Chemical Potential Driven Reorganization of Anions between Stern and Diffuse Layers at the Air/Water Interface

Ion adsorption and transfer at charged interfaces play key roles in various industrial and environmental processes. Molecular scale details of ion-ion, ion-water, and ion-surface interactions and their dependence on the character of the ion (ion-specific effects) are still debated. Complex ions, such as SCN- and SeCN-, are particularly interesting due to their unexpected adsorption trends which are ascribed to their permanent dipole moment and non-spherical shape. Here, we combine vibrational sum frequency generation (VSFG) spectroscopy and surface sensitive synchrotron X-ray studies to provide a detailed description of SeCN- adsorption at a charged surfactant monolayer at the air/aqueous interface. The results show that the increasing chemical potential may lead to further reorganization of the adsorbed ions, even though the total interfacial ion population does not change.