Electrostatic interactions at aqueous interfaces play a central role in controlling the organization and stability of charged particle-laden films. In particular, the explicit dependence of interfacial electrostatics on ionic strength and ion valence might enable to both finely modulate the structure and properties of charged aqueous surfaces and to better understand the interfacial partitioning of electrolytes. This requires a comprehensive characterization of the electrostatic properties at both the nano and macroscale. Here, we combine in situ grazing-incidence small-angle X-ray scattering (GISAXS), Langmuir compression isotherms, and vibrating-plate surface potential measurements to investigate the interfacial electrostatics of model negatively charged silica nanoparticles adsorbed at the air-water interface in the presence of mono-, di-, and trivalent electrolytes at various ionic strengths. 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.
Tomasella et al. (Sun,) studied this question.