Spectroscopic studies of proton-transfer and metal-ion binding of a solution-phase model for silica-immobilized 8-hydroxyquinoline Original Research Article

The proton-transfer and stepwise metal-ion complex formation reactions of the water-soluble 8HQ ligand, 5-(p-sulfophenylazo)-8-hydroxyquinoline, are investigated in order to understand the interfacial reactivity of the analogous ligand immobilized to silica by a phenylazo linker on the 5-position on the quinoline ring. Raman spectra of the ligand are acquired to determine whether the same tautomerization equilibrium exists for the solution-phase model compound as for the silica-immobilized ligand. UV-Vis absorption spectra of aqueous solutions of the ligand are acquired as a function of pH and metal-ion concentration. The stepwise reactions are monitored, equilibrium constants are determined, and component spectra are resolved using multivariable least-squares (MVLS) methods. These methods were shown to be capable of resolving component spectra made up of broad bands that differ by only a 3 nm band shift. The acid dissociation constants of the ligand were determined to be: pKA1 of 3.78±0.3 and pKA2 of 7.94±0.4. The equilibrium constants for ligand binding to Cu2+ were also measured and found to be K10=1.4 (±0.3)×1010 and K20=2.0 (±0.5)×107. The first ligand binding constant was compared to the complexation of the silica-immobilized 8HQ ligand. The formation constant for the 1:1 ligand:Cu2+ solution complex was ∼30-times greater than that of the silica-immobilized 8HQ ligand. This is likely due to interactions of the immobilized ligand with the nearby silica surface that change its affinity for metal-ion binding.