Light-scattering cross section of hydrophilic and hydrophobic silica aggregates

The aggregation dynamics of solid particles in liquid media is currently determined by optical-based particle sizing methods. Because it can be used in situ and applied to a wide particle size range, turbidimetry is acknowledged as one of the best methods for this characterization. Although much work has been done on aggregation, some aspects are less known and require additional experimental and theoretical research. This is particularly the case of aggregation of hydrophobic particles. Corresponding aggregates are three-phase objects (solid-liquid-gas) the morphology and optical properties of which are not known. The present work rests on the turbidimetric study of hydrophilic and hydrophobic silica samples in stirred aqueous solutions. Modeling involves different aspects: aggregate morphology, aggregate optical properties, and aggregation dynamics. This article particularly emphasizes the second aspect. Fractal-like models are proved to be representative of the aggregate morphology even at small size. Light-scattering cross section of the aggregates is calculated from their averaged projected area; effective refractive index is proved to be a good parameter for modeling the optical properties of both hydrophilic and hydrophobic aggregates. Classical models of porous aggregate formation (Kusters theory) are used for describing the aggregation dynamics.