Determination of Peptization Time of Particulate Sols Using Optical Techniques: Titania As a Case Study

Colloidal sol−gel technology is widely used as a powerful synthesis route. It consists of two main stages, hydrolysis and peptization. The quantification of peptization time is necessary to improve the processing control and enhance product reliability. However, the peptization time is normally estimated by simple visual inspection. The aim of this work is to establish for the first time a procedure to quantify the peptization time. For such purposes, different optical techniques have been used, in particular the measurements of particle size distribution by laser diffraction and by dynamic light scattering. The experimental mismatch between both series of data allows determining the peptization time, and the results are in good agreement with turbidimetry measurements through the transmission of near-infrared light source. The synthesis of titania has been selected as a case study because anatase presents a large interest in many different applications. It has been demonstrated that peptization time significantly increases as the synthesis temperature decreases: Sols prepared at 25 °C present a peptization time of ∼95 h, sols prepared at 30 °C need ∼72 h for a complete peptization, sols synthesized at 35 °C present a peptization time of ∼45 h, and those prepared at 50 °C show a peptization time of ∼9 h. The peptization time for sols prepared above 80 °C is quite short; the synthesis is less reproducible and the risk of uncontrolled gelation increases. The average particle size changes from ∼13 nm for sols prepared at 25 °C, 20 nm for sols prepared at 35 °C and, to ∼30 nm for sols prepared at 50 °C. The resulting xerogel has been characterized in order to demonstrate the suitability of the synthesis procedure.