Preparation of SiO₂ aerogel by water glass: effect of different sodium removal methods on aerogel properties

Removal of Na⁺ is a very important step in the SiO₂ aerogel preparation using water glass as precursor, which has a significant effect on the structure and properties of aerogel. In this study, three methods (Solution-crystallization, Water washing, and Ion exchange) were used to remove Na⁺ from the system and investigated the effects of different methods on the aerogel properties. The results show that all three aerogels have low bulk density (<0.075 g/cm³), low thermal conductivity (≤0.015 W/(m·K)), and well hydrophobicity (θ ≥ 137°). Among them, Na⁺ exists in the sol in the sodium removal process by water washing and effects the uniformity of gel network. The samples for sodium removal by solution-crystallization show better hydrophobicity and poorer high-temperature stability, which is related to the presence of abundant hydrophobic groups (Si-OC₂H₅) on the gel surface. The microstructure and the pore size distribution of the aerogels prepared by ion exchange are uniform, which makes them have optimal comprehensive performance. The mechanism of solution-crystallization effect in the process of sodium removal was explored, providing a workable idea for low-cost preparation of SiO₂ aerogel.