Department of Chemistry , University at Buffalo, State University of New York , Buffalo , New York 14260-3000 , United States.
Langmuir. 2018 Aug 7;34(31):9279-9288. doi: 10.1021/acs.langmuir.8b01813. Epub 2018 Jul 25.
We report direct spectroscopic measurements of the macromolecular organization of ionic surfactants on the surface of semiconducting single-walled carbon nanotubes (SWCNTs) within solution-processed thin films. By using vibrational sum frequency generation (VSFG) spectroscopy, sensitive measurements of interfacial surfactant ordering were obtained as a function of surfactant concentration for sodium dodecyl sulfate (SDS)-encapsulated (6,5) and (7,6) SWCNTs with and without excess electrolytes. Anionic surfactants are known to effectively stabilize SWCNTs. The current models suggest a strong influence of the dispersion conditions on the surfactant interfacial macromolecular organization and self-assembly. Direct experimental probes of such an organization using nanotubes of specific chirality are needed to validate the existing models. We found that as the bulk SDS concentration increases near the surfactant critical micelle concentration, the interfacial ordering increased, approaching the formation of cylindrical-like micelles with the nanotube at the core. At the higher surfactant concentrations measured here, the (6,5) SWCNTs produced more ordered structures relative to those with the (7,6) SWCNTs. The relatively larger-diameter (7,6) chiral tubes support enhanced van der Waals (vdW) interactions between the tube carbon surface and the surfactant methylene chain groups that likely increase the density of gauche defects. A new effect arises when the precursor solution is exposed to a small concentration of divalent Ca counterions. We postulate that a salt-bridging configuration on such highly curved surfaces decreases the ordering of interfacial surfactant molecules, resulting in compact, disordered structures. However, this phenomenon was not observed with excess Na ions at the same ionic strength. Instead, a modest increase in surfactant ordering was observed with the excess monovalent electrolyte. These results provide new insights for thin film solution processing of vdW nanomaterials and demonstrate that VSFG is a sensitive probe of surfactant organization on nanostructures.
我们报告了在溶液处理的薄膜中，直接对离子表面活性剂在半导体单壁碳纳米管（SWCNT）表面的大分子组织进行的光谱测量。通过使用振动和频产生（VSFG）光谱，我们获得了作为表面活性剂浓度函数的界面表面活性剂有序性的敏感测量结果，其中包括具有和不具有过量电解质的十二烷基硫酸钠（SDS）包裹的（6,5）和（7,6）SWCNT。阴离子表面活性剂被认为能有效地稳定 SWCNT。目前的模型表明，分散条件对表面活性剂界面大分子组织和自组装有很大的影响。需要使用特定手性的纳米管对这种组织进行直接的实验探测，以验证现有的模型。我们发现，随着体相 SDS 浓度在表面活性剂临界胶束浓度附近增加，界面有序性增加，接近形成以纳米管为核心的圆柱状胶束。在我们这里测量的较高表面活性剂浓度下，（6,5）SWCNT 产生的有序结构相对于（7,6）SWCNT 更多。相对较大直径的（7,6）手性管支持管碳表面与表面活性剂亚甲基链基团之间的增强范德华（vdW）相互作用，这可能增加 gauche 缺陷的密度。当前驱体溶液暴露于少量二价 Ca 反离子时，会出现一个新的现象。我们假设，在这种高度弯曲的表面上，盐桥的形成会降低界面表面活性剂分子的有序性，导致紧凑、无序的结构。然而，在相同离子强度下，用过量的 Na 离子没有观察到这种现象。相反，用过量的单价电解质观察到表面活性剂有序性略有增加。这些结果为 vdW 纳米材料的薄膜溶液处理提供了新的见解，并证明了 VSFG 是纳米结构表面活性剂组织的敏感探针。
