New Solvents for Nanotubes: Approaching the Dispersibility of Surfactants

We demonstrate dispersion and exfoliation of nanotubes in two new solvents for nanotubes, cyclohexyl-pyrrolidone (CHP) and 1-benzyl-2-pyrrolidinone (NBenP). Both solvents are structural analogues of the well-known nanotube solvent N-methyl-pyrrolidone. Each solvent can disperse nanotubes at high concentrations, up to 3.5 mg/mL for CHP. The nanotubes in these dispersions are highly exfoliated, even at high concentration. In cyclohexyl-pyrrolidone dispersions, the root-mean-square bundle diameter was ∼3 nm for a concentration of 2 mg/mL. The bundle diameter fell as the concentration was reduced, reaching 1.5 nm at concentrations below 10−3 mg/mL. These dispersions have very large populations of individual nanotubes and small bundles. For CHP the total population of one-dimensional dispersed objects exceeded 100 μm−3 for concentrations >2 mg/mL. Of these ∼10% were individual SWNTs. However, as the concentration was reduced, the fraction of individual SWNTs increased to ∼80% for a nanotube concentration of 10−4 mg/mL. Like other successful nanotube dispersing solvents, both of these new solvents are characterized by surface tensions close to 40 mJ/m2. We believe their ability to disperse and exfoliate nanotubes is due to the low energetic cost of exfoliation in such solvents. Finally, their relative lack of toxicity makes these solvents much more user-friendly than traditional nanotube solvents such as N-methyl-pyrrolidone or dimethyl-formamide.