Molecular-level insights into composition-dependent structure, dynamics, and hydrogen bonds of binary ionic liquid mixture from molecular dynamics simulations

Abstract

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.