| Recently,since the interface and confined systems have many potential applications in biomedical,catalytic and membrane materials fields,many experimental and theoretical workers pay more attention to the distribution of water molecules on graphene interface as well as the confined water in graphene nanocapillary.To accurately investigate the interface water on graphene surface and the confined water in graphene nanocapillary at molecular level by using polarizable force field is of great significane.In this paper,the complex of acene and one water molecule,the structure and phase behavior of water molecule in graphene nanocapillary,and the distribution of the confined water in a wider graphene slit pore are studied by using the fine ABEEM-7P water model and ABEEM graphene model.The characteristic of ABEEMσπpolarizable force field（PFF）is the electrostatic interaction.With the surroundings changing,the charge distribution of each site fluctuates to adapt the current structure,which is very close to the real situation.1)Study on Multiple Acenes as well as the Complexes of Multiple Acenes and Water.Eight structures,including C6H6,C24H12,C54H18,C96H24,C6H6-H2O,C24H12-H2O,C54H18-H2O and C96H24-H2O are the studied systems.We have obtained a set of ABEEM charge parameters which are suitable for graphene and graphene-water complexes.The ABEEM charge distributions of C6H6,C24H12,C54H18 and C96H24 as well as the complexes of C6H6-H2O,C24H12-H2O,C54H18-H2O and C96H24-H2O are in good agreement with those of the Mulliken charge distributions.Compared with the results of OPLS-AA and OPLS-AAP force field,the binding energies of C6H6-H2O,C24H12-H2O,C54H18-H2O and C96H24-H2O as well as the distance between the oxygen atom of the water molecule and the plane of acene obtained from ABEEMσπPFF are in better agreement with those obtained fromωb97xd/6-31+G（d,p）level.In addition,the potential energy curves of C6H6-H2O,C24H12-H2O,C54H18-H2O and C96H24-H2O yielded by ABEEMσπPFF are very close to those obtained from ab initio calculations.It can be concluded that a set of ABEEM charge parameters and force field parameters are obtained,which are very suitable for studying the complex systems of acenes and water molecules.This lays the foundation for studying water molecules adsorption on graphene surface.2)The Structure and Phase Behavior of the Confined Water in Graphene NanocapillariesThe structure and phase behavior of the confined water in graphene nanocapillaries are studied by molecular dynamics simulation by using ABEEMσπPFF.ABEEM-7P water model and ABEEM graphene model have more fine charge distribution.How the size of graphene slit pore,the density of the confined water,temperature and pressure influence on the the distribution of the confined water are discussed in detail.To study the effect of the size of graphene slit pore on the distribution of the confined water,the structures of monolayer,bilayer and trilayer ice simples in graphene nanocapillaries are observed,which are different from those previously reported.We find the ordered bilayer AB stacked square ice and trilayer ABA stacked square ice in 8.00 and 10.20?graphene slit pores,respectively.By changing the width of graphene nanocapillary from 7.80 to 8.60?every 0.10?interval,it has been found that the structure of bilayer ice experiences a rhombic-square-triangular configuration.By changing the width of graphene nanocapillary from 10.00 to 11.00?,a rhombic-square-triangular configuration of the trilayer ice is found.As a reference,we carry out a series of molecular dynamics simulations by using fixed charge force field with SPC/E water model and ABEEM force field parameters.However,only slightly distorted monolayer ice like square ice appears which is actually diamond-shaped.No ordered bilayer or trilayer structure appears.Hydrogen bond analysis shows that in ABEEMσπPFF each confined water forms four hydrogen bonds,and sometimes one lone pair electron or one hydrogen atom participates in formation of two hydrogen bonds.For bilayer AB stacked square ice,few hydrogen bonds form between layers.However,for AB stacked triangular ice,there are many hydrogen bonds between layers.For three layer ice simple,each confined water forms a tetrahedral hydrogen bonds,which always exists between layers.As a reference,we investigate the hydrogen bond distribution obtained from fixed charge force field with SPC/E water model.The ordered hydrogen bonds distribution is obtained only in monolayer triangular ice.Generally speaking,the hydrogen bond structure yielded by ABEEMσπPFF is more ordered than that obtained by fixed charge force field with SPC/E water model.This may be due to the polarization effect,the fine ABEEM model and the special electrostatic interaction in the hydrogen bond interaction region in ABEEMσπPFF.The density of the confined water plays an important role in the distribution of the confined water.However,the distribution of the confined water at different temperature shows little difference,which suggests that the temperature influence the conformation of the confined water little.Under different constant pressure,the distribution of the confined water shows an interesting phenomenon.It has been found that the monolayer triangular ice can transform to AB stacked bilayer square ice in a 6.50?graphene nanocapillary by changing the pressure from 0.50 to1.50 GPa or at 0.50 to 1.00 GPa by changing the width of the graphene nanocapillary from6.50 to 7.00?.Furthermore,we also observe that the ordered AB stacked bilayer triangular ice transforms to ABA stacked trilayer square ice when the graphene nanocapillary is 9.00?by changing the pressure from 0.50 to 1.50 GPa.Charge analysis shows that the electrostatic interaction between graphene and the confined water in ABEEMσπPFF is more moderate.Our analysis shows that in addition to van der Waals interaction,the size of graphene slit pore,the density of the confined water,polarization effect and pressure all play an vital role in the distribution of the confined water in graphene slit pore.Our results provide some important information for the formation of monolayer,bilayer and trilayer square ice in graphene nanocapillaries obtained by electron transmission spectroscopy（TEM）.3)The Structure and Phase Behavior of the Confined Water in Wider Graphene Slit PoresIn order to study the effect of width of the two graphene slabs and the density of the confined water on the distribution and phase behavior of the confined water,molecular dynamics simulations are carried out by using ABEEMσπPFF.ABEEM-7P water model and ABEEM graphene model are also utilized.The width of graphene slit pores are set to 14.00,17.00,20.00,30.00 and 40.00?,respectively.The densities of the confined water in 20.00?graphene slit pore are set to 0.64,0.80,1.00 and 1.20 g·cm-3,respectively,and the densities of the confined water in 30.00?graphene slit pore are 0.43,0.80,0.95,1.08 and 1.35 g·cm-3,respectively.The dynamic properties including diffusion coefficients,the structural properties including conformation of the confined water,radial distribution function（RDF）,number density of the confined water and O-O-O angle distribution of each layer confined water and the charge distribution of the simulation systems are analyzed.Diffusion coefficients analysis shows that when the graphene slit pore is wider and the density of the confined water is lower,the diffusion coefficients of the confined water is smaller.Subsequently,we predict that when the spacing between the two graphene plates equals to 50.00?and the density of confined water is 0.26 g·cm-3,the diffusion coefficient of the confined water will close to that of liquid water.Conformation analysis shows that in the 20.00?graphene slit pore,the confined water is prior to close to the graphene surface as many as possible,forming one or two layer confined ice simples.Interestingly,there exists several water molecules in the middle of the slit pore connecting with the water molecules on both graphene slabs with hydrogen bonds.In the 30.00?graphene slit pore,the confined water is still prior to close to the graphene surface as many as possible.When the number of the confined water is small,monolayer ice forms near the graphene surface.When the number of confined water molecule is large,the graphene surface forms only two or three layers ordered ice.The residual confined water in the middle of the graphene slit pore distributes in disorder.This verifies that the formation of the ordered confined ice depends entirely on the surface of graphene.All the monolayer ordered confined water forms triangular configuration,which suggests that the conformation of triangular is more stable.The radial distribution function and the number density distribution of the confined water fully confirm the conformation analysis of the confined water.The absolute values of charge distribution of the oxygen and hydrogen atoms of the confined water near graphene surface is less than those of liquid water,which corroborates the effect of shielding charge of graphene.When the confined water is distant from the two graphene plates,the charge distribution of the confined water is close to that of liquid water.The above analysis suggests that the wider graphene slit pore plays an vital role in the structure,phase behavior and the charge distribution of the confined water. |
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