Analyzing the sol-gel transition of colloidal silica suspensions using time-resolved rheometry

The sol-gel process, which involves the hydrolysis and condensation of silica precursors, is a widely used method for preparing silica gels in a practical, well-controlled, and cost-effective way. This study aimed to explore the feasibility of using time-resolved rheometry (TRR) to examine the sol-gel transition of colloidal silica suspensions (silica sols) and analyze the effects of varying silica solid content and pH levels. The silica content ranged from 0.8 to 15 wt%, with pH values between 4.0 and 6.2. Cyclic frequency sweep (CFS) measurements were conducted to monitor changes in linear viscoelastic properties during the sol-gel transition of the silica suspensions. The results indicated that increasing the solid content of silica sols led to a linear increase in transition time, while a rise in pH resulted in an exponential decrease in transition time. The significance of this work lies in the novel TRR measurement protocol, which can accurately measure transition times from minutes to several hours and storage moduli from 100 Pa to over 100 kPa. Additionally, the study demonstrates that the resulting silica gels exhibit viscoelastic behavior characteristic of colloidal glasses rather than colloidal gels, as evidenced by the decrease in loss tangent with increasing frequency.