Molecular-Level Understanding of Structures and Dynamics of Imidazolium-Based Ionic Liquids around Single-Walled Carbon Nanotubes: Different Effects between Alkyl Chains of Cations and Nanotube Diameters

摘要

Here, we have performed a series of molecular dynamics simulations to investigate the solvation structures and dynamics of imidazolium-based ionic liquids (i.e., [Emim][BF4] and [Bmim][BF4]) around single-walled carbon nanotubes (CNTs) with two different diameters. We make an effort to address the effects of both the alkyl chains of cations and the CNT diameters on the interfacial properties as well as the reasons behind these effects. Our simulation results demonstrate that increasing the CNT diameter can lead to a larger interaction between the ions and the CNTs so that more cations and anions tend to aggregate around the larger CNT, while the alkyl chain is found to have a negligible effect on the relevant structures in the first solvation shell. Meanwhile, the imidazolium rings of cations prefer to be almost parallel to the CNT surface and the preference can be further enhanced by the larger CNT diameter and the longer alkyl chain. On the other hand, either increasing the CNT diameter or the alkyl chain length can result in slower rotational motions of cations around the CNT, but the latter has a more considerable effect, which is significantly different from their effects on the solvation structures. In addition, the anions have the same dependence of their rotational motions around the CNTs on the alkyl chain length as the cations. Our simulation analysis further reveals that the same dependence results from that the alkyl chains of cations can affect the hydrogen bonds between cations and anions so that they have an indirect and considerable influence on the rotational motions of anions.