Molecular-Level Understanding of Surface Roughness Boosting Segregation Behavior at the ZIF-8/Ionic Liquid Interfaces

Abstract

Here, we perform a series of classical molecular dynamics simulations for two different [HEMIM][DCA] and [BMIM][BF4] ionic liquids (ILs) on the ZIF-8 surface to explore the interfacial properties of metal-organic framework (MOFs)/IL composite materials at the molecular level. Our simulation results reveal that the interfacial structures of anions and cations on the ZIF-8 surface are dominated by the surface roughness due to the steric hindrance, which is extremely different from the driving mechanism based on solid-ion interactions of ILs on flat solid surfaces. At the ZIF-8/IL interfaces, the open sodalite (SOD) cages of the ZIF-8 surface can block most of the large-size cations outside and significantly boost the segregation behavior of anions and cations. In comparison with the [BMIM][BF4] IL, the [HEMIM][DCA] IL has much more anions entering into the open SOD cages owing to the combination of stronger ZIF-8-[DCA]- interactions and more ordered arrangement of [DCA]- anions on the ZIF-8 surface. Furthermore, more and stronger ZIF-8-[BF4]- hydrogen bonds (HBs) are found to exist on the cage edges than the ZIF-8-[DCA]- HBs, further preventing [BF4]- anions from entering into SOD cages. By more detailed analyses, we find that the hydrophobic interaction has an important influence on the interfacial structures of the side chains of [HEMIM]+ and [BMIM]+ cations, while the π-π stacking interaction plays a key role in determining the interfacial structures of the imidazolium rings of both cations. Our simulation results in this work provide a molecular-level understanding of the underlying driving mechanism on segregation behavior at the ZIF-8/IL interfaces.