Charge inversion at minute electrolyte concentrations

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Anionic DMPA monolayers spread on LaCl₃ solutions reveal strong cation adsorption and a sharp transition to surface overcharging at unexpectedly low bulk salt concentrations. We determine the surface accumulation of La³⁺ with anomalous x-ray reflectivity and find that La³⁺ compensates the lipid surface charge by forming a Stern layer with ≈ 1 La³⁺ ion per 3 lipids below a critical bulk concentration, cₜ ≈ 500 nM. Above cₜ, the surface concentration of La³⁺ increases to a saturation level with ≈ 1 La³⁺ per lipid, thus implying that the total electric charge of the La³⁺ exceeds the surface charge. This overcharge is observed at ≈ 4 orders of magnitude lower concentration than predicted in ion-ion correlation theories. We suggest that transverse electrostatic correlations between mobile ions and surface charges (interfacial Bjerrum pairing) may account for the charge inversion observed in this dilute regime.