Departamento de Fisica dos Materiais e Mecanica, Universidade de Sao Paulo, 05315-970 Sao Paulo SP, Brazil.
ACS Nano. 2010 Feb 23;4(2):765-70. doi: 10.1021/nn901350s.
Several strategies aimed at sorting single-walled carbon nanotubes (SWNT) by diameter and/or electronic structure have been developed in recent years. A nondestructive sorting method was recently proposed in which nanotube bundles are dispersed in water-surfactant solutions and submitted to ultracentrifugation in a density gradient. By this method, SWNTs of different diameters are distributed according to their densities along the centrifuge tube. A mixture of two anionic amphiphiles, namely sodium dodecylsulfate (SDS) and sodium cholate (SC), presented the best performance in discriminating nanotubes by diameter. We present molecular dynamics studies of the water-surfactant-SWNT system. The simulations revealed one aspect of the discriminating power of surfactants: they can actually be attracted toward the interior of the nanotube cage. The binding energies of SDS and SC on the outer nanotube surface are very similar and depend weakly on diameter. The binding inside the tubes, on the contrary, is strongly diameter dependent: SDS fits best inside tubes with diameters ranging from 8 to 9 A, while SC is best accommodated in larger tubes, with diameters in the range 10.5-12 A. The dynamics at room temperature showed that, as the amphiphile moves to the hollow cage, water molecules are dragged together, thereby promoting the nanotube filling. The resulting densities of filled SWNT are in agreement with measured densities.
近年来，已经开发出了几种旨在通过直径和/或电子结构对单壁碳纳米管（SWNT）进行分类的策略。最近提出了一种非破坏性的分类方法，其中将纳米管束分散在水-表面活性剂溶液中，并在密度梯度中超离心。通过这种方法，不同直径的 SWNTs 根据它们在离心管中的密度沿着离心管分布。两种阴离子两亲物，即十二烷基硫酸钠（SDS）和胆酸钠（SC）的混合物，在通过直径来区分纳米管方面表现出最佳性能。我们对水-表面活性剂-SWNT 体系进行了分子动力学研究。模拟揭示了表面活性剂的区分能力的一个方面：它们实际上可以被吸引到纳米管笼的内部。SDS 和 SC 在纳米管外表面上的结合能非常相似，并且直径依赖性较弱。相反，在管内的结合强烈依赖于直径：SDS 最适合直径在 8 到 9 A 之间的管，而 SC 则最适合直径在 10.5-12 A 之间的较大管。在室温下的动力学研究表明，随着两亲物移动到空心笼中，水分子被一起拖曳，从而促进了纳米管的填充。填充的 SWNT 的密度与测量的密度一致。
