Determination of binding constants of various alkali metal cations with p-sulfonatocalix[8]arene by using inclusion equilibrium with thionine dye

The binding constants of various alkali metal cations with water-soluble p-sulfonatocalix[8]arene (Calix-S8) were determined spectrophotometrically by using the inclusion equilibrium of thionine (Th) dye as a chemical indicator. Depending on the kind of alkali metal cations, the inclusion constants of Calix-S8 for Th decrease steeply with an increase in salt concentrations. Alkali metal cations compete with the organic guest Th in the Calix-S8 inclusion. Based on a competitive binding experiment, the binding constants of alkali metal cations with Calix-S8 were evaluated to be 17, 60, and 11 dm⁶ mol^-2 for Na⁺, K⁺, and Cs⁺, respectively. We have demonstrated an absorption-based method of evaluating the binding constants of spectroscopically inert metal cations with Calix-S8 and shown the effects of salts on the molecular recognition of Calix-S8.