Abstract
The ionic liquids BMIM+I-, BMIM+BF4-, and BMIM+PF6- were simulated
by means of the molecular dynamics method over a time period of more
than 100 ns. Besides the common structural analysis, e.g., radial
distribution functions and three dimensional occupancy plots, a more
sophisticated orientational analysis was performed. The angular correlation
functions g(00)110(r) and g(00)101(r) are the first distance dependent
coefficients of the pairwise orientational distribution function
g(rij,Omega1,Omega2,Omega12). These functions help to interpret the
three dimensional plot and reveal interesting insights into the local
structure of the analyzed ionic liquids. Furthermore, the collective
network of ionic liquids can be characterized by the Kirkwood factor
Gkappa(r) J. Chem. Phys. 7, 911 (1939). The short-range behavior
(r<10 A) of this factor may be suitable to predict the water miscibility
of the ionic liquid. The long-range limit of Gkinfinity is below
1 which demonstrates the strongly coupled nature of the ionic liquid
networks. In addition, this factor relates the orientational structure
and the dielectric properties of the ionic liquids. The static dielectric
constant epsilon(omega=0) for the simulated system is 8.9-9.5. Since
in ionic liquids the very same molecule contributes to the total
dipole moment as well as carries a net charge, a small, but significant
contribution of the cross term between the total dipole moment and
the electric current to epsilon(omega=0) is observed.
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