Nonclassical Behavior in Competitive Ion Adsorption at a Charged Solid-Water Interface

Ion adsorption at solid-water interfaces is commonly described by interactions between specific surface sites and adsorbed ions in classical models. However, energetic contributions from non-site-specific ion-ion interactions have been less well understood. Here, we report nonclassical behaviors observed during competitive adsorption between Sr2+ and Na+/Rb+ at the negatively charged muscovite mica (001)-water interface, revealing apparent controls of adsorbed ion speciation over the interfacial reactivity. In the absence of competing cations, Sr2+ adsorbs in approximately equivalent proportions of inner-sphere and outer-sphere complexes, whereas it adsorbs predominantly as an outer-sphere complex in the presence of Na+/Rb+. This transformation of adsorbed Sr2+ speciation significantly decreases its adsorption strength, as indicated by the similar to 15-fold shift in the Sr2+ adsorption edge concentration, compared to that calculated from a classical Langmuir isotherm model developed on the basis of site-specific interactions. These observations highlight the importance of non-site-specific interactions in controlling the energetics of chemical reactions at the charged interface.