Sol–gel synthesis of porous and dense silica microspheres

An acidic silica sol (35 ± 2 wt% equivalent SiO2) having a gelling time of 9–10 min has been used as an aqueous phase for obtaining a w/o emulsion in CCl4 as oil phase in presence of a surfactant, Tween 80. The silica sol was allowed to form gel at room temperature via polycondensation among the –Si–OH groups forming the porous silica gel microspheres. The surface area of the microspheres heated at 500°, 700° and 900 °C was found to be 227 m2/g, 167 m2/g and 81 m2/g indicating the gradual densification. The decreased surface area and unchanged –Si–O–Si– asymmetric stretching vibration at 1084 cm−1 up to 700 °C probably indicate the formation of extensive cross-linked gel structure in the microsphere. The appearance of the –Si–O–Si– asymmetric stretching vibration at 1104 cm−1 and the absence of porosity while heating at 1000 °C indicate the formation of dense silica glass microspheres.