Structures and properties of silica sols and gels prepared by sol-gel method

Abstract

Silica gels were prepared by the hydrolysis and condensation of the mixture of tetraethoxysilane (TEOS) and water using ethanol and HCl or $NH_4$ OH as a solvent and a catalyst, respectively, and the structures and properties of the gels were analyzed. Hydrolysis and condensation of TEOS were studied under various pH ranges. A theory for the estimation of hydrolysis rate is proposed based on the temperature profile during the hydrolysis reaction. Differential temperature analysis was used to determine the hydrolysis rate constant and the activation energy of hydrolysis. A kinetic model was established on the basis of hydrolysis, esterification and water-alcohol forming condensation. Each reaction rate constant could be obtained from gas chromatography analysis and the kinetic model. Hydrolysis rate constants were turned out to be in proportion to acid concentration. Also esterification reaction should not be neglected during the sol-gel transition of TEOS in alcoholic solvent. From the gelatin time, the apparent value of activation energy of condensation reaction was also acquired. The viscosity was measured for a TEOS solution with different water content and catalyst. A base catalyzed TEOS solution was shown to have spherical particles. An acid catalyzed TEOS solution with a low water content has been judged to form linear polymers from the analysis of reduced viscosity. However, the TEOS solution with a high water content is supposed to form highly branched polymers because the reduced viscosity is almost independent of the $SiO_2$ concentration. It is suggested that the formation of silicon oxides is based on a ribbon of three joined siloxane chains. The rehologicl properties were studied for silica soils by measuring the viscosity change with shear rate. All soils in the closed system showed flow behaviors from Newtonian to thixotropic fluid via pseudoplastic fluid during the sol-gel transition. In the open system, the acidic sol prepared from a solution w...