Molecular-Level Understanding of Solvation Structures and Vibrational Spectra of an Ethylammonium Nitrate Ionic Liquid around Single-Walled Carbon Nanotubes

Molecular dynamics simulations have been performed to explore the solvation structures and vibrational spectra of an ethylammonium nitrate (EAN) ionic liquid (IL) around various single-walled carbon nanotubes (SWNTs). Our simulation results demonstrate that both cations and anions show a cylindrical double-shell solvation structure around the SWNTs regardless of the nanotube diameter. In the first solvation shell, the CH3 groups of cations are found to be closer to the SWNT 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 intensities of both C-H (the CH3 group of the cation) and N-O (anion) asymmetric stretching bands at the EAN/SVVNT interface are found to be slightly higher than the corresponding bulk values owing to the accumulation and orientation of cations and anions in the first solvation shell. More interestingly, the N-O stretching band exhibits a red shift of around 10 cm(-1) with respect to the bulk value, which is quite contrary to the blue shift of the O-H stretching band of water molecules at the hydrophobic interfaces. Such a red shift of the N-O stretching mode can be attributed to the enhanced hydrogen bonds (HBs) of the NO3- anions in the first solvation shell. Our simulation results provide a molecular-level understanding of the interfacial vibrational spectra of an EAN IL on the SWNT surface and their connection with the relevant solvation structures and interfacial HBs.