Mixing and packing of binary hydrophobic silica aerogels

Particulate hydrophobic silica aerogel (Cabot Nanogel) is extremely porous, has the lowest density and lowest thermal conductivity of any solid, and is a much better insulator than air. Therefore it is advantageous to mix coarse and fine Nanogel particles to produce a uniform mixture that has a packing density (or bulk density) that is higher than that of either of the particles by themselves. A simple method to mix binary particles together without segregation is to add a small amount of liquid in a slowly rotating tumbler mixer. However, if water is used, the hydrophobicity of the Nanogel surface will repel the water and if an organic solvent is used, the solvent will simply enter the pores of the Nanogel. In this paper we use an ethanol water solution that has a lower surface tension than water, and a contact angle close to 90° so that it can wet the surface of the coarse particles without entering the pores. The mixtures so produced are very uniform without segregation and have packing densities very close to the predicted theoretical maximum for binary particles.