Molecular dynamics studies on interfacial interactions between imidazolium-based ionic liquids and carbon nanotubes

We have investigated the basic mechanism of carbon nanotube (CNT) interactions with various room-temperature ionic liquids (RTILs) using molecular dynamics (MD) simulations. To understand the effects of the cation molecular geometry on the properties of the interface structure in the RTIL systems, we have studied a set of three RTILs with the same [BF₄]^- (tetrafluoroborate) anion but with different cations, namely, [EMIM]⁺ (1-ethyl-3-methylimidazolium), [BMIM]⁺ (1-butyl-3-methylimidazolium), [HMIM]⁺ (1-hexyl-3-methylimidazolium), and [OMIM]⁺ (1-octyl-3-methylimidazolium) ions. The simulation results showed that the imidazolium cations exhibit two distinct orientations (perpendicular and parallel to the CNTs surface) at the interface irrespective of the alkyl chain length of the cations. The average number of hydrogen bonds per cations inside the CNT was found to be higher for [OMIM][BF₄] (1.01), which suggests that [OMIM]⁺ imidazolium rings to be concentrated at the center of the CNT, which favors hydrogen bond. The reported results show the diffusion coefficients of ions in confinement are much lower in comparison to the bulk region. The interaction energy between [OMIM][BF₄] (-8.75 kcal.mol⁻¹.ion⁻¹) and CNT was found to be higher as compared to other ILs. The cations paralleling the CNT surface are thermodynamically significantly more stable because of the substantial interfacial π-π stacking interactions, as shown by a comparison with the calculated interaction energies between cations and the CNTs. Our simulation results provide a molecular-level understanding of the stabilization and dispersion of CNT bundles in ILs.