Non-Reciprocal Interactions Induced by Water in Confinement

TL;DR

This study reveals that water-mediated electrostatic interactions near graphene surfaces exhibit non-reciprocity and directional dependence, challenging traditional Coulomb's law assumptions and highlighting the complex role of water in nanoscale electrostatics.

Contribution

The paper uncovers non-reciprocal, directional ion interactions near graphene surfaces induced by water polarization, which are not explained by existing permittivity models.

Findings

Ion interactions near graphene are non-reciprocal and directional.

Confinement enhances attraction between oppositely charged ions.

Current permittivity models cannot explain observed effects.

Abstract

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.