Quantitatively elucidating the effect of supercritical drying conditions on silica aerogels’ structure and chemical composition

Aerogels, renowned for their exceptional thermal insulation, are widely used in the space industry and sustainable applications. However, their large specific surface area makes them highly sensitive to environmental conditions, such as relative humidity, which can be detrimental depending on their chemical composition. In this study we employ several tools to analyze and quantify the effect of supercritical drying conditions on silica aerogels. Raman scattering was used to assess molecular adsorbates signatures, Thermal Gravimetric Analysis to measure temperature dependent mass loss, Temperature Programmed Desorption to identify decomposed and adsorbed species, and Elemental Analysis to validate the elemental composition. The measurements show that hydrophilic silica aerogels lose ∼10% of their weight when heated to ∼150 o C, due to desorption of H 2 O molecules, which, at room temperatures, are manifested in the Raman scattering spectrum by a distinctive broad band of O-H stretching. Hydrophobic aerogels showed a negligible desorption of physisorbed H 2 O and lacked the O-H stretching fingerprint; the adsorption of ambient H 2 O is inhibited by the incorporation of exthoxy (-OCH 2 CH 3 ) groups on the surface. A non-negligible organic matter was also found in the hydrophilic aerogels (∼40% of that found in the hydrophobic aerogels), which is ascribed to ethanol that remain trapped in the pores; this calls for awareness of the difficulty of achieving pure silica aerogels. This study centers on the ability to derive fundamental, quantitative, and correlative insights from a diverse and complementary suite of experimental techniques, thereby informing the synthesis and design of aerogels tailored for specific applications.