Immobilization and molecular rearrangement of ionic liquids on the surface of carbon nanotubes

Abstract

Two imidazolium-based ionic liquids (ILs), i.e. [Bnmim][BF₄] (1-benzyl-3-methylimidazolium tetrafluoroborate) and [Bmim][AuCl₄] (1-butyl-3-methylimidazolium chloroauric acid), were adsorbed on the outer surface of multi-walled carbon nanotubes (MWCNTs) via a physical method. In order to understand the molecular packing of ILs immobilized on the MWCNTs surfaces, composite samples with different weight ratios of ILs were characterized by using X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). It was found that the rings of imidazolium cations of ILs are nearly parallel to the MWCNTs surfaces, and the packing orientation is determined by the substituent groups on the imidazolium-ring. Both the anions ([BF₄]⁻ and [AuCl₄]⁻) occupy an adjacent area to the imidazolium cation to form a neutral surface layer. Furthermore, for the immobilized ILs, the melting point (Tm) of [Bmim][AuCl₄] rises slightly, while the Tm of [Bnmim][BF₄] gets depressed. The molecular rearrangement and the melting point change of the immobilized ILs result from the complex interfacial interactions between the ILs and the MWCNTs surfaces.