Dispersion of single-walled carbon nanotubes in aqueous solutions of the anionic surfactant NaDDBS
Dispersion of single-walled carbon nanotubes in aqueous solutions of the anionic surfactant NaDDBS
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DOI：
10.1021/jp0365099
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发表时间：
2003-12-04
影响因子：
3.3
通讯作者：
Resasco, DE
中科院分区：
文献类型：
--
作者：
Matarredona, O;Rhoads, H;Resasco, DE
单壁碳纳米管(SWNT)在水或有机溶剂中的不溶性限制了这一独特材料的实际应用。最近的研究表明，使用适当的表面活性剂可以大大提高单壁碳纳米管的悬浮性能。尽管人们已经在不同程度上证明了阴离子、阳离子和非离子表面活性剂的有效性，但碳纳米管与不同表面活性剂相互作用的确切机制仍不清楚。为了加深对这种界面现象的理解，我们研究了表面化学修饰对纳米管-表面活性剂相互作用程度的影响。单壁碳纳米管表面化学的这种变化可以通过简单地改变前处理方法来实现，前处理方法可以是酸性的，也可以是碱性的。我们发现，提纯方法对PZC(零电荷点)等本征表面性质有很大影响。也就是说，单壁碳纳米管表面的电荷随周围介质的pH值而变化。然而，研究发现，在阴离子表面活性剂十二烷基苯磺酸钠(NaDDBS)在SWNT上的吸附过程中，库仑力并不起主要作用，而是被表面活性剂尾部和纳米管壁之间的疏水相互作用所克服。只有在远离PZC的pH值时，库仑力才变得重要。表面活性剂尾部和纳米管之间的疏水作用力决定了表面活性剂稳定的纳米管的结构。在这种结构中，每个纳米管被表面活性剂分子的单层覆盖，其中头部形成紧凑的外表面，而尾部保持与纳米管壁的接触。值得注意的是，尽管最终构型可以描述为中心有纳米管的圆柱形胶束，但这种结构的形成机理不是通过将纳米管掺入胶束中进行的，而是通过最终形成表面活性剂单分子层的两步吸附来进行的。
The insolubility of single-walled carbon nanotubes (SWNT) in either water or organic solvents has been a limitation for the practical application of this unique material. Recent studies have demonstrated that the suspendability of SWNT can be greatly enhanced by employing appropriate surfactants. Although the efficiency of anionic, cationic, and nonionic surfactants has been demonstrated to different extents, the exact mechanism by which carbon nanotubes and the different surfactants interact is still uncertain. To deepen the understanding of this interfacial phenomenon, we have investigated the effects of chemical modifications of the surface on the extent of nanotube-surfactant interaction. Such changes in the surface chemistry of the SWNT can be achieved by simply varying the pretreatment method, which can be acidic or basic. We have found that intrinsic surface properties such as the PZC (point of zero charge) are greatly affected by the purification method. That is, the electrical charge of the SWNT surface varies with the pH of the surrounding media. However, it has been found that during the adsorption of the anionic surfactant sodium dodecylbenzenesulfonate (NaDDBS) on SWNT Coulombic forces do not play a central role, but are overcome by the hydrophobic interactions between the surfactant tail and the nanotube walls. Only at pH values far from the PZC do the Coulombic forces become important. The hydrophobic forces between the surfactant tail and the nanotube determine the structure of the surfactant-stabilized nanotubes. In such a structure, each nanotube is covered by a monolayer of surfactant molecules in which the heads form a compact outer surface while the tails remain in contact with the nanotube walls. It is important to note that although the final configuration can be described as a cylindrical micelle with a nanotube in the center, the mechanism of formation of this structure does not proceed by incorporation of a nanotube into a micelle, but rather by a two-step adsorption that ends up in the formation of a surfactant monolayer.