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

A fast and simple, one-step synthesis for the preparation of silica aerogel powders is reported. A waterglass based sol was prepared without prior ion-exchange and stirred with an immiscible organic phase (heptane) that contains the hydrophobization agent (hexamethyldisilazane). The addition of nitric acid triggers gelation and subsequently the hydrophobization of this hydrogel, which causes the gel particles to transfer from the aqueous into the organic phase, whilst the sodium and nitrate ions are retained in the aqueous phase. The resulting organogel slurry is then converted into a silica aerogel powder by means of ambient pressure drying. The addition of phase transfer agents, ethanol and isopropanol in particular, strongly influences the final silica aerogel properties, including tap density, hydrophobicity, particle size distribution, surface chemistry (solid-state NMR), and microstructure (TEM, SEM, nitrogen sorption). The addition of intermediate amounts of phase transfer agent strongly improves the aerogel properties, but these positive effects disappear when larger quantities are added. Under optimal conditions, silica aerogel powders with high mesoporosity, surface areas above 800 m(2)/g, tap densities as low as 0.083 g/cm(3), and thermal conductivities below 20 mW m(-1) K-1 can be produced in a one-step process within just over two hours. The direct use of waterglass as an inexpensive silica precursor provides a great opportunity to reduce the production cost of silica aerogel powders for thermal insulation applications.