Coagulation/phase separation process in the silica/inorganic salt systems (1)—observation of state transformation—

Abstract

The authors investigated the transformation of state of silica particles and inorganic salts during the calcination of solid mixtures of silica/potassium halide (KX), which separated the mixture components producing microporous structures, at temperatures below the melting point of either materials. The silica sol (SiO2—Na2O)/KX solid was examined at high temperatures using thermal analysis, X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The data suggest the crystalline inorganic salt component transformed into an amorphous state ca. 100–200°C lower than its melting point, while the silica particle component coagulated, spontaneously separating the two components (“coagulation/phase separation”). Upon cooling, the inorganic salt was dispersed in water, resulting in porous silica with a narrow pore size distribution. However, at calcination temperatures above 800°C, formation of silica crystal existed as a separate domain without creating porous structures.