Simulation insight into water transport mechanisms through multilayer graphene-based membrane

Abstract

Although single-layer nanoporous graphene has proven to be effective as a reverse osmosis desalination membrane, multilayer nanoporous graphene (MNPG) is economically affordable to be synthesized. In this study, water transport through large cylindrical (LC) and small cylindrical (SC), as well as through large hourglass-shaped (LHGS) pore structures constructed by MNPG is investigated via molecular dynamics (MD) simulations. It was found that the number of occupancy states increases with increasing pressure in LC pore, whereas they decrease with increasing pressure in SC pore. At P=2katm, water molecules must overcome a very large free energy barrier in SC pore owing to large entrance effects, suggesting a dramatic reduction in net flux. The LHGS pore structure is suggested to be a more efficient design for achieving higher flux, compared to other structures.It was found that the hydrophilicity effect could nearly double the flux inside LHGS pore, owing to the strong hydrogen bonds. Moreover, the mean square displacement (MSD) profile in a hydrophilic pore shows larger displacement than a hydrophobic one, which facilitates water filling mechanism. It also indicated that the layers with hydrophilicity effect increase water concentration in the area close to the surface of the layers owing to strong hydrogen bonds. It is concluded that osmotic permeability of water molecules increases substantially inside hydrophilic LHGS pore.