Solubility and Debundling of Carbon Nanotubes

As with the fullerenes, the solubility of carbon nanotubes is an essential prerequisite for studying their properties. Numerous efforts have thus been made to progress in this field. Yet the size and structure of the nanotubes are a severe impediment to solubihzation both in aqueous and organic media. Firstly, they are very big objects already, with their length ranging in the micrometer scale. Consequently, the formation of a classic solution is rather unlikely, and any actual dissolution should better be considered a colloidal system or a dispersion. However, there are other effects besides their length that contribute to their insolubility. 

There are several strategies nevertheless to obtain solutions or at least stable suspensions of single- and multiwaUed nanotubes. The most important ones are presented below. 

The simplest, and at the same time the most effective way to achieve dispersion in water or aqueous media is the addition of amphiphihc molecules carbon nanotubes without any functional groups are hydrophobic. This characteristic may be employed to enclose them in micelles. The surfactant molecules arrange around the single tubes with their hydrophilic head directed outward and their hydrophobic tail oriented toward the nanotube in the center. In this manner, each individual tube is surrounded by an envelop that enables a dispersion, for example, in water. Yet complete solubilization requires considerable amounts of the surfactant. Solutions obtained this way may in fact contain up to 80% of detergent and only 20% of carbon nanotubes. Surfactants suitable to enclose nanotubes in micelles include, for example, sodium dodecylsulfate (SDS), Triton X-100, or octadecyl-trimethylammonium bromide (OTAB). 

Choosing a block-copolymer with both hydrophilic and hydrophobic domains as the amphiphilic substance, the appHcation of suitable linking methods can lead to the formation of an irreversibly closed capsule around every single nanotube. The reaction with a diamine linker may for instance initiate a crosslinking in the polyacryhc acid domains of an amphiphilic polystyrene/polyacrylic acid copolymer. Upon addition of water to a solution in DMF the latter form micelles with the nanotubes in the center. Contrary to the simple micelles mentioned in the previous paragraph, the crossHnked species can be dried and redispersed. 

Another strategy is pursued in the direct functionalization of nanotubes. The solubihty of the carbon material in different solvents can be controlled by the attachment of various functional groups. In Section 3.5, numerous examples for a solubility enhancement of single- and multiwalled carbon nanotubes are described. A rather simple functionalization usually occurs already in the first steps of purification. The reaction with concentrated mineral acids does not 

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