Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
J Phys Chem B. 2010 Dec 2;114(47):15616-25. doi: 10.1021/jp1076406. Epub 2010 Nov 4.
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 (SDS). 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.
最近，胆盐生物表面活性剂被广泛用于在水溶液中分散高重量分数的单壁碳纳米管（SWNTs），并借助超速离心对 SWNTs 按其电子性质进行分类。为了帮助阐明胆盐在 SWNT 分散过程中的作用，我们报告了第一个关于在水溶液中胆酸钠（SC）这种常见胆盐表面活性剂在 SWNT 上吸附和表面自组装的详细的大规模全原子分子动力学（MD）模拟研究。我们发现，胆酸盐离子像一个环一样包裹在 SWNT 上，并且有一个小的倾向垂直于 SWNT 的圆柱轴取向，这是一个独特的特征，尚未在常规线性表面活性剂（如十二烷基硫酸钠（SDS））中观察到。此外，我们进行了一系列模拟来计算两个平行 SC 覆盖的 SWNT 之间的平均力势（PMF）作为 tube 间分离的函数。通过将我们模拟的 SC 的 PMF 分布与文献中报道的 SDS 的 PMF 分布进行比较，我们发现，在饱和表面覆盖率下，SC 比 SDS 更能稳定 SWNTs，这一发现与 SC 广泛用于在水溶液中分散 SWNTs 是一致的。事实上，SC 在 PMF 分布中引起的更高排斥能垒和更浅的吸引力势阱使得 SC 比 SDS 具有更好的分散诱导稳定性。特别是，我们发现 SC 引起的较浅的吸引力势阱归因于 SC 的刚性、豆状结构，这使得这种胆盐表面活性剂能够更有效地适应 tube 间的间隙。
