The effect of mixing alkoxides on the Stoeber particles size

Submicrometric particles were obtained, through the Stoeber method, from alc. (ethanol) solns. of tetraethoxysilane (TEOS) and 3-aminopropyl-triethoxysilane (APTS), at total alkoxide concn. 0.15 M, in the presence of ammonia (0.1 M) and water (8.8 M) and fluorescein sodium (0.40 m). Nano-sized particles were obtained from solns. 0.15 M of TEOS or APTS. A surprising particle size increase was obsd. when mixing TEOS and APTS at const. overall alkoxide content (0.15 M). The result was explained, on the basis of the known aggregation model, by taking into account that the substitution of TEOS to APTS and vice-versa changes the ratio of nucleation and aggregation rates and that nuclei of different size are expected to form when the alkoxides are mixed. The mixing appears to change the evolution of particle population favoring the establishment of a size distribution wherein the dominant aggregation event can be between the freshly generated nuclei and large aggregates, which is the faster aggregation event. The closer is the TEOS/APTS ratio to 1 the sooner particles size differentiation would occur and the fewer but greater particles would form, in short times.