Ion-Specific Induced Charges at Aqueous Soft Interfaces*

Surface-sensitive X-ray scattering and spectroscopic techniques are employed to monitor ion binding specifically to Langmuir monolayers of densely packed carboxyl or phosphate groups. By systematically varying pH of Fe$^{3+}$, Fe$^{2+}$ and La$^{3+}$ solutions, we show that the critical surface pressure at the tilted (L2) to untilted (LS) transition is ionic specific and pH dependent. While the maximum density of surface bound La$^{3+}$ per carboxylic group is $\sim$ 0.3, the amount necessary to neutralize the fully charged surface, for Fe$^{3+}$ it is nearly 0.6. Furthermore, the binding of Fe$^{3+}$ is accompanied with a significant accumulation of Cl$^-$ co-ions implying interfacial charge inversion. Similar experiments with charged phosphate groups at the interface show that the bindings of Fe$^{2+}$ and La$^{3+}$ are electrostatically driven. Our results have implications on biomineralization processes and ionic functions at cell membranes.