Preferential Ionic Interactions and Microscopic Structural Changes Drive Nonideality in Binary Ionic Liquid Mixtures as Revealed from Molecular Simulations

Thermophysical
and structural properties of two binary mixtures
of ionic liquids were determined in this study using molecular dynamics
simulations at 353 K. The mixtures contains common cation and different
anions, combining 1-n-butyl-3-methylimidazolium [C4mim]+ chloride [Cl]− with two
other ionic liquids, namely, [C4mim]+ methylsulfate
[MeSO4]− and [C4mim]+ bis­(trifluoro­methane­sulfonyl)­imide [NTf2]−. Each mixture was characterized in terms
of thermodynamic quantities such as densities and excess molar volumes
and transport properties specifically self-diffusion coefficients
and ionic conductivities, using seven molar compositions (0:00, 10:90,
25:75, 50:50, 75:25, 90:10, 100:0). Excess molar volumes for the two
binary ionic liquid mixtures exhibited small deviations from ideality;
the Cl-[MeSO4] system showed negative deviation while positive
deviation was observed for the Cl-[NTf2] system. Structural
analysis elucidated in terms of radial distribution functions, orientations
of the anions around cation through angular distribution functions,
and spatial distribution functions revealed that the significant changes
in the ionic interactions occur within the first solvation shell of
the cations and these changes are responsible for the observed nonideality.
The self-diffusion coefficients of the ions were found to decrease
monotonically with Cl– concentration for both the
mixtures. Further, predictions of ionic conductivities using both
the Nernst–Einstein formalism and Einstein relationship pointed
to the presence of correlated ionic motion which was confirmed by
the long ion-pair relaxation time constants especially for the anion
present as a minor component.