Interfacial ion diffusion is also likely slowed by interactions between the ions and the graphene surface. Fig. shows the free energy of interfacial sodium and chloride ions as a function of position on the graphene surface. This quantity is defined as -k B T ln P ion , where P ion is the spatial probability distribution of an ion in the interfacial layer projected onto the graphene lattice. Here we observe that the sodium ion experiences a more corrugated energy landscape at the graphene surface than the chloride does. This difference in surface interaction is related to the fact that the sodium ions lose more of their first solvation shell when adsorbing to the interface than the chloride ions, as quantified in the coordination numbers presented in Fig. . We hypothesize that these differences in surface interactions and interfacial solvation give rise to the contrasting trends seen for the cation and ion, in which sodium ions diffuse most quickly in the center of the slit but chloride ions diffuse most quickly at the interface. Previous molecular dynamics simulations based on classical force fields have also noted that distorted hydration shells and reduced coordination numbers at an interface are associated with slower ion diffusion, suggesting that this is a general phenomenon which is not highly sensitive to ion-water and ion-pore interaction parameters.