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 are still debated. Complex ions, such as SCN–and SeCN–, are particularly interesting due to their unexpected adsorption trends. 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 floating charged monolayer. Polarimetry studies show that the average orientation of SeCN–anions with respect to the surface normal decreases from 45° to 22° with the increasing KSeCN concentration. Interfacial SeCN–coverage saturates at very low bulk concentrations, but their orientational organization, distribution between Stern and diffuse layers, and effects on the hydrogen-bonding network of the interfacial water continue to change with increasing bulk KSeCN concentration. These 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. The reorganization of the interfacial ions and the water may be very important in chemical separations of heavy metals, where metal–anion complexes drive the selective ion transfer at aqueous interfaces.