Pyrocatechol violet complexation at the boehmite-water interface

Acid/base reactions and pyrocatechol violet complexation at the boehmite-so lution interface have been investigated in 0.10 M K(Cl) solution at 298.2 K in the range 5.0 less than or equal to -log h less than or equal to 9.0. E quilibrium measurements were performed as potentiometric titrations complem ented by spectrophotometric analysis of a ligand (batch adsorption experime nts). The experimental data were evaluated on the basis of the extended con stant capacitance model, allowing for inner-sphere and/or outer-sphere comp lexation. The acid/base properties are described by the equilibria: H+ + =A lOH reversible arrow =AlOH2+; log beta(1,1,0(int)) = 7.46 +/- 0.04 and =AlO H reversible arrow =AlO- + H+; log beta(-1,1,0(int)) = -9.87 +/- 0.12. The specific capacitance was determined as 1.00 F m(-2). The binding of pyrocat echol violet {PCV, 2-[(3,4-dihydroxyphenyl)(3-hydroxy-4-oxocyclohexa-2,5-di ene-1-ylidene)methyl]benzenesulfonic acid}, H4L, was described by the forma tion of a single inner-sphere complex according to the reaction =AlOH + H4L reversible arrow =AlL3- + 3H(+) + H2O, Allowing for a charge distribution of this complex between the inner plane and beta-plane (-2, -1), a signific ant improvement of the fit was obtained. The adsorbed complex exhibited a d ark blue color over the pH range 5-8.5, indicative of proton loss from the 1,2-dihydroxyl moiety. Furthermore, DRIFT spectra showed the bonding enviro nment of PCV bound to the surface to exhibit similarities with correspondin g catechol adsorption to different aluminium (hydr)oxides. These spectral f eatures turned out to be very similar to soluble catechol complexes of Al(I II). Thus, it is suggested that PCV forms a mononuclear chelating complex a t the boehmite-water interface. The maximum surface coverage reached was ca , 33%, indicating that the high negative charge of the species may result i n surface charge "saturation" rather than saturation of surface binding sit es. Steric hindrances caused by the large size of PCV may also contribute t o the low surface coverage. (C) 2000 Academic Press.