(551c) Potential of Mean Force Between Aqueous Single Walled Carbon Nanotubes in Surfactant Solutions

Molecular dynamic simulations were conducted to calculate the effective potential of mean force (PMF) between two (6,6) SWNTs in the presence of aqueous surfactants. We employed two sodium dodecyl sulfate (SDS) concentrations, with SDS adsorbed on the (6,6) SWNTs, to calculate the PMF as a function of nanotube-nanotube distance. For comparison, we also computed the PMF between two pristine (6,6) SWNTs in water, as well as between two (6,6) SWNTs covered by flavin mononucleotide (FMN) surfactants. In the absence of surfactants our results show, as expected, a strongly attractive SWNT-SWNT PMF at short nanotube-nanotube separations. The presence of SDS surfactants modulates the PMF profile by introducing mid-range nanotube-nanotube repulsions. Because FMN surfactants, the phosphorilated form of vitamin B2, contain an aromatic isoalloxazine moiety and a chiral phosphate group, they couple more tightly with the SWNTs and yield a much more pronounced repulsive force between the approaching SWNTs than that observed in the presence of SDS. Our results will help us identify the surfactant properties that allow us to manipulate nanotube-nanotube effective interactions. This is the key for designing nanotube-specific dispersing agents.