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 ∼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.