Growth Mechanism of Cadmium Sulfide Ultrafine Particles in Water-in-Oil Microemulsion

Abstract

The growth mechanism of CdS ultrafine particles in water-in-oil microemulsions was investigated. The growth occurs via a irreversible particle coagulation process. Its rate constant is dependent on the agglomeration number of a particle n. At small agglomeration numbers, the rate constant k is about 10⁷ dm³mol^–1s^–1. It decreases dramatically to about 10³ dm³mol^–1s^–1 at a critical agglomeration number n_c followed by a further decrease in k above 2n_c. This causes sharpening of the size distribution near n_c and 2n_c. A simple model based on the coagulation process with n-dependent rate constants is proposed, which explains well the experimental results. The effects of initial mixing and the feed ratio of Cd²⁺ to S^2– are understood well by the proposed model. The relationship between n_c and the stable supramolecules composed of CdS particles was discussed.