A Molecular Dynamics Study of Single-Walled Carbon Nanotubes (SWCNTs) Dispersed in Bile Salt Surfactants

A Molecular Dynamics Study of Single-Walled Carbon Nanotubes (SWCNTs) Dispersed in Bile Salt Surfactants[*]

Frederick R. Phelan Jr.[†]^,1  and Huai Sun²

¹Materials Science and Engineering Division, NIST, Gaithersburg, MD 20899

²Aeon Technology Inc. and School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China

  Abstract

Separation of single-walled carbon nanotubes by chirality is an important aspect of nanotube metrology. Density gradient ultra-centrifugation (DGU) has emerged as one of the most important tools for chirality separations, and relies heavily on the use of bile salt surfactants to create buoyant density differences between tubes of different chirality, based on the size and packing structure of the hydration shell surrounding the dispersed tube [1-4]. However, even though DGU is extensively used, a greater understanding the surfactant self-assembly mechanism and packing structures for bile salt surfactants and co-surfactants of different functionality is greatly needed [3-4], as the efficacy  of the separations depends critically on the details of the SWCNT/surfactant complex that forms during the dispersion stage. In this work, we study the behavior of a family of bile salt surfactant and co-surfactant mixtures for a variety of metallic and semi-conducting SWCNT chiralities. The goal is to provide a greater understanding of how small changes in surfactant functionality, e.g., sodium deoxycholate (DOC), sodium taurodeoxycholate (TDOC), and sodium cholate (SC), promote different packing densities critical to DGU separations, and compare with recent experimental results [4]. The force field parameters used in the simulations are developed based on the Consistent Force-Field (CFF) potential using the Direct Force-Field 7.0 (DFF7) software [5], and will be stored in WebDFF, an open and extensible, smart force-field repository being developed at NIST in collaboration with Aeon Technology, as part of the Materials Genome Initiative (MGI).

References

[1]   M. S. Arnold, S. I. Stupp, and M. C. Hersam, “Enrichment of Single-Walled Carbon Nanotubes by Diameter in Density Gradients,” Nano Lett., vol. 5, no. 4, pp. 713–718, Apr. 2005.

[2]   M. S. Arnold, A. A. Green, J. F. Hulvat, S. I. Stupp, and M. C. Hersam, “Sorting carbon nanotubes by electronic structure using density differentiation,” Nat Nano, vol. 1, no. 1, pp. 60–65, Oct. 2006.

[3]   F. Bonaccorso, T. Hasan, P. H. Tan, C. Sciascia, G. Privitera, G. Di Marco, P. G. Gucciardi, and A. C. Ferrari, “Density Gradient Ultracentrifugation of Nanotubes: Interplay of Bundling and Surfactants Encapsulation,” J. Phys. Chem. C, vol. 114, no. 41, pp. 17267–17285, Oct. 2010.

[4]   J. A. Fagan, M. Zheng, V. Rastogi, J. R. Simpson, C. Y. Khripin, C. A. Silvera Batista, and A. R. Hight Walker, “Analyzing Surfactant Structures on Length and Chirality Resolved (6,5) Single-Wall Carbon Nanotubes by Analytical Ultracentrifugation,” ACS Nano, Mar. 2013.

[5]   “DFF 7, AeonTechnology.” [Online]. Available: http://www.aeontechnology.com/Product_DFF.php. [Accessed: 13-May-2013].

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[*] Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

[†] Corresponding author: NIST, 100 Bureau Dr., STOP 8542, Gaithersburg, MD 20899; (301) 975-6761; Fax: (301) 975-4924; frederick.phelan@nist.gov