Evidence of structural reorganization during aggregation of silica nanoparticles

Silica nanoparticles have been produced by neutralization of sodium silicate. The obtained suspension was unstable because of the high value of the ionic strength of the medium. Aggregation of the particles has been monitored thanks to a combination of in situ turbidity and dynamic light scattering (DLS) measurements. An optical model has been developed to extract both fractal dimension Df and primary particle radius rpp of the formed aggregates from these data. The obtained results clearly indicate a densification of the clusters as aggregation proceeds. A correlation between our experimental turbidity values and hydrodynamic radii was found. Comparison with calculated dimensionless numbers showed that a constant Df could not explain the observed trend. A fractal dimension dependent on the number of particles inside the aggregate Npp is thus suggested.