Non-Reciprocal Interactions between Ions Near a Graphene Surface

Ions in aqueous media near surfaces are ubiquitous in nature and technology, thereby quantifying electrostatic interactions near interfaces is of prime importance. Here we investigate the effective interaction between two oppositely charged ions in different positions in water confined between two graphene surfaces. We found that the attraction between oppositely charged ions physisorbed at the water/graphene interface is enhanced in the direction perpendicular to the interface and the attraction is further amplified by enhancing the confinement. This interaction becomes repulsive when one ion is intercalated into the graphene layers and the other is physisorbed at the interface. In the in-plane direction the interaction is almost unaffected while in the perpendicular direction the interaction energy changes by about 5 k_BT just by interchanging the ion’s position. Due to the symmetry breaking, the interfacial electrostatic interactions are referred to as nonreciprocal. Our results imply that electrostatic interactions near interfaces cannot be described using an inhomogeneous dielectric permittivity obtained via the water structure.