SKELETAL DENSITY OF SILICA AEROGELS DETERMINED BY HELIUM PYCNOMETRY

Density measurements of acid-catalysed aerogels have been carried out using helium pycnometry technique. The density increases from 1.85 to 2 when the volume percentage of tetramethoxysilane of the starting solution increases from 6 to 33. The aerogels submitted to an oxidation heat treatment show higher density values varying between 2 and 2.2. The variation of the skeletal density with the volume percentage of tetramethoxysilane is explained by a simple model taking into account the size of the primary particles which build the gel network. The observed difference between the measured density and the bulk density of amorphous silica (2.2) is attributed to the presence of organic residues located on the particles surface. The increase of skeletal density with the oxidation treatment is proposed to be due to the smoothing of the particle surface or to an increase of the particle size occurring during the oxidation treatment.