Fast Synthesis of Spherical Silica Aerogel Powders by Emulsion Polymerization from Water Glass

Abstract

In this study, a fast synthesis technique for silica aerogel powder was proposed that reduces the length of the tedious surface modification and solvent exchange processes. Spherical silica aerogel powder could be fabricated by an emulsion polymerization method from water glass and n-hexane. A stable water glass-in-hexane emulsion with a water glass to hexane ratio of 5:5 was fabricated using the surfactant, sorbitan monooleate (Span80). It was found that acetic acid and isopropanol catalysts for the hydrolysis and condensation of water glass droplets dispersed in the emulsion were crucial for the synthesis of the spherical silica aerogel powder with a narrow particle size distribution and for reducing production time. In addition, the particle size of the spherical silica aerogel could be controlled by changing the water glass droplet size.A silica (SiO 2 ) aerogel is an ultra-porous (90-99% of theoretical density) material with a 3-dimensional network structure of silica nanoparticles. [1] Because of its interesting characteristics such as high specific surface area, low thermal conductivity, low dielectric constant, and low refractive index, it has received wide attention within various fields as a promising material for catalysis, [2,3] adsorption, [4,5] heat insulation [6,7] and drug delivery. [8"9] Silica aerogels are normally synthesized via a sol-gel route from silicon alkoxides such as tetramethylorthosilicate (TMOS) or tetraethylorthosilicate (TEOS). A silica wet gel is formed as a result of a hydrolysis and condensation process of the silicon alkoxide molecules. The silica aerogel can then be obtained by a supercritical drying process with carbon dioxide (CO 2 ). [10,11] However, the alkoxide-derived silica aerogels use expensive precursors, and the supercritical drying process requires long processing times (low heating and depressurization rates) [12,13] which increases the overall cost of the aerogel.Ambient pressure drying (APD) is another option to produce silica aerogel, and water glass-based silica aerogels by APD has been extensively studied by many researchers. [14][15][16][17] The APD relies on surface modification and solvent exchange processes that are not only tedious but also economically unfavorable. Since surface modification agents (typically, trimethylchlorosilane (TMCS) or hexamethyldisilazane (HMDS)) can be transported within the silica hydrogel only by diffusion, APD requires processing times longer than a few or sometimes even tens of hours. [14][15][16] Recently, some groups have attempted to reduce the processing time of APD via a co-precursor method. [18][19][20] Huber et al. proposed a one-pot synthesis technique for a silica aerogel granulate, which could reduce solvent use and production time to within 4 h. However, they used an expensive precursor like polyethoxysiloxane (PEDS), and the morphology of the synthesized silica aerogel granulate was irregular. [21] Pan et al. prepared a silica aerogel powder via APD from water glass within 4 h. [22] Although the mechanical cru...

Thermal Gelation for Synthesis of Surface-Modified Silica Aerogel Powders

“…In our previous study focusing on catalysts for hydrolysis and condensation of water glass, we proposed a novel fast synthesis technique for spherical silica aerogel powders with a narrow particle size distribution. This synthesis technique reduced the total processing time to less than 2 h [ 15 ]. However, silica aerogel powders using the aforementioned fast synthesis processes exhibit some drawbacks.…”

Eco-Friendly Fabrication of Highly Stable Silica Aerogel Microspheres with Core–Shell Structure

“…Bhagat et al reported superhydrophobic silica aerogel powders with simultaneous surface modification, solvent exchange, and sodium ion removal from hydrogels [29]. Lee et al reported a fast synthesis technique for silica aerogel powders by an emulsion polymerization method using water glass, n-hexane, and a surfactant (Span80) [30]. Jiang et al prepared an amine-grafted silica aerogel microsphere by dropping the siliceous solution into a hot oil bath [31].…”

Phase transfer agents facilitate the production of superinsulating silica aerogel powders by simultaneous hydrophobization and solvent- and ion-exchange

“…In addition, because ions will partition preferentially into the aqueous phase, and the hydrophobic gel moves into the organic phase, this process can be carried out without any ion-exchange step prior to gelation. More recent variations of this approach to the synthesis of powders [27,[41][42][43][44][45] include gelation, hydrophobization, and solvent and ion-exchange in one single step. Hydrophobic silica aerogel powders are produced through the addition of an acid to a vigorously stirred two-phase mixture of waterglass and an organic phase (e.g.…”