Differences in the morphology and vibrational dynamics of crystalline, glassy and amorphous silica – commercial implications†
Quartz, bentonite, sodium silicate, precipitated and pyrogenic synthetic amorphous silica (SAS) were compared using high-resolution transmission electron microscopy and neutron vibrational spectroscopy. These materials span the full gamut of structures of silica: crystalline, disordered crystalline, glassy and completely amorphous respectively. Traces of water, together with the silanol groups, are of paramount importance for commercial applications, particularly the catalytic influence on the reaction kinetics of surface silanisation of SAS in tyre technology. The reaction of a bifunctional silane, (bis(3-triethoxysilylpropyl)-tetrasulfide), with precipitated silica eliminates accessible surface-silanol groups to improve SAS/polymer interaction and removes traces of water. The degree of reaction of ethoxy-functions was quantified. The removal of reactive protons by the silanisation reaction is even more effective than drying of SAS at 120 °C, however, residual traces of isolated silanols were detected in the interior structure of SAS, even after silanisation or high temperature (750 °C) drying. Some minor residual translational periodicity present in the bulk piece of amorphous solid water glass is completely missing in amorphous SAS powders.