Glass transition in dried aqueous sols

The structure of sodium silicate solutions can be described as colloidal dispersion or sol. A dispersion of pyrogenic SiO 2 or Al 2 O 3 particles (size: 10-15 nm) in NaOH solutions can also be addressed as sol. Both types of sols were dried at temperatures between 40 and 100 °C to amorphous solids. Dried sodium silicate solutions remained transparent during the whole drying process when access of CO 2 is avoided. Dried pyrogenic silica dispersions remained amorphous for shorter drying periods (up to 14 d). After longer drying periods (up to 90 d) crystallization and cracking was observed. The thermal analysis of both types of silicatic materials revealed transition effects typical of a glass transition.