Insight into the stabilization mechanism of imidazole-based ionic liquids at the interface of the carbon nanotubes: A computational study

Abstract

Ab initio calculations have been performed to systematically investigate the structures, binding energies, vibrational spectra, as well as the non-covalent interactions of imidazolium-based ionic liquids (ILs) at the interface of carbon nanotubes (CNTs). The effects of alkyl length of imidazolium cations, types of anions as well as diameters of CNTs on the relevant interfacial structures and properties have been explored in this study. Our calculations demonstrated that the interfacial imidazolium cations display two different orientations (cations paralleling and perpendicular to the CNTs surface) irrespective of the alkyl chain length of cations, types of anions and the diameters of CNTs. By comparison with the calculated binding energies between cations and the CNTs, the cations paralleling to the CNTs surface are thermodynamically much more stable due to the significant interfacial π-π stacking interactions. In addition, our results indicated that there is significantly difference for the infrared (IR) spectra of the imidazolium cations (between two orientations) on the CNTs surface. Therefore, with the IR spectra, the specific orientations of imidazolium cations on surface of CNTs can be better understood. Moreover, interaction region indicator (IRI) as well as symmetry-adapted perturbation theory (SAPT) analysis were employed to deep understanding of the non-covalent interactions between imidazolium-based ILs and CNTs. Our calculations explore that the paralleling cations is stabilized on the CNTs surface by the dispersion (especially for the π-π stacking) interactions. However, for the cations perpendicular to the CNTs surface, electrostatic, induction as well as the dispersion interactions are all very important. Our calculated results provide a molecular level understanding the stabilization mechanism of imidazole-based ILs at the interface of the CNTs and will be a bit favorable to clarify the existing debate on the stabilization mechanism from various experimental observations.