ARTICLE NO. CS964593 Scanning Transmission X-Ray Microscopy: A New Method for the Investigation of Aggregation in Silica

tion of reactive groups on the surface of the particles is During the preparation of silica by acidi®cation of water glass, stochastic, directions of particle–particle bonds are rather primary silica particles form extended and rami®ed aggregates. arbitrary and therefore particle–particle interactions result The growing aggregates form a gel, a tenuous network of intercon- in tenuous aggregates with an amorphous structure. Interac-nected aggregates. After aging and drying of the wet gel, porous tions are not restricted to particle–particle or particle–aggre-silica is obtained. To study the extremely vulnerable aggregates gate bonds, and interactions between aggregates or clustersonly noninvasive methods are allowed. Moreover, because of the of particles are also possible, leading to highly ramified andcolloidal scale many methods based on (atomic or molecular scale) extended aggregates.spectroscopy are not informative. Scanning transmission x-ray mi-Polymerization or particle formation and aggregation,croscopy (STXM), using high-brilliance synchrotron radiation at 3.25 nm (380 eV) and 2.60 nm (480 eV) as an X-ray source, pro- however, are not strictly sequential processes: both are based vides a new technique to obtain direct images of wet or solved on the same (condensation) reaction and therefore are to be aggregates at a 50±5000 nm scale. The 50 nm resolution is suf®- considered parallel processes. Moreover, because OH0 is cient to provide excellent images of fractal structures. In this paper both a reagent and a catalyst for the condensation/hydrolysis the principles of STXM are discussed in relation to investigations reaction, reaction rates at low pH are much lower than at hig