Effect of precursors, methylation agents and solvents on the physicochemical properties of silica aerogels prepared by atmospheric pressure drying method

The properties of aerogels dried at ambient pressure prepared using different synthesis conditions have been studied. The effect of type of precursor (tetramethoxysilane (TMOS), tetraethoxysiliane (TEOS) and sodium silicate (`water glassʹ)), solvent (n-hexane, n-heptane and toluene) and surface chemical modification agent (trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDS) and hexamethyl-disilazane (HMDZ)), on transparency, monolithicity and physical properties of the aerogels are studied. Low density aerogels (0.14–0.3 g/cm3) could be prepared by using the present TMOS and `water glassʹ based recipes and TMCS or HMDZ as surface modifiers. Using the TMOS precursor, TMCS; and for `water glassʹ precursor, HMDZ resulted in the best quality silica aerogels in terms of monolithicity, visual transparency and lowest density. HMDS surface modification resulted in hydrophilic high density aerogels for all the precursors, hence this chemical is not recommended for effective surface modification. The `water glassʹ based gels experienced the highest degree of monolithicity because these wet gels had the highest stiffness and the largest pore size of the three precursors studied. Using n-hexane or n-heptane as the solvent resulted in aerogels with the lowest density and the best visual transparency.