Understanding of Competitive Hydrogen Bond Behavior of Imidazolium-Based Ionic Liquid Mixture around Single-Walled Carbon Nanotubes

Here we have performed molecular dynamics simulations to investigate the structures, dynamics, and hydrogen bonds (HBs) of the imidazolium-based ionic liquid (IL) mixture containing equimolar 1-ethyl-3-methylimidazolium tetrafluoro-borate ([Emim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) around the single-walled carbon nanotubes (CNTs) with three different diameters of 5.42, 10.85, and 21.70 angstrom. Our simulation results reveal for the first time that the [BF4](-) and [PF6](-) anions have an obvious competitive HB behavior for the IL mixture around the CNTs, and such HB competition can be further enhanced as the CNT diameter increases. The HBs between the [PF6](-) anions and cations are always more and stronger than those between the [BF4](-) anions and cations regardless of the CNT diameter, although the enhanced HBs are found for both kinds of anions in the first solvation shell around larger CNTs. Nevertheless, the interfacial [PF6](-) anions show a larger HB enhancement than the corresponding [BF4](-) anions as the CNT diameter increases. Besides the competitive interactions between the different anions and the CNT, such competitive HB behavior in the IL mixture leads to more [PF6](-) anions but less [BF4](-) anions aggregating around the larger CNTs, which is significantly different from the previous aggregation behavior of pure ILs around the CNTs. Furthermore, there exist the opposite diameter-dependent changes of average HB number per cation with the [BF4](-) and [PF6](-) anions. In addition, the enhanced HBs between cations and anions around the larger CNTs result in slower rotations of both the [BF4](-) and [PF6](-) anions and more pronounced orientational distribution of both the imidazolium cations in the first solvation shell around the CNTs.