Transformation of CdS colloids: Sols, gels, and precipitates

Highly concentrated CdS colloids (about 30 wt %) are prepared in acetone through the grafting of 4-fluorophenylthiol at their Surface. ¹⁹F NMR measurements show that the controlled oxidation by an aqueous solution of hydrogen peroxide leads to a mixture of dithiol and fluorosulfonate species. Aggregation of particles produces either transparent gels or opaque precipitates, depending on the nature and the concentration of the oxidant. The sol-gel transformation results from an aggregation mechanism, which requires the release of surface groups because of their interaction with water molecules. Considering the formation of mass-fractal clusters, small-angle X-ray scattering measurements show, at least in the first steps, that the growth kinetics is in agreement with the usual reaction limited cluster aggregation mechanism previously observed in the silica system. This can be followed by a saturation effect, enhanced at low water concentration, which is due to the poisoning of the surface by chemical groups produced by oxidation.