Role of the Bile Salt Surfactant Sodium Cholate in Enhancing the Aqueous Dispersion Stability of Single-Walled Carbon Nanotubes: A Molecular Dynamics Simulation Study
Role of the Bile Salt Surfactant Sodium Cholate in Enhancing the Aqueous Dispersion Stability of Single-Walled Carbon Nanotubes: A Molecular Dynamics Simulation Study
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DOI：
10.1021/jp1076406
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发表时间：
2010-12-02
影响因子：
3.3
通讯作者：
Blankschtein, Daniel
中科院分区：
文献类型：
--
作者：
Lin, Shangchao;Blankschtein, Daniel
近年来，胆盐生物表面活性剂被广泛应用于单壁碳纳米管(SWNTs)在高质量分数的水溶液中的分散，以及利用超速离心法根据单壁碳纳米管的电子性质进行分类。为了帮助阐明胆盐在单壁碳纳米管分散过程中的作用，我们报道了第一个详细的大规模全原子分子动力学(MD)模拟研究，研究了常见胆盐表面活性剂胆酸钠(SC)在水溶液中在单壁碳纳米管上的吸附和表面自组装。我们发现胆酸盐离子像一个环一样缠绕在SWNT周围，并且有一个小的趋势，垂直于SWNT的圆柱轴，这是传统的线型表面活性剂如十二烷基硫酸钠(SOS)所没有观察到的独特特征。此外，我们还进行了一系列的模拟来计算两个平行的SC覆盖的单壁碳纳米管之间的平均力势(PMF)随管间距离的变化。通过比较我们模拟的SC的PMF分布和文献报道的十二烷基硫酸钠的PMF分布，我们发现，在饱和表面覆盖率下，SC是一种比十二烷基硫酸钠更好的稳定剂，这一发现与SC在水介质中分散SWNTs的广泛使用是一致的。事实上，SC的色散诱导稳定性优于十二烷基硫酸钠，这是因为在PMF分布中，SC具有较高的排斥能垒和较浅的吸引能。特别是，我们发现SC诱导的较浅的吸引能是由于SC坚硬的豆状结构，这使得这种胆盐表面活性剂能够更有效地适应管间间隙。
Very recently, bile salt biosurfactants have been utilized extensively to disperse individual single-walled carbon nanotubes (SWNTs) in aqueous solution with high weight fractions, as well as to sort SWNTs according to their electronic properties with the aid of ultracentrifugation. To help elucidate the role of bile salts in the SWNT dispersion process, we report the first detailed large-scale all-atomistic molecular dynamics (MD) simulation study of the adsorption and surface self-assembly of a common bile salt surfactant, sodium cholate (SC), on a SWNT in aqueous solution. We find that the cholate ions wrap around the SWNT like a ring and have a small tendency to orient perpendicular to the cylindrical axis of the SWNT, a unique feature that has not been observed for conventional linear surfactants such as sodium dodecyl sulfate (SOS). In addition, we carry out a series of simulations to compute the potential of mean force (PMF) between two parallel SC-covered SWNTs as a function of the intertube separation. By comparing our simulated PMF profile of SC with the PMF profile of SDS reported in the literature, we found that, at the saturated surface coverages, SC is a better stabilizer than SDS, a finding that is consistent with the widespread use of SC to disperse SWNTs in aqueous media. Indeed, the superior dispersion-induced stability of SC over SDS results from a higher repulsive energy barrier and a shallower attractive energy well induced by SC in the PMF profile. In particular, we found that the shallower attractive energy well induced by SC is due to the rigid, bean-like structure of SC which allows this bile salt surfactant to more effectively accommodate the intertube gap.