Polydispersity during the formation and growth of the Stober silica particles from small-angle X-ray scattering measurements

The early stages of formation of Stober silica particles have been investig ated in situ during the hydrolysis and condensation of tetraethylorthosilic ate under base-ammonia conditions in different alcoholic solvents. Time-res olved ultra-small-angle X-ray scattering by the entities produced in the so lutions is used for structural characterization and monitoring of the growt h kinetics of the particles. Our primary focus is to assess the polydispers ity of the formed colloidal particles and its evolution as a function of ti me. We first applied a maximum entropy analysis of the scattering data to d etermine the size distribution and the time evolution of the size distribut ion of the colloidal particles. Second, we extended the cumulant method to analyze our earlier small-angle X-ray scattering data (H. Boukari, J. S. Li n, and M. T. Harris, J. Colloid Interface Sci. 194, 311, 1997; Chem. Mater. 9, 2376, 1997) and search for the presence of a distribution of fractal pa rticles. The maximum entropy analysis indicates that there is a continuous nucleation of particles during the synthesis, and that this takes place wit hin a relatively narrow size distribution. The cumulant analysis shows that , except at later times, the data are not adequate to confirm conclusively the presence of a distribution of fractal dimension at any time during the experiment. We discuss the impact of these results on growth kinetic models proposed for this system. (C) 2000 Academic Press.