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 BF₄^-, PF₆ ^-, and NTf₂^- (N,N-bis((trifluoromethyl) sulfonyl)amide) by metathesis and afforded functionalized MWCNTs, f-3b (BF ₄^-), f-3c (PF₆^-), 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 (BF₄^-) > f-3c (PF₆^-) > f-3d (NTf₂^-), and the water-soluble f-3a was phase-transferred from the aqueous phase to the chloroform phase via simple anion exchange with NTf₂^-. All of the IL-functionalized MWCNTs, including the water-soluble f-3a, exhibited a preferential solubility in an ionic liquid, [bmim][NTf₂].