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.
Highlights
Water and ions, two basic components of living organisms and ubiquitous in numerous natural and technological processes, are strongly interconnected [1,2,3]
Our results show that in confinement the ion-ion effective interaction is directional and position dependent with respect to the graphene surface due to a nonzero persistent interfacial water polarization
We identify this property as nonreciprocity, which describes the directionally dependent interactions and nonequivalent change of interactions upon exchange of the ions’ positions near a confining surface
Summary
Two basic components of living organisms and ubiquitous in numerous natural and technological processes, are strongly interconnected [1,2,3]. The capability of a medium to be polarized and the resulting attenuation of electrostatic forces are typically quantified by the dielectric permittivity This dielectric permittivity normalized by that of the vacuum (ε0), called the relative permittivity (εr), is frequently assumed to be a distinctive property of a material and is often termed the dielectric constant.
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