Structure And Surface Tension Of The Water In The External Electric Field:A Molecular Dynamics Study

The study on the properties of water surface in the influence of the external electric field is of great significance to the understanding of many chemical and biological processes.In this thesis,the constant potential method(CPM)is used to simulate the liquid water under the electric field,which mainly focuses on the effect of an external electric field on the microstructure,kinetics,surface potential,polarization behavior and surface tension of liquid water.Key findings of the thesis are as following:1.The surface structure of liquid water was studied by using the method of mMolecular Dynamics and the Intrinsic Sampling Method(ISM).Different from the traditional gibbs-dividing surface,the instantaneous surface of the simulated system water with schedule is obtained by the intrinsic analysis of the water surface.The stratified structure at the water surface is found,the coefficient of instantaneous fluctuation of water surface is obtained,and the correlation between surface capillary fluctuation amplitude and wave vector is established.Obtained the thermodynamic properties(such as pressure,stress,surface tension,etc.)based on the intrinsic structure of water,and the molecular orientation and thermodynamic properties of the liquid water surface were shown to be anisotropic.The determination of intrinsic surface is of great significance to the analysis of microscopic structure and thermodynamic properties of liquid water surface.2.Using the constant potential method,we have simulated the liquid water under the external electric field at ambient temperature by the molecular dynamics.The electric potential of the plate is strictly controlled,so that the electrode can maintain a constant difference between the plates,so as to meet the conditions of the constant external electric field.With the increase of the external electric field,the density distribution of the water surface and bulk water is not obvious.The orientation of molecule and dipole direction are gradually consistent with each other in the water surface,and the surface stress and pressure have a great fluctuation.In addition,we find that when the potential difference?? > 2V between the plates,the bulk water produces polarization behavior,and the change of the surface structure and thermodynamic properties of the water become obvious.3.The influence of the external electric field on the surface potential of the water is studied,and the variation of electric potential of water surface under the external field is obtained.By using the probe method,we obtained one dimensional potential distribution along the electric field direction,and it is found that when the potential difference ?? > 2V between the plates,the bulk region of the water appears potential gradient.Under the zero external electric field,the surface potential of water near the cathode and anode is the same.with the increase of the external field,the surface potential of the water is different,the surface potential of the water near the cathode decreases gradually,and the surface potential of the water near the anode increases.4.With the intrinsic and the mean statistical perspective,we studied the surface structure and thermodynamic properties of the water under different external electric fields systematically,which based on the capillary waves theory and Gibbs-dividing interface respectively.compared the relationship between surface tension(the definition of mechanics and capillary waves theory)and the external electric field.Through calculation and analysis,it is found that the intrinsic statistical perspective can reflect the surface structure of water more accurately,especially the change of surface structure and thermodynamic properties under the external electric field interference.Under the zero electric field,the two definitions of surface tension are basically the same;with the increase of the applied electric field,under the influence of the low electric field,the surface tension has the same decreasing trend under the mechanical definition and the capillary wave theory definition.Two start to produce the difference under the high electric field.In summary,the polar molecular liquid water is simulated by constant potential method and molecular dynamics under the external electric field.From the intrinsic and average statistical analysis perspective,obtained the surface structure and thermodynamic properties of liquid water in the influence of external electric field.The essential characteristics of this helps to reveal both the micro scale surface structure and the thermodynamic properties of water under external electric field.