Saturation of surfactant structure at the single-walled carbon nanotube surface

Density gradient ultracentrifugation (DGU) and fluorescence spectroscopy are used to probe the limiting behaviors of the dynamic response of surfactant structure at the single-walled carbon nanotube (SWNT) surface to reorganizing forces, including changes in surfactant concentration and electrolyte screening. DGU results indicate that, as surfactant (sodium dodecyl sulfate, SDS) concentration is increased, SDS adsorbed on metallic SWNTs becomes limited in its ability to reorganize before SDS adsorbed on semiconducting species. A diameter-dependent enhancement is observed in photoluminescence intensities from semiconducting SWNTS upon initial titration with NaCl. This response to electrostatic screening diminishes as SDS concentration is increased. The results are understood as a saturation of the surfactant structural response, defined as both a loss in ability to increase SDS loading at the SWNT surface and a loss in ability to reorient surface structure in response to a reorganizing force. Saturation of response is found to be reversible and also occurs as a result of restricting SDS mobility. These results confirm several aspects of recent molecular dynamics simulations of SDS behavior on SWNTs and have important implications for tunability of density-based separation approaches using cosurfactant systems that include SDS.