Scanning Transmission X-Ray Microscopy: A New Method for the Investigation of Aggregation in Silica

During the preparation of silica by acidification of water glass, primary silica particles form extended and ramified aggregates. The growing aggregates form a gel, a tenuous network of interconnected aggregates. After aging and drying of the wet gel, porous silica is obtained. To study the extremely vulnerable aggregates only noninvasive methods are allowed. Moreover, because of the colloidal scale many methods based on (atomic or molecular scale) spectroscopy are not informative. Scanning transmission x-ray microscopy (STXM), using high-brilliance synchrotron radiation at 3.25 nm (380 eV) and 2.60 nm (480 eV) as an X-ray source, provides a new technique to obtain direct images of wet or solved aggregates at a 50-5000 nm scale. The 50 nm resolution is sufficient to provide excellent images of fractal structures. In this paper the principles of STXM are discussed in relation to investigations of wet gel systems like silica gel.