Tuning the Surface Ordering of Self-Assembled Ionic Surfactants on Semiconducting Single-Walled Carbon Nanotubes: Concentration, Tube Diameter, and Counterions
Tuning the Surface Ordering of Self-Assembled Ionic Surfactants on Semiconducting Single-Walled Carbon Nanotubes: Concentration, Tube Diameter, and Counterions
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DOI：
10.1021/acs.langmuir.8b01813
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发表时间：
2018-08-07
期刊：
影响因子：
3.9
通讯作者：
Velarde, Luis
中科院分区：
文献类型：
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作者：
Algoul, Soha T.;Sengupta, Sanghamitra;Velarde, Luis
我们报道了溶液处理薄膜中离子表面活性剂在半导体单壁碳纳米管(SWCNTs)表面的大分子结构的直接光谱测量。利用振动和频(VSFG)光谱技术，研究了十二烷基硫酸钠(SDS)包覆的(6，5)和(7，6)单壁碳纳米管在有无过量电解液的情况下，界面表面活性剂有序度随表面活性剂浓度的变化规律。已知阴离子表面活性剂能有效稳定单壁碳纳米管。目前的模型表明分散条件对表面活性剂界面大分子的组织和自组装有很大的影响。为了验证现有的模型，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。我们发现，随着表面活性剂临界胶束浓度附近的体积浓度的增加，界面有序性增加，接近形成以纳米管为核心的圆柱状胶束。在较高的表面活性剂浓度下，(6，5)单壁碳纳米管比(7，6)单壁碳纳米管产生了更多的有序结构。相对较大的直径(7，6)手性管支持增强的范德华(VDW)相互作用之间的管碳表面和表面活性剂亚甲基链基，这可能增加高切缺陷的密度。当前体溶液暴露在小浓度的二价钙离子反离子中时，会产生新的效应。我们假设，在这种高度弯曲的表面上的盐桥构型减少了界面表面活性剂分子的有序性，导致了紧凑、无序的结构。然而，在相同离子强度下，过量的Na+离子不能观察到这一现象。相反，在过量的单价电解液中，表面活性剂的有序度略有增加。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是表面活性剂在纳米结构上组织的灵敏探针。
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 Ca2+ 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.