Molecular Simulation Studies On Competitive Hydrogen Bond Behaviors Of Imidazolium-Based Ionic Liquid Mixtures

Pure ionic liquids（ILs）have been widely used in catalysis,synthesis,extraction and electrochemistry due to their high thermal stability,low vapor pressure and wide electrochemical window.However,the high viscosity and low conductivity of pure ILs also hinder their applications,especially in the field of electrochemistry.The physical and chemical properties of ILs can be adjusted by changing the structures and composition of cation and anion,which makes the ILs also being called"design solvents".By comparison with other synthesis methods,relevant studies have shown that mixing different ILs is an economical and convenient method to synthesize substances with specific physical and chemical properties.When the ILs change from pure phase to mixtures,there is an obvious increase in the complexity of interactions between ionic species,which makes it difficult for the experiments to directly detect the structures,dynamics and hydrogen bonds（HBs）properties of IL mixtures.In addition,most of present research works mainly focus on the behaviors of bulk IL mixtures,yet there is a lack of explorations at the interface.On the other hand,the differences in the properties for ILs at the interface and in the bulk phase are still unclear.Therefore,the main goal of this thesis is to explore the structures,dynamics and HBs properties of imidazolium-based ILs in bulk and at the interface by means of classical molecular dynamics（MD）simulation.In Chapter 2,we selected equimolar IL mixtures of 1–ethyl–3–methylimidazolium tetrafluoroborate and 1–butyl–3–methylimidazolium tetrafluoroborate with different alkyl chain lengths([Emim]_0.5[Bmim]_0.5[BF₄]),and special attention was paid to the effects of alkyl chain length on the structures and dynamics properties.Our simulation results show that the alkyl chain length has negligible effect on the structures properties,as well as the association/dissociation dynamics properties between cations and anion.However,further analysis on HBs networks between cations and anions reveal that[Bmim]⁺cations have more ability to form stronger HB with anions compared to[Emim]⁺cations,which contributes to the slower diffusion and rotation motions of[Bmim]⁺cations than[Emim]⁺cations.The order of diffusion velocity is[Emim]⁺>[Bmim]⁺>[BF₄]^–,and the order of rotation motion is[BF₄]^–>[Emim]⁺>[Bmim]⁺.In Chapter 3,we explored[Emim][BF₄]_x[NTF₂]_（1-x）IL mixtures with five different concentration of anions,and analyzed the effects of different anions on the structures,dynamics and HBs properties of the IL mixtures.As the[BF₄]^–anion concentration increases,the weaker average interaction energy between cations and anions leads to the weaker aggregation tendency for cation–anion and the rapider diffusion motions of ions.Therefore,the order of diffusion rate always follows[Emim]⁺>[BF₄]^–>[NTF₂]^–,and the order of rotation rate is[BF₄]^–>[Emim]⁺>[NTF₂]^–.Besides,the rotation relaxation time of[Emim]⁺and[NTF₂]^–is at least one order more than that of[BF₄]^–,which attributes to the fact that the rotation motions of spherical[BF₄]^–anions are mainly affected by the continuous HBs dynamics,while the rotation motions of[Emim]⁺and[NTF₂]^–are mainly affected by intermittent HBs dynamics.The weaker HBs between cation and anion further verified the faster rotation of ions.In Chapter 4,we mainly explore the structures,dynamics and HBs properties of equimolar[Emim][BF₄]and[Bmim][PF₆]IL mixtures at the interfaces of carbon nanotubes（CNTs）with three different diameters.Our simulation results for the first time reveal that[BF₄]^–and[PF₆]^–anions in the IL mixtures around CNTs have obvious HBs competition behavior,and the competition phenomenon becomes more obvious with increasing the diameter of CNTs.The HBs of cation–[PF₆]^–are always greater than that of cation–[BF₄]^–,and the HBs between cation and anion are gradually enhanced as the diameter of CNTs increases,Nevertheless,the HBs of cation–[PF₆]^–show a greater increase when compared with the corresponding that of cation–[BF₄]^–.The competitive advantage of HBs for[PF₆]^–leads to the aggregation tendency of more[PF₆]^–anions and less[BF₄]^–anions around larger CNTs,which is obviously different from the aggregation behavior of pure ILs around CNTs.Above competitive HBs behavior leads to the opposite trend of the average HBs number of the two anions with respect to cations as the increase of CNTs diameter.In addition,the stronger HBs between the cations and anions at the larger CNTs interface result in the slower rotation motion of ions.In the graduate stage,we have determined the structures,dynamics,and HBs properties of binary IL mixtures in bulk and at the interface from the molecular level,and an in-depth discussions have been performed among their relationship,which is helpful for experimental scientists to further explore the physical and chemical properties of IL mixtures and prepare chemical substances with specific applications.