(707g) Mixing of Silica Aerogel (Cabot Nanogel) Particles of Different Sizes

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 that is higher than that of either of the particles by themselves. Here we present two different methods for mixing coarse and fine Nanogel particles without segregation and simultaneously increasing the packing density. The first method combines a negative pressure (vacuum) with an external force field, such as sound waves or vibration to fill the voids between the coarse particles with fine particles. The second method adds a small amount of liquid to the particles 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 enter the pores of the Nanogel. To overcome this problem, we used an ethanol- water solution that has a lower surface tension than water, and a contact angle close to 90 degrees. The mixtures produced by both methods are uniform without segregation and have packing densities very close to the predicted theoretical maximum for binary particles.