Role of the bile salt surfactant sodium cholate in enhancing the aqueous dispersion stability of single-walled carbon nanotubes: a molecular dynamics simulation study.

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.