Molecular Dynamics Simulations Of Solvation Structures And Vibrational Spectra Of Protic Ionic Liquids At The Interfaces Of Single-walled Carbon Nanotubes

In the past decades, single-walled carbon nanotubes（SWCNTs） have always attracted intensive attention because of their unique chemical, mechanical, thermal, electrical, and optical properties. However, additional stabilizers are always introduced to the traditional solvents to prepare stable dipsersions of SWCNTs because of their hydrophobic nature and the strong van der Waals attractions among them. Recently, room-temperature ionic liquids（RTILs） have been regarded as a promising alternative to these commonly used solvents for making stable SWCNT dispersions without the addition of stabilizers. Meanwhile, it’s also reported that the ILs confined in carbon nanotubes play a critical role in energy, environment and catalysis. Here, a basic scientific issue that how to understand the internal and external interfacial solvation structures of ionic liquid aroud the SWCNTs can be couluded from the pratical applications of above two aspects. Wherein, the former application is mainly related to the external solvation structure of ionic liquid around the SWCNTs, namely external interface. And the latter application mainly involves the internal structures of ionic liquid under the confinement, namely internal interface. In addition, constructing a response relationship between the interfacial solvation structures and corresponding vibrational spectra from molecular level is beneficial for experimental scientists to analyze the interfacial microstructure properties of ionic liquid by means of vibrational characterization. To this end, we have proposed that employing molecular dynamics（MD） simulation to systematically investigate the external and internal solvation structures and vibrational spectra of protic ionic liquid around the SWCNTs.First, MD simulations have been performed to explore the external solvation structures and vibrational spectra of an ethylaammonium nitrate（EAN） IL around various SWCNTs. Our simulation results demonstrate that both cations and anions show a cylindrical double-shell solvation structure around the SWCNTs regardless of the nanotube diameter. In the first solvation shell, the CH3 groups of cations are found to be closer to the SWCNT surface than the NH₃⁺ groups because of the solvophobic nature of the CH3 groups, while the NO3- anions tend to lean on the nanotube surface, with three O atoms facing the bulk EAN. On the other hand, the asymmetric stretching vibration modes of both C-H（the CH3 group of the cation） and N-H（amino group） at the EAN/SWCNTs interface are almost identical with those in bulk EAN. Additionally, the N-O stretching band exhibits a red shift of around 10 cm-1 with respect to the bulk value, which can be attributed to the enhanced hydrogen bonds（HBs） of the NO3- anions in the first solvation shell.Then, the internal confined structures and relevant vibrational spectra of EAN IL inside the SWCNTs with various diameters have been investigated in detail by using MD simulation. Our simulation results demonstrate that the EAN IL confined in larger SWCNTs can form well-defined multishell structures with an additional cation chain located at the center. However, a different single shell hollow structure has been found for both the cations and the anions in the 1 nm SWCNT. For the cations confined in SWCNTs, the CH3 groups stay closer to the nanotube walls because of their solvophobic nature, while the NH₃⁺ groups prefer to point toward the central axis. Accordingly, the NO3- anions tend to lean on the SWCNT surface with three O atoms facing the central axis to form HBs with the NH₃⁺ groups. In addition, in the 1 nm SWCNT, the CH3 groups of cations exhibit an obvious blue shift of around 16 cm-1 for the C-H stretching mode with respect to the bulk value, and the N-H stretching mode of NH₃⁺ groups is split into two characteristic peaks with one peak appearing at a higher frequency. Such a blue shift is attributed to the existence of more free space for the C-H bonds of confined CH3 groups, while the splitting phenomenon is due to the fact that more than 60% of the confined NH₃⁺ groups have one dangling N-H bond. For the anions confined in the 1 nm SWCNT, the N-O stretching mode of NO3-has a maximum red shift of around 24 cm-1 with respect to the bulk value, which is attributed to enhanced HBs between anions and cations. Overall, our simulation results not only have characterized the external interfacial as well as the internal confined solvation structures and vibrational spectra from molecular level, but also have constructed a relationship between both of them for the first time.