B Bogdanov, P Ivanov, Y Hristov, R Raykova
2013 Science & Technologies   unpublished
Highly porous hydrophilic and hydrophobic silica xerogels were synthesized by surface modification of silica hydrogels at ambient pressure drying. The silica hydrogels were prepared by a sol-gel polymerization of an inexpensive silica precursor (sodium silicate) under atmospheric conditions. In order to minimize shrinkage due to drying, the hydrogel surface was modified using trimethylchlorosilane (TMCS) in the presence of ethanol/n-hexane solution before ambient pressure drying (APD).
more » ... ing (APD). Properties of the final product were investigated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR). Highly porous hydrophilic xerogels were obtained after heat-treating the modified xerogels. At temperatures above 400 o C the surface alkyl groups (-CH 3) were significantly oxidized and, consequently, the properties of the resulting xerogels were altered. Products obtained via the proposed inexpensive approach have superior properties and the method exploits an inexpensive silica source (sodium silicate). INTRODUCTION Aerogels are the most highly porous nanostructured materials. They exhibit large surface area (~1200 m 2 /g), high porosity (80-98%), low bulk density (~0.03 g/cm 3), extremely low thermal conductivities (0.005 W/mK), and unique acoustic properties (sound velocities as low as 100 m/s) [1,2]. Because of these properties, aerogels are utilized as thermal super-insulators in solar energy systems, refrigerators, and thermal flasks [3]. Despite these applications, the high production costs have thus far prevented their commercial use. Meanwhile, applications for porous silica xerogels continuously expand as their production costs decrease and their properties improve. Hydrophobic and hydrophilic silica xerogels with superior physical properties such as high surface area and large pore volume have potential applications in fields such as adsorbents, separations, biomedicine, sensors, drug delivery systems, catalyst carriers, thermal insulation, glazing, paints, and oil spill clean-up [4-8]. Conventional silica xerogels have relatively high density, low surface area, and small pore volume, restricting their applications. Recent observations suggest that the properties of porous materials improve following modification with silica gels (alcogel or hydrogel) during synthesis before the ambient pressure drying (APD) [9-13]. Moreover, silylating hydrogels and drying at ambient pressure can give less-dense silica xerogels. During the drying process, non-polar alky groups (which repel each other) replace surface OH groups, resulting in the ""spring back-effect"", which preserves the silica gel network and, hence, the porosity [14]. Surface modification of silica hydrogels by alkyl groups has been reported to preserve the porous network even after drying at ambient pressure [15]. Prakash et al. [16] have synthesized silica aerogel films at ambient pressure via solvent exchange and surface modification processes. Solvent exchange is a lengthy and tedious process because it simply depends on diffusion of the solution within the gel. Hence, its take several days to produce silica aerogels at ambient pressure. Schwertfeger et al. [17] developed a new synthesis for sodium silicate-based silica aerogel at ambient pressure. Since then, many researchers have focused on synthesizing sodium silicate-based silica aerogels at ambient pressure. Nevertheless, the solvent exchange process, which is required for silica aerogel synthesis at ambient pressure, makes it a tedious process. Recently, Shi et al. [18] reported a new method, called