Interaction of Surfactants Containing a Sulfuric Group with a (5,5) Carbon Nanotube

Sulfuric acid is a good nanotube disperser but surfactants containing a sulfuric head group with a Na atom are better for dispersing carbon nanotubes. Nevertheless, there is no explanation for the importance of the sodium atom in the head group. We have studied the interaction of sulfuric acid, sodium bisulfate and sodium butyl sulfate molecules with a (5,5) single-walled carbon nanotube using density functional theory (DFT), calculating equilibrium configurations, binding energies, charge transfers, and densities of states. In all cases there is charge transfer from the tube to the molecules when these are adsorbed, as has been previously found for sulfuric acid adsorbed on graphene. The presence of these molecules with their hydrogen atoms pointing to the tube does not affect the density of states of the tube close to the Fermi energy, but the presence of sodium bisulfate or sodium butyl sulfate with the Na atom close to the tube enhances the density of states in the region below and near the Fermi energy. This difference could play a role in the superior surfactant properties a molecules with a Na atom in the head group.