Molecular Dynamics Study of Water Diffusivity in Graphene Nanochannels

Transport properties of nano-confined fluids such as diffusivity can exhibit utterly distinctive characteristics compared to the transport properties in the bulk due to the interactions between atoms in the solid walls and fluid atoms as well as the confinements. In this paper, the diffusivity of water confined in the graphene nanochannels is calculated by molecular dynamics simulations through the Einstein equation, and the results show that the diffusivity of nano-confined water is obviously anisotropic, i.e., the perpendicular (vertical to the graphene walls) diffusivity is obviously lower than the diffusivity in the parallel plane. By studying the Lagrangian dynamics of molecules in the confined region, we realize that the anisotropy can be attributed to the trapping of water molecules in the potential wells near the graphene walls, resulting in the inhibition of the molecular mobility in the perpendicular direction. Meanwhile, the proportion of confined water molecules decreases with increasing channel height and the contributions of the trapped water molecules on the inhibited mobility in the perpendicular direction are weakened. As a result, the diffusivity in all directions approaches the bulk values at high channel heights. The obtained results are helpful in revealing the mechanisms of water diffusion in nanospaces from the molecular level.