Stability of colloidal silica (aerosil) hydrosols. I. Preparation and characterization of silica (aerosil) hydrosols

Abstract

The preparation and extensive characterization of silica (Aerosil) hydrosols using a combination of five different methods are described in detail. The reproducibility of dispersion of Aerosil powders was tested using rheology measurements in a concentric cylinder viscosimeter. Different dispersion methods were tried and ultrasonication proved to be the most suitable for providing standardized, stable Aerosil-hydrosols. Standardized ultrasonicated Aerosil-hydrosols were characterized by determining the viscosity/concentration dependence in a concentric cylinder viscosimeter, the sedimentation coefficient in an analytical ultracentrifuge, the partial specific volume in an oscillating u-tube of a high precision density meter, the translational diffusion coefficient with a photon correlation spectrometer, and the dimension and shape of the Aerosil particles by transmission electron microscopy. The combination of these data shows that the particles obtained by ultrasonication consist of many smaller, nearly spherical primary particles irreversibly fused together in aggregates of a limited size. The characteristic parameters derived from this combination, such as diameter of the diffusion-equivalent sphere, number of primary particles in an aggregate, porosity of the aggregate, etc., make possible a comparison with morphological parameters from extrapolated literature data for the aggregation process. Such a comparison results in a very good agreement between experimental and theoretical data. This work, therefore, demonstrates that colloidal clusters which have been previously mainly described by electron micrographs and computer simulations can be more realistically characterized by a combination of hydrodynamic techniques.