Role of Surfactant Molecular Structure on Self-Assembly: Aqueous SDBS on Carbon Nanotubes

Stabilizing single-walled carbon nanotubes (SWNTs) monodispersed in diameter and chirality in aqueous media remains elusive. Surfactants have proven useful in deploying ultracentrifugation techniques for separating carbon nanotubes, but the molecular mechanism responsible for the effectiveness for such technique remains not fully understood. On the basis of recent molecular simulation results, it appears that the morphology of self-assembled surfactant aggregates on carbon nanotubes strongly affects the effective potential of mean force between pairs of interacting carbon nanotubes. In the present work, the effect of surfactant molecular structure on the properties of aqueous surfactant self-assembled aggregates was investigated using all-atom molecular dynamics simulations. To quantify how the surfactant molecular structure affects self-assembly, sodium dodecylbenzenesulfonate (SDBS) surfactants with the headgroup located either on the fifth or on the twelfth carbon atom along the dodecyl tail were considered. All simulations were conducted at room conditions for different surface coverages on (6,6), (12,12), and (20,20) SWNTs. The results suggest that the surfactant molecular structure strongly affects the packing of surfactants on the nanotubes, therefore modulating effective nanotube-nanotube interactions. In qualitative agreement with experiments, no strong effects due to nanotube diameter were observed.