Can SWNTs Swim Apart?

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Science  09 May 2008:
Vol. 320, Issue 5877, pp. 719
DOI: 10.1126/science.320.5877.719a

Single-walled carbon nanotubes (SWNTs) form as bundles, and to facilitate their processing, they are usually solubilized by some sort of surface treatment, such as coating with surfactants or protonation with superacids. However, unadorned SWNT bundles, when sonicated in dilute N-methyl-pyrrolidone (NMP) suspensions, have been observed to disperse and even exfoliate. Bergin et al. now show that this process represents true thermodynamic solubility; in other words, the free energy of mixing is negative. This situation is unusual for SWNTs in that the entropy term of mixing for such large molecules is very small, so the enthalpy of mixing must be near zero or even negative. The authors found that as the degree of dilution of SWNT samples in NMP increased, an analysis of bundle sizes (via atomic force microscopy of droplets evaporated on substrates) showed a decrease in bundle diameter, indicating that the samples were approaching equilibrium in solution through spontaneous desorption of SWNTs from the bundles. Light-scattering studies afforded a measurement of the Flory-Huggins parameter, whose sign indicated that the enthalpy of mixing was negative. The authors note that the surface energy of NMP is similar to that of the graphitic surface of the nanotubes; scanning tunneling microscopy (STM) images of the SWNTs after sonication in NMP showed evidence of NMP molecules adsorbed on the SWNTs migrating from inside the bundles to the silicon deposition surface. The authors argue that strong physisorption of NMP onto the SWNT surfaces drives solu- bilization and that this observation will be key in developing better solvents. — PDS

Adv. Mater. 20, 10.1002/adma.200702451 (2008).

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