Surfactant-nanotube interactions in water and nanotube separation by diameter: atomistic simulations |
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Authors: | E JF Carvalho and M C dos Santos |
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Institution: | (1) Department of Chemistry and Richard E. Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, TX 77005, USA |
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Abstract: | A non-destructive sorting method to separate
single-walled carbon nanotubes (SWNTs) by diameter was recently
proposed. By this method, SWNTs are suspended in water by
surfactant encapsulation and the separation is carried out by
ultracentrifugation in a density gradient. SWNTs of different
diameters are distributed according to their densities along the
centrifuge tube. A mixture of two anionic surfactants, namely
sodium dodecylsulfate (SDS) and sodium cholate (SC), presented the
best performance in discriminating nanotubes by diameter.
Unexpectedly, small diameter nanotubes are found at the low
density part of the centrifuge tube. We present molecular dynamics
studies of the water-surfactant-SWNT system to investigate the
role of surfactants in the sorting process. We found that
surfactants can actually be attracted towards the interior of the
nanotube cage, depending on the relationship between the
surfactant radius of gyration and the nanotube diameter. The
dynamics at room temperature showed that, as the amphiphile moves
to the hollow cage, water molecules are dragged together, thereby
promoting the nanotube filling. The resulting densities of filled
SWNT are in agreement with measured densities. |
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