Effect of precursor,solvvent,and water molar ratios on surface area and porosity of tetraethoxysilane silica aerogels

For most applications, not only are monolithic and transparent silica aerogels required but they should also have low density, large surface area, and high porosity (> 90%). The aerogels have been prepared by hydrolysis and polycondensation of tetraethoxysilane (TEOS) using an organic acid (citric acid) catalyst, followed by supercritical drying in an autoclave. The molar ratio of ethanol/TEOS (A) and H₂O/TEOS (B) cause significant structural rearrangement of citric acid catalysed TEOS silica aerogels, for example, changes in surface area, pore diameter, porosity, etc. The experimental results presented here show the effect of precursor, solvent, and water molar ratios on surface area and porosity of TEOS silica aerogels. The values of A and B were varied from 2.5 to 20 and 1 to 10 respectively. It has been found that as the value of A increases up to 5, the surface area, pore diameter, and per cent porosity increase, and then remain almost constant for A > 5. On the other hand, the lower (<6) and higher (> 9) values of B resulted in lower surface area, smaller pore diameter, and lower per cent porosity, whereas medium values of B (6 < B < 9) resulted in larger surface area, pore diameter, and the per cent porosity. For values of A between about 4 and 6 and values of B between about 6 and 9, monolithic and transparent silica aerogels with larger surface area, pore diameter, and, per cent porosity were observed. It has been found that A and B values of ∼ 6 resulted in narrow and uniform pore size distribution, which reduces the differential pressures during supercritical solvent extraction and leads to monolithic silica aerogels.