  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
modulated by nanoconfinement affects ion adsorption and transport, biomolecular
self-assembly, and surface chemical reactions. Therefore, it is of paramount
importance to understand how water-mediated interactions change at the
nanoscale. Here we show that near the graphene surface anion-cation
interactions do not obey the translational and isotropic symmetries of
Coulomb's law. We identify a new property, referred to as non-reciprocity,
which describes the non-equivalent and directional interaction between two
oppositely charged ions near the confining surface when their positions with
respect to the interface are exchanged. Specifically, upon exchange of the two
ions' positions along the surface normal direction the interaction energy
changes by about 5$k_BT$. In both cases, confinement enhances the attraction
between two oppositely charged ions near the graphene surface, while
intercalation of one ion into the graphene layers shifts the interaction to
repulsive. While the water permittivity in confinement is different from that
in bulk, the effects observed here via molecular dynamics simulations and X-ray
reflectivity experiments cannot be accounted for by current permittivity
models. Our work shows that the water structure is not enough to infer
electrostatic interactions near interfaces.
