Role of oxalic acid in structural formation of sodium silicate-based silica aerogel by ambient pressure drying

Recently, the demand of sodium silicate-based silica aerogels has decreased due to its inadequate physical properties when compared to those made by other silicon alkoxides. To avoid this problem, introduction of a drying control chemical additive (DCCA) in the sol has received great attention. DCCA is one of the additives that can control the rate of hydrolysis and the condensation reaction in sol state by the formation of hydrogen bonds between DCCA and silanol groups of silica sol. A control over these reactions results in a uniform pore size distribution, which, in turn, decreases the drying stress with a decrease in pore size, in accordance with the Young–Laplace equation. Therefore, the structure of the silica aerogel can be maintained by minimizing the volume shrinkage due to drying stress. In this research, oxalic acid was first used as DCCA in the formation of sodium silicate-based silica aerogel by ambient pressure drying. The physical properties of these silica aerogels can be changed by changing the molar ratio of oxalic acid/Na₂SiO₃ in the sol state. When the oxalic acid:Na₂SiO₃ molar ratio was 15 × 10⁻⁴ in the sol state, aerogels with high specific surface area (623.2 m²/g), pore volume (4.271 cm³/g), average pore diameter (27.41 nm), high porosity (94.3%), high contact angle (144.09°), and high optical transmittance (75%) were obtained.