Molecular Simulation Studies Of Solvent Structure And Ion Transportation In Graphene Two-dimensional Nano-channel

There are many advantages of membrane separation technology,such as low energy consumption and high efficiency.Graphene 2-dimensional membrane,which is stacked by graphene sheets,with abundent 2-dimensional nano-channels,has an excellent mechanical property and stable chemical property,and has broad prospects in the fields of seawater desalination and pollution control.Molecular simulation is able to observe the motion of ions and molecules from microscopic view,therefore can provide unique insights in the study of mass transportation in nano-channel.In order to provide theoretical instructions for the design of graphene 2-dimensional nano-membrane and to develop graphene 2-dimensional membrane,this paper studies the mechanism of ion’s transportation in graphene 2-dimensional nano-channel by molecular dynamics.The main content of this paper is as follow.To accurately simulate the ion-πinteraction between graphene and ion in bulk solution through molecular dynamics,new ion-graphene force fields are developed by adjusting the ion-graphene（C atom）interaction parameterε_i-C(optimizedε_i-Cparameter)in fixed charge force field（FF）.Furthermore,potential of mean force（PMF）of graphene-（single）ion in the bulk solution is decomposed into the contribution of ion-graphene interaction（ion-graphene PMF of anhydrous system）and the contribution of ion-solvent interaction(ion-graphene PMF in bulk solution withε_i-C=0).Ion-water interaction plays an important role in the adsorption of ions by graphene.In graphene-electrolyte solution of 1 mol/L concentration,the adsorption of cations and anions with newε_i-C parameter is observed at graphene-solution interface.And the positions of ions’adsorption peak are in good agreement with the position of PMF minima of graphene-（single）ion.In graphene 2-dimensional nano-channel（with a height of 1.005 nm）,water molecules are arranged in anisotropic double layers,and the water density between two layers of water is as low as 1/4 of the density in bulk solution（240 kg/m³）.The distribution of ions,which is either close to water layer or located in the middle of nano-channel,is determined by both ion-water interaction and ion-graphene interaction.When ion with small radius locates in the middle of channel,the shape of the first hydration shell of ion is much alike the evenly distributed sphere when in bulk solution,and ion’s force came from water is similar with that in bulk solution,and the residential time of water（τ）in ion’s hydration shell changes little,therefore the force auto-correlation function（FACF）of the ion in nano-channel is also similar with the FACF of ion in the bulk solution,diffusion coefficient（D）of ion in nano-channel does not change much comparing with D in bulk solution.On the contrary,when the radius of ion is large or ion locates in water layer,the hydration shell of ions is arranged in layers and compressed into one ring（ion located in water layer）or two rings（ion with large radius located in the middle of channel）because of the influence from graphene wall,τin ion’s hydration shell is significantly larger than that in bulk solution,both the correlation of water configuration around ion in time and the correlation of ion force in time are enhanced,and the tail of ion’s FACF is enlarged,therefore the integral of FACF（reflecting ion’s friction）increases,and D of ion drops to 0.78（the ratio of D of water in nano-channel to D of water in bulk solution）times of D in bulk solution.Further research found that,D of ions in bulk solution simulated by molecular dynamics is consistent with D of the ions in infinitely dilute solution mesured by experiment,which proves the correctness of simulation method in this paper;and D of ions and conductivity of ions are in good agreement with Stokes relation in both bulk solution and graphene nano-channel.In addition,the ion force mainly comes from the interaction between ion and its hydration shell,but the solvents out of ion’s hydration shell also has a an important influence on ion’s FACF.After reducing the density of water in graphene nano-channel,D of water molecules increases,andτin ion’s hydration shell becomes shorter,which weakens the correlation of water configuration around ion in time and the correlation of ion force in time,therefore the tail of ion’s FACF becomes smaller and ion’s friction decreases.