Molecular Dynamics Simulation Of Ionic Solution Under The External Electric Field And The Shear Flow Field

Ionic solution has played an important role in academia and industry because of its universality.However,the understanding of the micro mechanism of ionic solution is still challenging in the experiment,while molecular dynamics simulation can well solve the exploration and verification of basic scientific problems,to fundamentally help the design and development of new materials containing ionic solution.This paper abandons the method of using a simple dielectric constant to deal with the solvents and adopts the newly developed Stockmayer-like fluid model.Through coarse-grained mo-lecular dynamics simulation,starting from the multi-body interaction of ion-ion,ion-dipole and dipole-dipole and focusing on the electrostatic correlation effect and ion solvation effect,the orientation,structure and dynamic properties of ionic solution in equilibrium,under the applied electric field and the shear flow field were studied.The main contents are as follows:1.The orientation,structure and dynamics of ionic solution in equilibrium:By selecting six solvents with different dipole moments（μ*=0.5,1.0,1.5,2.0,2.5,3.0）and two different ion concentrations((8₄)=0.5 M,2.0 M),the average orientation of dipolar solvents around a single ion is calculated first by coarse-grained molecular dy-namics simulation.It is shown that the dipole orientation in the solvent solvation layer varies with solvents in different dielectric properties,which can not be captured by the traditional dielectric constant.With the increase of dipole moment,the structure of ions in solution changes from ion cluster to complete dispersion,and then to local aggregation.The internal cause is the coupling of ion-ion,ion-dipole,dipole-dipole multibody interaction,and there is a competitive relationship between the electrostatic correlation effect and ionic solvation effect.The mean square displacement and diffu-sion coefficient of ions and solvents were calculated.The study shows that the diffusion behaviors of ions and dipolar solvents are a non-monotonic relationship with the dipole moment,which is consistent with the law of the change of ionic structure with dipole moment.2.Effect of applied electric field on orientation,structure and kinetic properties of ionic solution:In this work,we selected two kinds of electric field strength（E*=0.1,0.5）.Firstly,we explored the influence of the external electric field on the average ori-entation of dipolar solvents.It is shown that the interaction between the external electric field and different species leads to the reorientation of the dipolar solvent around the ion,and the rotation of the whole dipolar solvent is also limited by the external electric field.Then,the influence of the external electric field on ion structure is explored.The result shows that the influence of external electric field on ion structure in solvents with different dipole moments is non-monotonic,and the coupling between electrostatic cor-relation effect and ion solvation effect is changed.In solvents with a low dipole moment,ionic clusters are elongated into sheets or wedges,which can provide a self-assembly strategy for materials containing ionic solutions.In terms of dynamics,the application of an external electric field will expand the nonmonotonic relationship between the dif-fusion drift ability of ions and dipole solvents in equilibrium and the dipole moment.3.The orientation,structural,mechanical and kinetic properties of ionic solution under steady-state shear:By selecting four shear rates（?=0.1,0.5,1.0,2.0）,the effect of the shear flow field on the average orientation of dipolar solvent was studied.The results show that the flow field will change the orientation of the dipolar solvent around the ion,which will affect the ion solvation.In terms of structural properties,the struc-ture of the ion solution system is basically consistent with the equilibrium state at a low shear rate.With the increase of shear rate,strong flow field will lead to the dispersion of ion aggregation state,the electrostatic correlation effect of ions will be reduced,and the solvation of ions will be enhanced.The shear viscosity and stress-strain curves of the system are calculated to explore the mechanical properties of ionic solution.The simulation demonstrates that the ionic solution system in this paper is a Newtonian fluid at a low shear rate,and a shear-thinning fluid with the increase of shear rate.We verify the Taylor formula for correcting the effect of the flow field,and the results are in good agreement.The diffusion coefficient after correction of the flow field is calculated.The simulation shows that the flow field accelerates the diffusion of cations,and this accel-eration effect is verified by the change of ionic structure induced by the flow field.