Heavy Anionic Complex Shows Unexpected Adsorption Behavior Coupled with Unique Interfacial Water Structure at a Soft Charged Interface*

The molecular-scale behavior of ions at aqueous charged interfaces is coupled with their hydration, and is fundamentally important to many macroscopic phenomena, including biophysics, atmospheric chemistry and chemical separations. Although lighter ions have been widely studied in this context, heavy anionic complexes, that are crucial for modern energy and lighting technologies, are underexplored. Here, we study the adsorption of PtCl₆^2- at a positively charged Langmuir monolayer. Surface-sensitive x-ray techniques show that PtCl₆^2- adsorbs in the diffuse layer and the Stern layer at low and high bulk concentrations, respectively [1]. This is contrary to the expectations from classical mean-field theories. Sum frequency generation spectroscopy shows that PtCl₆^2- partially retains its hydration sphere after adsorption in the Stern layer. This unique interfacial hydration is likely responsible for its unexpected adsorption behavior. Our experimental and computational studies provide important clues to solving the mystery of specific ion effects.

[1] Uysal, A.; Rock, W.; Qiao, B.; Bu, W.; Lin, B. J. Phys. Chem. C 2017, DOI: 10.1021/acs.jpcc.7b09350