Anionic DMPA monolayers spread on LaCl$_3$ solutions reveal strong cation adsorption and a sharp transition to surface overcharging at unexpectedly low bulk salt concentrations. We determine the surface accumulation of La$^{ 3+}$ with anomalous x-ray reflectivity and find that La$^{ 3+}$ compensates the lipid surface charge by forming a Stern layer with $\approx 1$ La$^{ 3+}$ ion per 3 lipids below a critical bulk concentration, $c_t \approx 500 \mathrm{ nM}$. Above $c_t$, the surface concentration of La$^{ 3+}$ increases to a saturation level with $\approx 1$ La$^{ 3+}$ per lipid, thus implying that the total electric charge of the La$^{ 3+}$ exceeds the surface charge. This overcharge is observed at $\approx$ 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.
Categories: cond-mat.soft