Nonclassical Behavior in Competitive Ion Adsorption at a Charged Solid–Water Interface
Sang Soo Lee, Changyong Park, Neil C. Sturchio, Paul Fenter
Abstract
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.