Tracing Dynamics, Self-Diffusion, and Nanoscale Structural Heterogeneity of Pure and Binary Mixtures of Ionic Liquid 1-Hexyl-2,3-dimethylimidazolium Bis(fluorosulfonyl)imide with Acetonitrile: Insights from Molecular Dynamics Simulations.

All-atom molecular dynamics (MD) simulations of the ionic liquid (IL) 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide ([C6mmim][FSI]) and its binary mixtures with acetonitrile (ACN) have been reported for the first time. The presence of ACN as a cosolvent, similar to the effect of increasing temperature, causes enhancements in the ion translational motion and fluidity of the IL, leading to significant improvement of ionic conductivity and self-diffusion, which is well explained by a microscopic structural analysis. In neat IL and a concentrated IL mixture, self-diffusion of the cation is higher than that of the corresponding anion; however, further addition of ACN into the diluted mixtures with IL molar fractions (xIL's) below 0.50 results in more weakened interactions among the nearest ACN-anion neighbors compared to those among the ACN-cation neighbors so that the number of isolated anions is more than that of isolated cations under this condition, and the anions diffuse faster than the cations, as expected on the basis of their relative sizes. The velocity autocorrelation function analysis indicates an inverse relation between xIL and the mean collision time of each species. Additionally, at a fixed xIL, both the mean collision time and velocity randomization time of ACN are shorter than those of the ions. Gradual addition of ACN changes the morphology of nanosegregated domains and tends to disrupt ionic clusters (i.e., it scatters and decomposes both the polar and nonpolar domains) compared with pure IL, whereas both the radial and spatial distribution functions show the stabilization role of ACN in the close-contact ion-pair association. On the other hand, increasing ACN causes weakening of the structural correlations of the cation-cation and anion-anion neighbors in the solutions. ACN molecules appeared as a bridge, with balanced affinities between the polar and nonpolar domains, and no indication was observed for aggregation of ACN molecules in the studied mixtures that can rationalize good miscibility with imidazolium-based ILs.