Kinetics of the complexation of Ni2+ ions by 5-phenyl-azo-8-hydroxyquinoline grafted on colloidal silica particles

This paper aims at describing the reactivity of colloidal suspension of organo-modified silica nanoparticles (fumed silicas with given specific surface areas of 200 and 390 m(2) g(-1), FS200 and FS390 respectively in the following) grafted with 5-phenyl-azo-8-hydroxyquinoline (5Ph8HQ), toward nickel ions. After describing the multistep functionalization process and characterizing the modified silica solids, dynamic light scattering was used to evaluate the influence of time, pH, and chemical modification on the stability of the colloidal suspensions obtained by dispersing the particles in water. The kinetics of complexation of Ni2+ by the grafted 5Ph8HQ moieties in modified silica suspensions was studied by the stopped-flow technique. The results are discussed in comparison to those obtained in micellar solutions (TX-100, CTAB) of the same ligand. A non linear dependence of the optical density with complex concentration has to be taken into account to get appropriate fittings of the optical absorbance vs time data collected from the stopped-flow experiments. Ni2+ complexation rates were found to vary with the state of the ligand in the following order: solubilized in neutral micelles (TX-100) >= grafted solid particles > solubilized in cationic micelles (CfAB). Chloride and acetate counter-anions were also considered, showing always faster reaction in acetate medium than in the presence of chloride ions. The ionic strength was also found to influence the reaction rates but mainly for 5Ph8HQ-grafted silica nanoparticles and not significantly for the ligand solubilized in micelles. Considering 5Ph8HQ being grafted on solid nanoparticles instead of being solubilized in micelles does not induce major changes in the kinetic scheme of complexation of Ni2+. ions. Pseudo-first order conditions seem to be likely reached although the local concentration of covalently immobilized ligand was much higher than those of nickel ions. (C) 2011 Elsevier B.V. All rights reserved.