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

Abstract

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 CH 3 groups of cations are found to be closer to the SWNT surface than the NH 3 + groups because of the solvophobic nature of the CH 3 groups, while the NO 3 − 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 CH 3 group of the cation) and N−O (anion) asymmetric stretching bands at the EAN/SWNT 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 NO 3 − 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.

Unique Structures and Vibrational Spectra of Protic Ionic Liquids Confined in TiO2 Slits: The Role of Interfacial Hydrogen Bonds

“…In addition, the confined cations have a shoulder before the conventional asymmetric N–H stretching vibrational peak at 3344 cm –1 . Above vibrational spectra are different from those of other interfaces such as EAN–air, EAN–carbon nanotube, and water–Au nanoparticle. ,− Some previous works , have confirmed that the characteristic asymmetric stretching vibration peak of N–H at 3344 cm –1 is due to HBs between EAN ion pairs. For the EAN IL molecules confined inside the TiO 2 slits, the EA + cations can form two types of HBs: one with the TiO 2 wall and the other with the NO 3 – anions.…”

“…R c ON /θ c ONH corresponds to the distance/angular criterion for HB formation, respectively. According to previous works, the values for R c ON and θ c ONH were set to 3.7 Å and 30°, respectively. We calculated the average HB number between the EA + cations and NO 3 – anions and between the EA + cations and the TiO 2 walls in the first layers of the four systems, as shown in the Table .…”

“…Accordingly, 128, 256, 384, and 512 EAN ion pairs were randomly placed inside TiO 2 slit pores with widths of 0.7, 1.4, 2.1, and 2.8 nm, respectively. In the current work, the EAN IL was described by the all-atom force field developed by Acevedo and Tirado-Rives , and is suitable for the interface between ILs and solid materials. , The force-field parameters for TiO 2 were taken from refs , . The nonbonded interactions, including van der Waals (vdW) and electrostatic interactions, between different species were described by Lennard-Jones (L-J) 12–6 and Coulombic potentials.…”

“…In recent years, scientists have made some progress in understanding the IL/carbon nanotube system using experiments together with theoretical simulations. Zhou et al studied the vibrational spectra of protic ILs surrounding single-walled carbon nanotubes (SWNTs) and found that the red-shift phenomenon of interfacial anions was induced by the IL/SWNT interaction. Similar vibrational spectra were observed in the Fourier transform infrared experiments on [C 6 mim] [PF 6 ] dispersed on the surface of SWNTs .…”

Nanostructure domains, voids, and low-frequency spectra in binary mixtures of N,N-dimethylacetamide and ionic liquids with varying cationic size

“…We calculated the vibrational spectra from molecular dynamics simulations by Fourier transform to velocity autocorrelation function (VACFs). The normalized velocity autocorrelation function is dened as [72][73][74][75]…”