Hollow Silica Colloids as Building Blocks for Highly Porous Water Borne Aerogels with Improved Mechanical Stability

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Fabrication of aerogels remains a challenge. Their importance originates from the fact that heat transport reduces 5 to 8 times when the pore sizes of such materials become smaller than the mean free path of the gas molecules (Knudsen effect). This project aims on preparation of hollow silica colloids and their uncongested assembly to a material with a thermal conductivity that approaches the properties of an aerogel. Other than established syntheses of aerogels, our concept uses assembly of defined micro-objects as building blocks to yield a hierarchically organized structure that enables the combination of high porosities with mechanical strength superior to established aerogels. For this purpose, we will develop a two-stage process. First, we attend to a micro-emulsion synthesis of hollow silica spheres with sub-micron sized oil droplets as a template around which silica deposits. We hypothesize that the shell strengthens mechanically by consolidation while still in the dispersion, and we build on preliminary results that capsules can be dried by evaporation of the water and oil without destruction by capillary forces. In a second step, and before removal of the solvent, the dispersed hollow spheres are mixed with an aqueous solution of silsesquioxanes, which are hydrophilic silica precursors. The addition of the silsesquioxanes serves two objectives: The hollow sphere colloids become sticky and can gel the aqueous dispersion and the silsesquioxanes control pore formation between the hollow spheres upon evaporation of the liquid components. The template synthesis of porous and hollow silica colloids is based on our development of silica precursor compounds that serve initially as surfactants to reduce the interfacial tension between oil and water to values below 0,5 mN/m and which allow spontaneous formation of a micro emulsion. Subsequent hydrolysis of the precursor yields a silica shell around the oil droplets. Hydrophilic or amphiphilic silsesquioxanes as a precursor for silica are an original development of the Russian partner and offer new possibilities to control pore formation during drying which is complemented by the order of the evaporation of the solvents (alcohol first, water second, oil last). Application relevant benchmarks are: (1) control of the porosity and pore sizes of the silica colloids in order to approach the transition from xerogels to aerogels with average pore diameters less than 60 nm (Knudsen number 1 for air). (2) Stability against capillary forces during drying. (3) Hydrophobisation of the gels by components contained in the templating oil droplets in order to prevent sorption of water by the final materials during use in humid atmosphere. (iv) The possibility to add fortifying components such as exfoliated layered silicates, nanofibers and organic gelators in the second gelation step in order to achieve mechanical strength that can withstand a load of 1 kg/cm2 at little and recoverable compliance.