| By means of molecular dynamics simulation, as well as with the assistance ofquantum chemistry, this article aims to simulate and investigate room-temperatureionic liquids, in particular focused on its heterogeneity of spatial structure. Due to thefact that the present experiment techniques fail to precisely probe the movement andstructure properties of molecules in liquids, it is vital to build certain way bridgingbetween the microscopic and macroscopic worlds well. Experimentalists often findsome novel phenomena or unique properties related to liquids, which are not able tobe clearly explained owing to not being able to directly observe the micro-mechanismon a nanoscale. Therefore, it is a pressing issue to reveal how the micro-mechanismyields special macroscopic phenomenon. Molecular dynamics simulation, as a way ofsuccessfully modeling the movements of the molecules in a liquid, has been widelyemployed to investigate the intrinsic mechanism of those distinctive macroscopicphenomena.In this article, based on two special phenomena of room-temperature ionicliquids, how to probe the micro-mechanism by molecular dynamics simulation iselaborated, in particular revealing the role of the existent unique space structure inroom-temperature ionic liquids.These two special phenomena are as follow:(1) Fluorescent molecules (2-amino-7-nitrofluorene, ANF), as a solute, are putinto room-temperature ionic liquids, which shows the absorption wavelength-dependent emission spectra different from ANF placed in traditional organicsolvents such as alcohol. In addition, this dependent spectrum phenomenonwill disappear under replacing ANF using other fluorescent molecules thatpossess the considerably longer lifetime of the excited-state.(2) Graphene or carbon nanotube is able to be well dispersed in room- temperature ionic liquids, further forming “gel”, which has been widely usedin solar battery, fuel cell, sensor etc. fields. However, there has been muchcontroversy regarding the intrinsic reason why the rich-π-electron carbonnanomaterials such as graphene and carbon nanotubes can be dispersed inionic liquids. As carbon nanotubes and graphene are rich in π-electrons,strong cation-π interaction between carbon nanomaterials and ionic liquids issuspected to be responsible for the mechanism of dispersion of carbonmaterials in ionic liquids. In contrast, spectroscopic evidence in theexperiment by Li and coworkers has suggested that there might not beelectronic interaction between carbon materials and ionic liquids.In terms of the first phenomenon, I took imidazolium ionic liquid as an example,and then concluded that because of the heterogeneity of spatial structure in ionicliquids, ANF molecules in different domains will present diverse optical responses.Finally, the relationship between spatial structure and optical response was given.Similarly, for the second phenomenon, the system, grapene in imidazolium ionicliquids, was taken as an example. Firstly, by quantum mechanical calculation, I drewthe conclusion of the consistence between the cation-π interaction and van der Waalsinteraction. I.e. using classical molecular dynamics parameters can cover the cation-πinteraction on the level of quantum mechanics. Secondly, by means of directmolecular dynamics simulations, the reason why graphene sheets are difficult to gettogether is ascribed to the strong force imposed by ionic liquid molecules betweentwo graphene sheets. The integration of this strong force will yield huge energybarrier to prevent the graphene sheets into aggregation. In addition, herein, the effectof unique space structure in room-temperature ionic liquids on the dispersion ofgraphene sheets is revealed. |
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