LIESEGANG PATTERN-FORMATION BY GAS-DIFFUSION IN SILICA AEROGELS

Molecular gas diffusion of HCl and NH3 has been studied both in air an d in aerogel. The two gases react to form NH4Cl which precipitates whe n a threshold concentration is reached. When the gases diffuse from op posite ends of a tube, the precipitate appears as a white ring whose p osition is determined by the ratio of the gas diffusion constants and follows Graham's law. When the diffusion process takes place in an aer ogel, NH4Cl precipitates in sheets that span a cross-section of the ge l. As the diffusion process continues, more bands are formed. The asse mbly 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.