Dissolution of pristine single walled carbon nanotubes in superacids by direct protonation

Strong tube-tube van der Waal attractions that hinder the dissolution of single walled carbon nanotubes (SWNTs) as individuals have been overcome by the direct protonation of the tubes by superacids. The pristine nanotubes disperse as individuals in 100% H2SO4, oleum, trifluromethanesulfonic acid, and chlorosulfonic acid to varying solubility levels. Optically homogeneous solutions with concentration as high as 45 g/L (2.5 wt %) at room temperature have been observed in the case of chlorosulfonic acid, the strongest among the series of superacids investigated. A dissolution model is proposed wherein the solubilized tubes exist as protonated polycarbocations, charge balanced by corresponding conjugate base anions. The removal of electrons from the SWNT, leading to a fractional positive charge on the carbons in the protonated state, is reversible upon deprotonation. With increasing concentration of the solubilized nanotubes, anion-mediated attractions at lower concentrations and spatial constraints at high concentrations guide the transformation of the SWNT-acid system into a nematic mesophase. Highly anisotropic, nematic mesophases formed in the dissolution process are promising precursors for the fabrication of macroscopic forms of SWNTs with high alignment of the carbon nanotubes.