CHARACTERIZATION OF SALICYLATE-ALUMINA SURFACE COMPLEXES BY POLARIZEDFLUORESCENCE SPECTROSCOPY

Speciation of salicylate anions (o-hydroxybenzoate) adsorbed on aqueou s colloidal alumina (delta-Al2O3) was determined by polarized fluoresc ence excitation spectroscopy. Adsorption isotherms and pH and ionic st rength edges suggest the existence of both inner-sphere and outer-sphe re salicylate surface complexes. Spectroscopic characteristics of inne r-sphere surface-salicylate complexes tone bidentate and two monodenta te) were identified through comparison of suspension spectra with rema rkably similar fluorescence and excitation spectra of solution phase A l-salicylate complexes. The large fluorescence Stokes gap of the aqueo us 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 a t extremely low surface coverage and the relative distributions are de pendent on pH, ionic strength, and the relative concentrations of alum ina and salicylate. The bidentate complex, however, is the predominant species at low surface coverages. Fluorescence anisotropy measurement s, both steady-state and time-resolved, demonstrate that the bidentate and monodentate surface complexes do not undergo rotational reorienta tion on the time-scale of the fluorescence (tau(f) = 4.0 ns), consiste nt with inner-sphere, polar covalent binding of these salicylate compl exes to alumina surface sites. At high surface coverage, rime-resolved anisotropy measurements suggest the existence of a surface salicylate species that is rotationally hindered (tau(r) = 31 ps) relative to fr ee solution phase salicylate ions (tau(r) = 20 ps). This behavior is c onsistent with an electrostatically bound outer-sphere complex suggest ed by the pH and ionic-strength sorption edges. Copyright (C) 1998 Els evier Science Ltd.