Structure And Dynamics Properties Of Imidazolium-based Ionic Liquid At Gas-liquid Interface:a Molecular Dynamics Study

Ionic liquid（ILs）are receiving increasing attention in green synthesis,catalysts,gas molecular storage,and separation due to their excellent physical properties.Compared to the ILs in the bulk region,more complex structure and function are possible for ILs at the gas-liquid interface.However,most of the experimental and computational studies are available in the literature regarding the structure and dynamics of the ions in the bulk phase,the understanding for surfacial structure and dynamics are rare.Therefore,in this work,we present the molecular dynamics（MD）studies of structure,dynamics and hydrogen bond（HB）of imidazolium-based IL at the gas-liquid interface.In the second chapter,we report a series of molecular dynamics simulations for the orientations and rotational dynamics of the 1-butyl-3-methyl-imidazoliumhexa fluorophosphate（[Bmim][PF₆]）IL at the gas–liquid interface,which is one of the most representative IL.As shown from simulation results,compared to the bulk phase,the[Bmim]⁺cations at the interface prefer to orientate themselves with their imidazolium rings perpendicular to the gas–IL interface plane and their butyl chains pointing toward the vacuum phase.Such a preferential orientation can be attributed to the combined effect of the hydrophobic interactions and the optimum loss of hydrogen bonds（HBs）.More interestingly,our simulation results demonstrate that the butyl chains of cations exhibit a two-stage rotational behavior at the interface,where the butyl chains are always in the vacuum phase at the first stage and the second stage corresponds to the butyl chains migrating from the vacuum phase into the liquid phase.A further detailed analysis reveals that their rotational motions at the first stage are mainly determined by the weakened HB strength at the interface while those at the second stage are dominated by their hydrophobic interactions.In the third chapter,we study the surface segregation for the imidazolium-based binary IL mixtures containing 1-ethyl-3-methylimidazolium tetrafluoroborate（[Emim][BF₄]）and 1-butyl-3-methylimidazolium tetrafluoroborate（[Bmim][BF₄]）at the gas-liquid interface by using molecular dynamics（MD）simulation.Our simulation results demonstrate that the preferential enhancement of the longer chain[Bmim]⁺cations at the outermost surface layer promoted more[Emim]⁺cations to be located near the side of liquid phase.According,these two kinds of imidazolium-based ILs show significant surface segregation in the surface region.In addition,Compared to the pure IL-vacuum interface,the[Emim]⁺cations,having stronger HBs interaction,show slower rotation motion in the interfacial layer,whereas,the rotation rates for[Bmim]⁺cations with weaker HB interaction become faster.Therefore,our simulation results provide a molecular-level understanding of the structure,rotation motion and HB of imidazolium-based IL at the gas-liquid interface,and extend the knowledge of surface structure properties.Our simulation result is of great benefit for experimental scientists to understand the surfacial properties of IL.