Aqueous sol-gel process in the silica-metasilicate system. A microrheological study

The electrochemical quartz crystal microbalance (EQCM) was used to investig ate the sol-gel process from an aqueous sodium metasilicate solution at 25 degreesC and pH 3. From the EQCM data it was possible to obtain information on the changing rheological properties of the system during the whole proc ess. Besides sensing changes in the rheological properties, the EQCM detect ed film formation on the sensor surface. This additional information was us ed to estimate, using a model for film formation kinetics, the sol particle size (the model prediction was confirmed by light scattering measurements) . The particle size estimation was of importance in the analysis of the sol viscosity behavior during the earlier stages of the particle aggregation p rocess. The analysis, based on a classical model for the viscosity of a dis persion of charged particles in an electrolyte, provided some insight into the initial aggregation phenomena and microgel formation. Determination of the gelling point was made by examining the evolution of the shear storage modulus of the system. The gelation point was identified as being the time at which the storage modulus rose abruptly from zero. The rheological behav ior of the system beyond the gelation point was analyzed in terms of the lo ss tangent, which value decreased noticeably before becoming constant. The observed decrease in the loss tangent, as a measure of the increasing impor tance of the elastic energy storage relative to the energy dissipation in t he gel, was evidence that Si-O-Si bond formation continued to take place fo r some time after the sol-gel transition.