Structures and dynamics of protic [EtNH3][NO3] and aprotic [Emim][NO3] ionic liquid mixtures around the single-walled carbon nanotubes: The critical role of imidazolium-based cations

Abstract

Molecular dynamics (MD) simulations and a series of ab initio calculations have been applied to systematically investigate the structural and dynamic properties for ionic liquid (IL) mixture of equimolar protic [EtNH3][NO3] and aprotic [Emim][NO3] around the single-walled carbon nanotubes (SWCNTs) of (4,4), (8,8) and (12,12). Our simulation results demonstrated that ions of IL mixtures display a well-defined shell structures at the interface irrespective of the diameters of SWCNTs. The aprotic [Emim]+ cations adsorb preferentially over protic [EtNH3]+ at the interface of SWCNTs irrespective of its diameters. Further analysis reveals that these two cations and the [NO3]– anions display ordered arrangement in the first solvation shell. Accordantly, the rotational dynamics of interfacial [Emim]+ and [EtNH3]+ cations decay much slower than those of the bulk curves, suggesting the rotational motions of these two cations at the interface of the SWCNTs are significantly restricted. Interfacial [Emim]+ cations decay much slower rotational motions as the SWCNTs diameter increases. However, for interfacial [EtNH3]+ cations, because they are far from the interface, they display similar rotations regardless of the SWCNTs diameters. More importantly, the HB lifetime of interfacial [Emim]+–[NO3]– and [EtNH3]+–[NO3]– are much longer than the corresponding bulk values due to the restrictions on the rotational motions of cations, suggesting the strength of these two HBs are enhanced at the interface. The binding energies for the above HBs were determined by the ab initio calculations, which are in accordance with the MD simulation results. Our results provide a molecular-level understanding the structural and dynamic properties of [EtNH3][NO3] and [Emim][NO3] IL mixtures around the SWCNTs, which highlighted the critical role of the imidazolium-based cations.