Laboratoire Francis PERRIN
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The role of surfactants in carbon nanotubes density gradient separation Séminaires SPAM LFP
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04/12/2008 à 11h00
LIDYL Bât 522, p 138 CEA-Saclay
Maria Cristina dos Santos - Instituto de Fisica, Universidade de Sao Paulo (USP) Sao Paulo, Brésil
Several strategies aimed at sorting single-walled carbon nanotubes (SWNT) by diameter and/or electronic structure have been developed in recent years [1]. In the non-destructive sorting method proposed by Arnold et al.[2], nanotube bundles are dispersed in water-surfactant solutions by ultrasonication and the resulting solutions are submitted to ultracentrifugation in a density gradient. By this method, SWNTs of different diameters are distributed according to their densities along the centrifuge tube. A mixture of two anionic amphiphiles, namely sodium dodecylsulfate (SDS) and sodium cholate (SC), presented the best performance in discriminating nanotubes by diameter. It has been pointed out by Rinzler[3], however, that nanotube distribution in the density gradient occurs in the opposite order. The density of surfactant-encapsulated nanotubes should decrease with increasing nanotube diameter, admitting that the nanotubes are empty and that the surfactant coverage at the outer surface is uniform. As a consequence, larger diameter tubes were expected to be found up in the low density part of the centrifuge tube, but they are actually found in the high density part. We present molecular dynamics studies of the water-surfactant-SWNT system.
The simulations revealed one aspect of the discriminating power of surfactants: they can actually be attracted towards the interior of the nanotube cage. The binding energies of SDS and SC on the outer nanotube surface are very similar and depend weakly on diameter. The binding inside the tubes, on the contrary, is strongly diameter dependent. The simulations showed that SDS fits best inside tubes with diameters ranging from 8 to 9 Ǻ, while SC is best accommodated in larger tubes, with diameters in the range 10.5 to 12Ǻ. 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 [4].

[1] M.C. Hersam, Nature Nanotech., doi:10.1038/nnano.2008.135 (2008).
[2] M.S. Arnold, A.A. Green, J.F. Hulvat, S.I. Stupp, and M.C. Hersam, Nature Nanotech. 1, 60 (2006).
[3] A.G. Rinzler, , Nature Nanotech. 1, 17 (2006).
[4] E.J.F. Carvalho and M.C. dos Santos, submitted.