MTMS-based aerogel gelation kinetics - experimental study and theoretical model parametrisation of skeleton mass growth

Abstract

Despite years of studying the synthesis and parameters of silica and organosilica aerogels, many gaps in knowledge remain. Most studies on silica aerogels focus on how specific variables affect the material’s final morphology or other properties. Only a few articles have attempted to correlate the influence of synthesis variables with condensation kinetics. However, these articles often only analyse changes in gelation time under specific reaction conditions or reagent types and ratios. This paper introduces a novel approach using UV-Vis spectroscopy to observe and analyse the condensation kinetics of methyltrimethoxysilane (MTMS)-based gels. This method allowed for real-time monitoring of skeletal mass growth by observing increases in the absorbance curve. Investigating eight samples, differing solely in the solvent-to-water ratio, which affected the final morphology of the samples, allowed for analysis of the connection between gelation kinetics and the microscopic phase separation mechanism. The experimental results showed that, for each sample, the growth of skeletal mass during condensation, as observed via UV-Vis, could be divided into three subsequent stages, with durations varying between samples. Combining these results with a proposed theoretical framework enables linking the final material’s morphology with the reaction order (ranging between 2 and 3) and reaction rate constants of the condensation process. Furthermore, differences between gelation time and skeletal mass growth time were explained through microscopic phase separation mechanisms and the role of hydrophobic interactions.