Covalent Modification of Multiwalled Carbon Nanotubes with Imidazolium-Based Ionic Liquids:  Effect of Anions on Solubility

Multiwalled carbon nanotubes (MWCNTs) were covalently modified with imidazolium salt-based ionic liquids (ILs). Coupling of acid chloride-functionalized MWCNTs with commercially available (3-aminopropyl)imidazole, followed by the reaction with n -butyl bromide, afforded 1-butylimidazolium bromide salt-functionalized MWCNTs ( f -3a ). The bromide anion of f- 3a could be exchanged with BF 4 -, PF 6 -, and NTf 2 - ( N, N -bis((trifluoromethyl)sulfonyl)amide) by metathesis and afforded functionalized MWCNTs, f -3b (BF 4 - ), f -3c (PF 6 - ), and f -3d (NTf 2 - ), bearing different counteranions. Raman, IR, NMR, TGA, and XPS analyses clearly confirmed covalent functionalization and imidazolium salt formation as well as anion exchange of the IL-functionalized MWCNTs. UV−vis data imply that counteranions have an effect on the relative solubility of the IL-functionalized MWCNTs in water: the solubility of f -3 in water is of the order f -3a (Br - ) > f -3b (BF 4 - ) > f -3c (PF 6 - ) > f -3d (NTf 2 - ), and the water-soluble f -3a was phase-transferred from the aqueous phase to the chloroform phase via simple anion exchange with NTf 2 - . All of the IL-functionalized MWCNTs, including the water-soluble f -3a, exhibited a preferential solubility in an ionic liquid, [bmim][NTf 2 ].