Ion-Specific Induced Charges at Aqueous Soft Interfaces

Ionic specificity effects, i.e., ions of the same valence leading to different macroscopic effects, are studied by considering a Langmuir monolayer of arachidic acid over a solution containing either Fe3+ or La3+. We systematically vary pH levels as a way to control the interfacial surface charge and characterize the system by surface-sensitive x-ray scattering and spectroscopic techniques. We show that the critical surface pressure at the tilted (L2) to untilted (LS) transition is ionic specific and varies with pH. While the maximum density of surface bound La3+ per head group of arachidic acid is similar to 0.3, the amount necessary to neutralize the surface charge, for Fe3+ it is nearly 0.6 and it is accompanied with a significant accumulation of the coions Cl- as revealed by surface x-ray spectroscopy. We account for the experimental observations by a statistical mechanical model including ion specificity.