Dispersing Carbon Nanotubes with Ionic Surfactants\nunder Controlled Conditions: Comparisons and Insight

Abstract

A fundamental\nunderstanding of the mechanisms involved in the surfactant-assisted\nexfoliation and dispersion of carbon nanotubes (CNTs) in water calls\nfor well-controlled experimental methodologies and reliable comparative\nmetrics. We have assessed the ability of several ionic surfactants\nto disperse single and multiwalled carbon nanotubes, resorting to\na stringently controlled sonication-centrifugation method for the\npreparation of the dispersions. The CNT concentration was accurately\nmeasured for a wide range of surfactant concentration, using combined\nthermogravimetric analysis and UV–vis spectroscopy. The obtained\ndispersibility curves yield several quantitative parameters, which\nin turn allow for the effects of nanotube morphology and surfactant\nproperties (aromatic rings, chain length, headgroup charge, and <i>cmc</i>) to be assessed and rationalized, both in terms of dispersed\nnanotube mass and surface area. The data also indicate that the CNT-surfactant\nassociation follows patterns that are markedly different from other\nequilibrium processes governed by hydrophobicity (such as micellization);\nin particular, the surfactant concentration needed for maximum dispersibility, <i>c</i>_s,max, and the number of surfactant molecules\nper unit CNT area at <i>c</i>_s,max are shown\nto depend linearly on chain length. The results further suggest that\nthe presence of micelles in the exfoliation process is not a key factor\neither for starting CNT dispersibility or attaining its saturation\nvalue.