面活性剂起作用。
