R. Wengeler, A. Teleki, M. Vetter, S. E. Pratsinis, H. Nirschl

The influence of primary particle diameter and degree of agglomeration of flame-made silica agglomerate suspensions in aqueous solutions is studied by high pressure dispersion (up to 1500 bars) through a nozzle of 125 µm inner diameter. These particles were produced (4-15 g/h) by oxidation of hexamethyldisiloxane (HMDSO) in a co-flow diffusion flame reactor. Their average primary particle size (10-50 nm) and degree of agglomeration were controlled by varying the oxygen and precursor flow rates. The particles were characterized by nitrogen adsorption, electron microscopy and small angle X-ray scattering. Hydrodynamic stresses break-up soft agglomerates and yield hard agglomerate sizes in the range of 100 to 180 nm as characterized by dynamic light scattering.  Soft-agglomerates exhibited decreasing light scattering diameters with increasing dispersion pressure while hard agglomerates were insensitive.