Switching silica aerogels from transparent to opaque

Adsorption of 2-propanol in the pores of a silica aerogel with a porosity of about 87% and a (breakthrough) pore diameter of 25 nm is accompanied by very strong light scattering. With an uptake of only a few vol.% of 2-propanol, the direct-hemispherical transmission is reduced by about 50% in the visible spectral range. The observed effect is present on both adsorption and desorption and is fully reproducible. This sorption-induced scattering vanishes as soon as the sample is either completely liquid filled or dried. From the experimental light scattering data we deduce that the mean chord length of the entities responsible for the strong light scattering is about 60–100 nm; these values indicate a structural feature significantly larger than the breakthrough pore diameter, and 25–50% smaller than the characteristic size of the density fluctuations found for the silica aerogel with no 2-propanol adsorbed.