title: Preferential Ionic Interactions and Microscopic Structural Changes Drive Nonideality in Binary Ionic Liquid Mixtures as Revealed from Molecular Simulations
author: Utkarsh Kapoor
description: 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.