Zwitterionic polymers functionalised nanoporous graphene for water desalination: a molecular dynamics study

In this paper, one nanoporous graphene grafting several zwitterionic polymer chains was designed as the osmosis membrane for seawater desalination. Using molecular dynamics simulation, the efficiency and mechanism of salt rejection were discussed. The simulated results showed that the zwitterionic polymer chains on nanoporous graphene can form the charge channel to block Na⁺ and Cl⁻ ions pass through, and the slat rejection efficiency of functionalised graphene can reach to about 90%. In the simulation, the steric hindrance and electrostatic interaction are the main factors for the salt rejection. With time evolution, the charge channel formed by the soft polymer chains can decrease the effective pore area of membrane, leading to the increase of steric hindrance; the positive and negative centres of polymer chains can adsorb Na⁺ and Cl⁻ ions under electrostatic interaction in the solution, contributing into the increase of charge density above the membrane. These conclusions are consistent with experimental report. Our designed osmosis membrane about the graphene is helpful for improving the potential application of defect graphene in water desalination and reducing the trouble of obtaining appropriate graphene sheet with small aperture.