Organic surface modification of TEOS based silica aerogels synthesized by co-precursor and derivatization methods

The experimental results on the organic modification of tetraethoxysilane (TEOS) based silica aerogels synthesized by co-precursor and derivatization methods are reported and discussed. In order to obtain silica aerogels with better physicochemical properties in terms of higher hydrophobicity, optical transmission and thermal stability, eight organosilane compounds ( hydrophobic reagents) of the type RnSiX4-n have been used. The molar ratio of tetraethoxysilane (TEOS), ethanol (EtOH), water (0.001 M oxalic acid catalyst) was kept constant at 1: 5: 7 respectively. The organically modified silica aerogels were produced by two different methods: (i) Co-precursor method and (ii) Derivatization method. In the former method, the molar ratio of hydrophobic reagent (HR) to TEOS was varied from 0.1 to 0.6. In the later method, derivatization of the wet gels was carried out using 20% hydrophobic reagent in methanol. The merits and demerits of both these methods have been presented. The organic surface modification of the aerogels was confirmed by the Fourier Transform Infrared (FTIR) spectroscopic studies and the contact angle measurements. In the co-precursor method, with the increase in hydrophobic reagent/TEOS molar ratio, the hydrophobicity increases (theta = 136degrees) and the optical transmission decreases (approximate to5%), whereas in the derivatization method the optical transmission is very high ( T approximate to 85%) but the hydrophobicity is low (theta = 120degrees). The thermal stability of the hydrophobic aerogels ( the temperature up to which the hydrophobicity is retained) was studied in the temperature range of 25 - 800degreesC. The aerogels based on the co-precursor method retained the hydrophobicity up to a temperature as high as 520degreesC and on the other hand, the derivatized aerogels are hydrophobic only up to a temperature of 285degreesC. For the first time, TEOS based hydrophobic silica aerogels have been obtained with negligible volume shrinkage using the trimethylethoxysilane (TMES) co-precursor. The aerogels were characterized by Fourier transform infrared spectroscopy (FTIR), optical transmittance, Scanning Electron Microscope (SEM), thermogravimetric (TG) and differential thermal (DT) analyses and the contact angle measurements.