Characterization of Salicylate-Alumina Surface Complexes by Polarized Fluorescence Spectroscopy

Abstract

Speciation of salicylate anions (o-hydroxybenzoate) adsorbed on aqueous colloidal alumina (δ-Al2O3) was determined by polarized fluorescence excitation spectroscopy. Adsorption isotherms and pH and ionic strength edges suggest the existence of both inner-sphere and outer-sphere salicylate surface complexes. Spectroscopic characteristics of inner-sphere surface-salicylate complexes (one bidentate and two monodentate) were identified through comparison of suspension spectra with remarkably similar fluorescence and excitation spectra of solution phase Al-salicylate complexes. The large fluorescence Stokes gap of the aqueous salicylate anion is highly sensitive to complexation, resulting in spectral shifts characteristic of aluminum binding in the three inner-sphere salicylate complexes. These species appear to be present even at extremely low surface coverage, and the relative distributions are dependent on pH, ionic strength, and the relative concentrations of alumina and salicylate. The bidentate complex, however, is the predominant species at low surface coverages. Fluorescence anisotropy measurements, both steady-state and time-resolved, demonstrate that the bidentate and monodentate surface complexes do not undergo rotational reorientation on the time-scale of the fluorescence (τf = 4.0 ns), consistent with inner-sphere, polar covalent binding of these salicylate complexes to alumina surface sites. At high surface coverage, time-resolved anisotropy measurements suggest the existence of a surface salicylate species that is rotationally hindered (τr = 31 ps ) relative to free solution phase salicylate ions (τ r = 20 ps ). This behavior is consistent with an electrostatically bound outer-sphere complex suggested by the pH and ionic-strength sorption edges.