Direct Observations of Ion Densities at Functionalized Interfaces to Test Hypotheses Regarding the Origin of Specific...

Interactions of anions with protonatable groups were investigated using X-ray fluorescence near total reflection (XFNTR) on floating monolayers at the surface of water. The number of ions attracted to the interfacial region, which XFNTR measures directly, is ion-specific as well as monolayer-specific. Our observation of the distinctly different behaviors of ClO₄^- and ReO₄^-, two ions with the same tetrahedral structure and almost the same sizes and hydration enthalpies, challenges current theories of ion specificity. Our observations are inconsistent with not only the Gouy-Chapman model (as expected) but also size-modified Poisson-Boltzmann theory and the "law of matching water affinity". We suggest that factors other than ion size and ion-water interactions, including possibly ion-ion interactions and lateral ordering at the interface, must be considered to account for specific ion effects.