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The local structure in BmimPF6/acetonitrile mixture: The charge distribution effect

Abstract

The changes of the local structure in the binary mixture of 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6) ionic liquid and acetonitrile are investigated over the entire composition range. Two charge distribution models of the ions are considered: in the first one, the atomic fractional charges of the cations and anions are kept equal with those in the neat ionic liquid, and hence they are independent from the mole fraction of the ionic liquid, while in the second one the charge distribution is scaled up by a mole fraction dependent factor. The sum of these charges converge to +1e and 1e on the cation and anion, respectively, at infinite dilution. All the other interactions and geometry parameters of the ions (i.e., Lennard-Jones, bond stretching, angle bending and dihedral parameters) are identical in the two cases. The effect of the fractional charge distribution on the hydrogen bonding between the counterions themselves and between the ions and solvent molecules, as well as on the stacking interactions between the cations is analyzed. To this end, two distances, characteristic to the hydrogen bond formed by the donor moiety and its nearest neighbor acceptor, as well as a coordinate system that defines unambiguously the orientation between a reference cation and its nearest neighbor are introduced. It is shown that, with the variable charge model, the neighboring cation-anion pairs maintain their relative arrangement similar to the neat ionic liquid down to the ionic liquid mole fraction of xIL = 0.10, whereas in the case of the constant charge model such changes occur already at xIL = 0.20.. These findings correlate with experimental results concerning the behavior of many physical chemical properties (e.g., excess volume, excess viscosity, chemical shift, infrared and Raman vibrational mode shifts, diffusion, etc.) that were found to undergo a drastic change in this mole fraction range. Our results show that in this composition range a transition occurs from the situation when the macroscopic physical chemical properties of the system are determined primarily by the cation-anion hydrogen bonding interactions to that when they are determined by the solvation of the cation and the anion by the molecular solvent.

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Publication details

The article was received on 05 Jun 2018, accepted on 07 Aug 2018 and first published on 09 Aug 2018


Article type: Paper
DOI: 10.1039/C8CP03546F
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    The local structure in BmimPF6/acetonitrile mixture: The charge distribution effect

    V. A. Koverga, O. N. Kalugin, F. A. MIANNAY, Y. Smortsova, K. Goloviznina, B. Marekha, P. Jedlovszky and A. IDRISSI, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP03546F

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