Na2SiO3 + H2SO4 + H2O -> Si(OH)4 + Na2SO4 Eq.1
Using Kistler’s method, the aqueous silica hydrogels were repeatedly rinsed with volumes of fresh anhydrous methanol to remove all but trace amounts of water. Kistler brought the contents of the gel past the critical point of methanol (240°C and 1600 psi; making the solvent system “supercritical”) in a high- temperature autoclave and slowly depressurized the system at a temperature that prevented recondensation of methanol within the porous silica gel structure. This method has numerous disadvantages, particularly in the toxicity of methanol, and in the handling of a flammable solvent at very high temperatures and pressures. In the seven decades since Kistler’s seminal work, there have been significant advances made in both the use of new precursor materials, and in the removal of solvent from them. For instance, Teichner and others established in the 1960’s that silicon alkoxides (e.g. tetraethylorthosilicate or TEOS) are the preferred soluble silica source for formation of silica gels because the need for water/alcohol solvent exchange could be avoided.[2,3] Ethanol based processing using silicon ethoxide derivatives such as TEOS and polydiethylsilicate (PDEOS) have become the preferred precursors to silica aerogels over the last few decades. With these materials, water is added to liberate alcohol and silicic acid. The silicic acid is very sensitive to condensing with itself, and rapidly building sol particles. As the process continues, a three dimensional gel network is formed, filling the mold volume and entraining all of the liquid solvent. The gels can then be further strengthened and treated in an alcohol solution, avoiding the need for solvent exchange if desired. In the past 20 years, the use of supercritical carbon dioxide as a solvent for drying of gels containing organic solvent has made the process safer and more economical. Hunt pioneered the use of supercritical carbon dioxide as the solvent medium for aerogel processing , further reducing hazards associated with venting of superheated methanol vapor.