Different Hydrogen Bond Changes Driven by Surface Segregation Behavior of Imidazolium-Based Ionic Liquid Mixture at...

In this work, molecular dynamics (MD) simulations were carried out to study the behaviors of a binary ionic liquid (IL) mixture consisting of equimolar [C₂C₁Im][BF₄] and [C₄C₁Im][BF₄], as well as two corresponding pure ILs, at the liquid-vacuum interface. Our simulation results show that the competition of nonpolar interactions between different alkyl chains of two cations results in an obvious surface segregation behavior of the IL mixture at the interface, indicating an enhanced aggregation of the [C₄C₁Im]⁺ cations but a weakened aggregation of the [C₂C₁Im]⁺ cations at the outermost surface. More interestingly, different hydrogen bond (HB) changes between two imidazolium cations at the interface can be driven by such surface segregation behavior, where the [C₂C₁Im]⁺ cations rather than the [C₄C₁Im]⁺ ones have more and stronger HBs with the [BF₄]^- anions by comparison with the corresponding pure ILs at the interface. Meanwhile, it is interesting to find that such a stronger HB would lower the rotations of the imidazolium rings of interfacial [C₂C₁Im]⁺ cations. By contrast, the [C₄C₁Im]⁺ cations at the outermost surface rotate faster owing to their weaker HB. In addition, the orientation analysis uncovers that there is a major decrease for the orderliness of interfacial [C₂C₁Im]⁺ cations, but a minor decrease for that of interfacial [C₄C₁Im]⁺ cations, from the pure IL to the IL mixture. Such distinct results are closely related to the surface segregation between the [C₂C₁Im]⁺ and [C₄C₁Im]⁺ cations in the IL mixture and their interfacial HB properties. Thus, our simulation results afford a deep insight into the surface segregation effect on the HB behavior of the imidazolium-based IL mixture at liquid-vacuum interface.