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

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.