《Role of the Bile Salt Surfactant Sodium Cholate in Enhancing the Aqueous Dispersion》.pdf

15616 J. Phys. Chem. B 2010, 114, 15616–15625 Role of the Bile Salt Surfactant Sodium Cholate in Enhancing the Aqueous Dispersion Stability of Single-Walled Carbon Nanotubes: A Molecular Dynamics Simulation Study Shangchao Lin†,‡ and Daniel Blankschtein*,† Departments of Chemical Engineering and Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Recei Ved: August 12, 2010; ReVised Manuscript ReceiVed: October 15, 2010 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 saturat