Liesegang pattern formation by gas diffusion in silica aerogels

Molecular gas diffusion of HCl and NH3 has been studied both in air and in aerogel. The two gases react to form NH4Cl which precipitates when a threshold concentration is reached. When the gases diffuse from opposite ends of a tube, the precipitate appears as a white ring whose position is determined by the ratio of the gas diffusion constants and follows Graham's law. When the diffusion process takes place in an aerogel, NH4Cl precipitates in sheets that span a cross-section of the gel. As the diffusion process continues, more bands are formed. The assembly of bands is called a Liesegang pattern. This phenomenon is known to occur in diffusion reaction systems. Numerical simulations based on the Ostwald nucleation theory qualitatively agree with experiments. (C) 1998 Elsevier Science B.V. All rights reserved.