Molecular-Level Insights Into Composition-Dependent Structure, Dynamics, And Hydrogen Bonds Of Binary Ionic Liquid Mixture From Molecular Dynamics Simulations

In this work, we have performed a series of molecular dynamics (MD) simulations to explore structure, dynamics, and hydrogen bonds (HBs) of the imidazolium-based ionic liquid (IL) mixture containing [Emim] [BF4](x) [NTF2]((1-x)), where the molar fraction x is 0.0, 0.25, 0.50, 0.75, and 1.0, respectively. Our simulation results demonstrate that the association extent between cations and both anions become weaker and weaker as the concentration of [BF4](-) anion increases due to the weakened interactions between them. Meanwhile, all ions in the IL mixture are found to diffuse faster at the higher concentration of [[BF4]- anion while the order of diffusion rate is always [Emim](+) > [BF4](-) > [NTF2](-) owing to different molecular weights regardless of the composition. Furthermore, the weakened HBs between cations and anions are found to be at the higher concentration of [BF4](-) anion, leading to a faster rotation for all ions in the IL mixture. Compared to the diffusion rates among different ions, however, there is unexpectedly a much larger difference in their rotation rates with the fixed order of [BF4](-) > [Emim](+) > [NTF2](-), where the rotational relaxation times of [Emim](+) and [NTF2](-) are much more than that of [BF4](-) by at least one order of magnitude. This can be attributed to that the rotational motions of spherical [BF4](-) anions only require the transient HB breakage with cations so that their rotations should be affected by the continuous HB strength, which is significantly different from those of both [Emim](+) and [NTF2](-) dominated by the intermittent HB strength. Our simulation results provide a molecular-level understanding composition-dependent structure, dynamics, and HBs in IL mixtures.