Determination of the Surfactant Density on SWCNTs by Analytical Ultracentrifugation
We report on the extensive characterization of single-walled carbon nanotubes (SWCNTs) dispersed in a variety of surfactants, such as sodium dodecyl benzene sulfonate (SDBS), sodium cholate (SC), and three synthesized perylene-based surfactants, by using differential sedimentation in H2O and D2O. Multidimensional evaluation of the absorption profiles over radius, wavelength, and time allows the determination of the anhydrous specific volumes of the SWCNT–surfactant complexes as well as the concentration of the surfactant reservoir in free micelles with very slow sedimentation coefficients (<1 Svedberg). Among the perylene bisimide surfactants, the smallest derivative is densely adsorbed on the nanotube backbone with an anhydrous specific volume significantly above that of SC or SDBS. Bulky Newkome dendritic groups on one or both ends of the perylene moiety gradually reduce the adsorption density, in accord with the absolute adsorption between 0.66 and 1.7 mmol surfactant per gram SWCNTs. Furthermore, hydrodynamic analysis reveals that SDBS favors the “tails-on” configuration. The distribution of sedimentation coefficients of SWCNTs prepared by high-pressure carbon monoxide decomposition (HiPco) is broader and shifted to faster sedimentation than those prepared by using cobalt–molybdenum catalysis (CoMoCAT), which reflects the polydispersity in diameter and length.