Ageing process of some pyrogenic silica samples exposed to controlled relative humidities: Part I: Kinetic of water sorption and evolution of the surface silanol density

• Pyrogenic silicas are, at room temperature, sensitive to RH humidity.
• The surface roughness induces clearly a decrease of the surface reactivity.
• Modification of surface properties of the silicas in a highly humid environment.
• The new silanol groups appears in the vicinity of the pre-existing silanol groups.

The kinetic of the ageing process of 3 pyrogenic silica samples, exhibiting increasing surface areas (Wacker N20, T30 and T40), was followed through gravimetric measurements, during about 1000 h at 22.5 °C, under different relative humidities (RH) from 62% up to100%. Silanol chemical titration, and IR spectroscopy were also called on to evidence the change of the surface chemistry induced by the ageing process.This study evidenced that pyrogenic silicas are, at room temperature, sensitive to RH humidity over a threshold of about 50% of RH. Exposed to high RH, a strong change of the surface functionality occurs bound to an important chemisorption of water. At RH of 100%, the silanol density could increase up to quite 40%. It is more important for low specific area samples than for the higher ones. The limit value of 3.5–3.8 SiOH/nm2 suggests that no polysilicic layer are formed. Moreover IR spectroscopy indicates that the new silanol groups appear in the vicinity of the pre-existing silanol groups at the border between hydrophilic and hydrophobic domains. It is hence evidenced that the patch like distribution of the silanol groups and the surface morphology, flat or rough play a main role on the ageing process.

The ageing process of 3 pyrogenic silica samples was studied under various relative humidities (RH). On the left figure, one sees that the ageing process induced an significant inceased of the silanol surface density, whereas Infrared spectroscopy evidences a relative quick increasing of the hydrogen bound silanol groups due to an increase of the local silanol density. These evolutions are explained taking into account a patch like distribution of the silanol groups on the pyrogenic silica surface and the surface morphology: flat or rough.Figure optionsDownload as PowerPoint slide