Study of aging of pyrogenic silicas by gravimetry and microcalorimetry

The study of the aging of synthetic pyrogenic silicas, in the presence of a water containing atmosphere was led on two pyrogenic silicas, having respectively a specific surface area of around 200 m2/g (N20) and 300 m2/g (T30) were aged under a relative humidity (RH) of 94% at 25 °C. The evolution of their surface properties was followed by gravimetry and immersion microcalorimetry. The aging phenomenon, under a RH of 94%, induces an important water intake of 25% for the N20 sample and 38% for the T30 one, but the water sorption kinetic is quite identical. The chemisorbed water amount due to the aging process was assessed, it corresponds to an increase of the silanol surface density of about 80%. On the contrary, the two silicas behave very differently when measuring the heats of immersion of the dried or water precovered aged samples in water. This difference was related to their surface morphology, N20 being flat at a molecular level whereas T30 exhibits a rough surface.