Molecular Dynamic Simulation Of Stretched Water In Nanochannels

Water confined in nanochannels is to be stretched with the variation of the external pressure,leading to unusual properties in comparison with bulk water.In order to unravel the impacts of stretching on the structural and dynamical properties of water confined in hydrophobic/hydrophilic nanochannels with various diameters,molecular dynamics simulations were performed.It was found that the water bilayers that are closer to interface S1 of hydrophilic graphene oxide nanochannels were observed in contrast to the monolayer at the hydrophobic graphene.The location of the first water layer is independent of the channel diameter and stretching coefficient.Moreover,water in the first layer tended to orient themselves parallel to the channel surface regardless of their diameter and hydrophilicity.With the increasing stretching coefficient,water molecules in hydrophobic nanochannels tended to slightly tilt toward the central region,whereas the orientation of water molecules in hydrophilic nanochannels was nearly unaffected by stretching.In addition,according to the tetrahedral order parameter,the more disordered water under stretching was observed,which could be attributed to the increased void fraction during stretching.Compared with bulk water,the hydrogen bond lifetime of confined water was increased,whereas it was decreased with the increase of the channel diameter.Owing to the more stabilized hydrogen bonds between hydrophilic nanochannel and water,the hydrogen bond lifetime of hydrophilic channels was longer and barely influenced by stretching coefficient.Besides,the hydrogen bond lifetime of water in hydrophobic nanochannels is shorter.Regarding the dynamical properties,the water self-diffusion coefficient was higher in hydrophobic graphene,and the water of interfacial region diffused faster than that in central bulk-like region.The reverse trends were observed for stretched water in hydrophilic channels,which may be attributed to the reduced water mobility resulting from the stronger coulomb interaction between hydrophilic surface and water.However,the diffusion coefficient is increased with the increase of stretching coefficient regardless of the hydrophobicity of nanochannels.The oscillating tangential pressure profile inside nanochannels is closely related to the water density distribution,with a nearly constant normal pressure.