Multiscale Simulation of the Interaction and Adsorption of Ions on a Hydrophobic Graphene Surface

The adsorption of ions on a graphene surface is very important to control relevant graphene-based processes. In this work, a multiscale simulation was carried out to study the adsorption of Na⁺/Cl⁻ ions on graphene by combining quantum mechanics calculations and molecular dynamics (MD) simulations. The interaction energies of the ions with graphene were computed using density functional theory (DFT). It was found that the ions show strong interaction with a graphene cluster and the overwhelming portion of the interaction energy is the ion-π orbital interaction. The large orbital interaction can be ascribed to the two contributions arising from the ion-induced polarization of graphene and the charge transfer between ion and graphene. Their different contribution degrees reveal that the polarization effect plays a main role on the orbital interaction for ion adsorption. Comparatively, for Na/Cl atom adsorption, the charge transfer shows large part to the orbital interaction with weak atom-induced polarization. The obtained interaction energies were applied to develop new interaction potentials between ion and graphene, and then MD simulations were used to study the interfacial adsorption behavior of Na⁺/Cl⁻ aqueous solution onto the graphene surface. Due to enhanced ion-π interactions, Na⁺/Cl⁻ cooperatively demonstrates a strong ion adsorption layer through direct contact with the hydrophobic graphene surface. Our simulation result presents a new understanding of ion-graphene interactions.