Synthesis of Hydrophobic Nanoporous Silica Aerogels with a Fast and Cost Effective Method

Silica aerogels have received significant attention in many fields such as catalysis, adsorption, thermal insulation and etc [1, 2]. They are unique nanoporous materials consisting of more than 90% air and less than 10% solid silica in the form of highly cross-linked network structure [1-4]. The pore size distribution is between 5 to 100 nm, open nature with interconnectivity and a BET surface area between 600 and 1000 m2/g [1]. Because of such pore structure they have unique properties, i.e. large surface area, very low bulk density (0.003-0.2 g/cm3) and very low thermal conductivity. Conventionally silica aerogels are prepared by supercritical drying of wet gels (at high pressure and temperature) and usually using expensive TEOS (tetraethylorthosilicate) as silica source [1, 2, 6]. Supercritical drying process can avoid capillary stress which is the reason of drying shrinkage and dense material (xerogel) formation [1]. Supercritical drying process is so energy intensive and dangerous that real practice and commercialization are difficult and the synthesized aerogels are hydrophilic and become wet with the moisture and water and changed to dense xerogels [1, 3, 5]. So it is very necessary to synthesize silica aerogels by an ambient pressure drying (APD) technique at a reasonable cost and hydrophobic surface. The main methods adopted for APD include network strengthen [1] and solvent exchange, surface modification [1-3] of wet gels. Multiple solvent exchange steps are diffusion based and need long time (several days) to accomplish and much volume of solvent is consumed in process [1,3,5,6]. Some researchers used one step solvent exchange/surface modification of wet gels and APD process [1-3]. All of them uses sodium ions removal step for synthesizing transparent aerogels [1-6]. In many applications of silica aerogels transparency is not necessary. In this study, we report upon our efforts to synthesize silica aerogels from cheap waterglass precursors by using ethanol (EtOH)/trimethylchlorosilane (TMCS)/n-Hexane solution for modification of wet gels in one step via APD process without ion removal step.