Effect of the flexibility and the anion in the structural and transport properties of ethyl-methyl-imidazolium ionic liquids
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Date
2007
Authors
Rey Castro, Carlos
Tormo, A. L.
Vega, Lourdes F.
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Abstract
This work summarizes some results obtained through equilibrium molecular dynamic simulations regarding the structure and transport properties
of several ionic liquids (ILs). The Green–Kubo relationships were employed to evaluate the cation/anion diffusion coefficients, electrical conductivity
and shear viscosity at 400 K. The ILs investigated were 1-ethyl-3-methylimidazolium salts of Cl−, NO3
− and PF6
− using two different force fields
for the cation: a rigid ion model of 1-ethyl-3-methylimidazolium, studied in a previous work [C. Rey-Castro, L.F. Vega, J. Phys. Chem. B 110 (2006)
14426–14435] and the flexible model of Urahata and Ribeiro [S.M. Urahata, M.C.C. Ribeiro, J. Chem. Phys. 120 (2004) 1855–1863]. Regarding
the anions, the most evident difference between the local structures in the three ILs is the position of the first peak of the radial distribution function,
reflecting the differences in anion sizes and shapes. The velocity autocorrelation functions are particularly sensitive to the relative weights of anion
and cation, although the integrated self-diffusion coefficients do not show significant differences between the Cl−, NO3
− and PF6
− salts. The electric
conductivity predicted by the rigid ion model of [emim]Cl is lower than the experimental value, whereas the model overestimates the viscosities.
In contrast, the flexible model leads to diffusion rates and conductivities that are one order of magnitude higher at the same temperature. The
shear viscosities obtained from simulations of the flexible model are in very good agreement with experimental data. The calculated conductivities
are compared with values obtained from the diffusion coefficients through the Nernst–Einstein relation in order to determine the importance of
cross-correlation among ions. The stress tensor and the distinct van Hove correlation functions indicate that the dynamics of the local structure of
the fluid relaxes faster in the flexible model.
Citation
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Fluid Phase Equilibria, 2007, vol. 256, núm. 1-2, p. 62-69