Modifying the surface energy and hydrophobicity of the low-density silica aerogels through the use of combinations of surface-modification agents

Aerogels are lightweight, highly transparent, thermally insulating materials comprising interconnected nanostructured pores. Low surface energy aerogels were prepared from ambient pressure drying of sodium silicate-based gels by modifying the pore surfaces with silylating agents including trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO), and hexamethyldisilazane (HMDZ), in combination with each other. Hydrophobic properties of the resulted aerogels were studied by contact angle measurements. Fourier-transform infrared spectroscopy (FTIR) was used to monitor the changes in chemical bonds within the aerogels due to surface modification. The microstructure was studied by transmission electron microscopy (TEM). Effect of temperature on the hydrophobicity of the aerogels was studied by thermogravimetric analysis/differential thermal analysis (TGA-DTA). Surface modification of silica gels with various mixtures of surface-modification agents showed different behaviors. Aerogels made by HMDZ and HMDSO combination comprised 5 nm pores and particles and showed a high surface energy, whereas aerogels prepared by HMDSO and TMCS combination had lower surface energy with relatively larger particle and pore sizes with a more uniform distribution of both. The properties of the latter sample were attributed to a greater degree of surface modification and negligible condensation of OH groups. This preparation produced silica aerogels with a low density (0.042 g/cc), low surface energy (3.39 N cm⁻¹), low thermal conductivity (0.050 W K⁻¹ m⁻¹), high optical transmission (85% at 700 nm) and hydrophobic (154° contact angle) with high hydrophobic thermal stability (425 °C). Moreover, the contact angle for materials prepared by this method decreased negligibly over 12 months’ storage in ambient conditions.