Interfacial Diffusion of Hydrated Ion on Graphene Surface: A Molecular Simulation Study

Hydration plays an important role in the diffusion and sieving of ions within nanochannels. However, it is hard to quantitatively analyze the contribution of hydration to the diffusion rates due to the complex hydrogen-bond and charge interactions between atoms. Here, we quantitatively investigated the interfacial diffusion rates of a single hydrated ion with different number of water molecules on graphene surface through molecular dynamics simulation. The simulation results show the ballistic diffusion mode by analyzing the mean-square displacement, and the diffusion rates change nonmonotonically with the hydration number. The potential energy profiles with the changing position of the hydrated ion on graphene surface were further analyzed, which shows the dominant factor for interfacial diffusion changing from ion-graphene interaction to water-graphene interaction as the number of water molecules increases. Besides, it was found that the surface hydrophilicity weakened the influence of hydration number on the diffusion rates of hydrated ion. Finally, the diffusion properties of different hydrated ions on graphene surface were investigated, and the hydrated Li⁺, Na⁺, and K⁺ containing three, four, and five water molecules, respectively, show the fastest diffusion rate. This work demonstrates the interfacial diffusion behavior and mechanism of hydrated ions at the molecular level, which can provide valuable guidance in nanosensors, seawater desalination, and other hydrated ion-related fields.