The interplay between nanoscale dipoles and ion screening drives the self-regulation of charged aqueous surfaces

We show that nanoparticle monolayers self-regulate their 2D organization to reach a common electrostatic steady state, characterized by a constant interfacial surface potential (ΔV ≈ 150 mV) and a fixed single-particle repulsive energy (U ≈ 2 × 10^-23 J), independently of ionic strength and ion identity. Variations in electrolyte composition instead control the nanoparticle surface excess, which adjusts to compensate changes in the interfacial dipolar strength. Further, both the interfacial dipole moment and the effective screening constant follow ion-independent power-law scaling with ionic strength, while ion-specific effects appear only as concentration-independent pre-factors. These results i) demonstrate that electrostatic interactions at the interface are governed by a confined electrolyte environment which deviates from classical bulk mean-field predictions and ii) provide a quantitative framework for predicting and tuning the structure and stability of charged aqueous interfaces.