Pressurized carbon dioxide-assisted hydrophobic modification of silica aerogel to reduce thermal conductivity

Abstract

This study proposed a method employing pressurized carbon dioxide (CO₂) to improve the hydrophobicity of silica aerogel. The effects of aging time, reaction temperature, and reaction time on the extent of hydrophobic modification were investigated systematically using scanning electron microscopy, water contact angle measurements, Fourier transform infrared analyses, thermogravimetric analyses, and thermal conductivity measurements. The results indicated that the use of CO₂ pressurization significantly promoted the hydrolysis of hexamethyldisilazane (HMDS) and participated in the hydrophobic modification of the aerogel, enhancing the substitution of Si-OH on the aerogel surface with Si-CH₃ to provide excellent thermal stability. Under the optimal reaction conditions, the water contact angle, specific surface area, and average pore diameter of the silica aerogel reached 150.76°, 645.95 m²/g, and 18.07 nm, respectively. Furthermore, an electrostatic potential analysis confirmed that (CH₃)₃Si-OH generated by the pressurized CO₂-promoted hydrolysis of HMDS exhibited highly nucleophilic and electrophilic sites, making it more susceptible to substitution reactions with Si-OH. Critically, the use of CO₂ pressurization helped to discharge the water inside the silica aerogel, thereby reducing the capillary force and drying stress while improving the skeletal structure to obtain an aerogel with excellent hydrophobic properties and a spatial structure that considerably reduced its thermal conductivity. Therefore, this study demonstrated a new approach for the preparation of hydrophobic silica aerogels with low thermal conductivity.