Nonreciprocal interactions induced by water in confinement

Water mediates electrostatic interactions via the orientation of its dipoles around ions, molecules, and interfaces. This induced water polarization consequently influences multiple phenomena. In particular, water polarization affects ion adsorption and transport, biomolecular self-assembly, and surface chemical reactions. Therefore, it is of paramount importance to understand water-mediated interactions modulated by nanoconfinement at the nanoscale. Here we investigate the effective interaction between two oppositely charged ions in different positions in water confined between two graphene surfaces. We find that the attraction between physisorbed ions is enhanced in the surface normal direction while the in-plane interaction is almost unaffected. The attraction in the surface normal direction is further enhanced by decreasing the confinement distance. Conversely, when one ion is intercalated into the graphene layers, the interaction becomes repulsive. Moreover, upon exchange of the ions' positions along the surface normal direction, the interaction energy changes by about 5k(B)T. The nonequivalent and directional properties found here, referred to as nonreciprocal interactions, cannot be explain by current water permittivity models in confinement. Our x-ray reflectivity experiments of the water structure near a graphene surface support our molecular dynamics simulation results. Our work shows that the water structure is not enough to infer electrostatic interactions near interfaces.