Basic characteristics and applications of aerosil: 30. The chemistry and physics of the aerosil Surface

Abstract

With the aid of special measuring techniques, it was possible to determine the molar absorptivity ϵ of the OH-stretching vibration band of the free silanol groups at 3750 cm −1 and the total silanol group density of hydrophilic Aerosil. In most cases the silanol group density is independent of the BET surface area (exception: Aerosil OX 50); the density of the free silanol groups decreases as the BET surface area increases (a deviation from the linear course occurs with Aerosil OX 50 and Aerosil 380). A temperature increase up to 600°C results only in the elimination of bridged silanol groups; higher temperatures are required before the isolated silanol groups are eliminated. Freshly manufactured Aerosil has a high content of isolated silanol groups; during storage, this content of isolated silanol groups decreases, and the silanol group density increases. The bonding of dimethylsilyl groups on the surface of Aerosil R 972 is discussed and the temperature stability of the surface groups of Aerosil R 972 and Aerosil R 812 is demonstrated.