Rapid reduction in gelation time and impregnation of hydrophobic property in the tetraethoxysilane (TEOS) based silica aerogels using NH 4F catalyzed single step sol–gel process

Abstract

The experimental results on the synthesis and physical properties of tetraethoxysilane (TEOS) based superhydrophobic silica aerogels, obtained by single step sol–gel process, are reported and discussed. Superhydrophobic silica aerogels were prepared by varying the molar ratio of MeOH/TEOS ( M) from 11 to 275, molar ratio of NH 4F/TEOS ( A) from 1.2 to 9.6 and concentration of NH 4F from 0.001 to 1 M, respectively. All the alcogels were dried under supercritical conditions. The remarkable results were obtained in terms of drastic reduction in the gelation time which is less than 1 min, using single step sol–gel process. Further, the obtained aerogels are hydrophobic ( θ = 155°) without use of any hydrophobic reagents. The obtained hydrophobic property of the aerogels was improved using mono, di and tri-functional methoxysilanes as co-precursor such as, trimethylmethoxysilane (TMMS), dimethyldimethoxysilane (DMMS) and methyltrimethoxysilane (MTMS), respectively. It was observed that as the order of functional groups of methoxysilane decreases, the contact angle of the water droplet on the aerogel surface increases, indicating the aerogel become more hydrophobic. The highest contact angle of ( θ = 168°) was observed for trimethylmethoxysilane as a co-precursor. The aerogels were characterized by bulk density, volume shrinkage, porosity, optical transmission and thermal conductivity measurements. The effect of humidity on the aerogel samples was studied using programmable environmental test chamber. The hydrophobicity of the aerogels was quantified by contact angle measurements and FTIR studies while microstructural studies were done using Transmission Electron Microscopy (TEM).