Dynamics of Surfactant-Suspended Single Walled Carbon Nanotubes In a Centrifugal Field

Nitish Nair1, Woo-Jae Kim1, Richard D. Braatz2, and Michael S. Strano3. (1) Department of Chemical Engineering, Massachusetts Institute of Technology, Building 66-580, 25 Ames St, Cambridge, MA 02141, (2) Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Box C-3, 293 Roger Adams Laboratory, Urbana, IL 61801-3602, (3) 66-566 Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

A hydrodynamic model is used to describe the motion of surfactant-suspended single walled carbon nanotubes in a density gradient, while being subjected to a centrifugal field. The number of surfactant molecules adsorbed on each nanotube determines its effective density, and hence, its position in the gradient after centrifugation has been completed. Analysis of the spatial concentration distributions of CoMoCAT nanotubes suspended with 2 w/v% sodium cholate yielded 2.21, 2.36 and 2.74 surfactant molecules adsorbed per nanometer along the length of the (6,5), (7,5) and (8,7) nanotubes, respectively. The estimates are commensurate with experimental values reported in the literature. Based on these results, a linear relation is proposed between SWNT diameter and the adsorbate population, which can be used to predict the fate of a species in the separation process. Since the density of the surfactant-nanotube assembly is highly sensitive to the number of adsorbed molecules, a perturbation would cause it to be enriched at a different location in the gradient. The level of sensitivity is also reflected in the 95% confidence levels that are reported in this work.