Molecular dynamics simulation of diffusion of nitrobenzene in 3-methylimidazolium hexafluorophosphate ionic liquids

The diffusions of nitrobenzene (PhNO2) in the three kinds of ionic liquids (ILs), i.e. 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]), 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), and 1-hexyl-3-methylimidazolium hexafluorophosphate ([Hmim][PF6]) were examined by molecular dynamics (MD) simulation. Diffusion coefficients of PhNO2, imidazole (Mim) cation and PF6 anion were calculated based on the Einstein equation. The influence of the different ionic liquids on the diffusion of PhNO2 was discussed. The results showed that at the same simulation concentration, the diffusion coefficients of all the species decrease with an increase of the side chain of 3-methylimidazolium hexafluorophosphate ionic liquids in the model system. These simulation results are in good agreement with the electrochemical experiment reported by Chen et al. Analyses of the pair correlation functions (PCFs) of the simulation systems showed that the non-bond and hydrogen bond interactions are formed between PhNO2-ionic liquid systems. The Coulomb and van der Waals (vdW) interactions are helpful for the diffusion of PhNO2 in 3-methylimidazolium hexafluorophosphate ionic liquids and the contribution from the vdW interaction is significantly greater than that from the Coulomb interaction in the system. From the related coefficient (R) of mean square displacement (MSD) curve and PCFs, the diffusion of PhNO2 in [Hmim][PF6] ionic liquids is weaker than the other two ionic liquids. This study has provided a theoretical direction for the experiments on the mass transfer of PhNO2 in ionic liquids.