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Shown in the top picture is a simple experimental configuration similar to ones first investigated in the mid- and late-1800's. The bottom of a test tube is filled with a gel based on a salt solution, and after the gel has cast, a second salt solution with a mutually-insoluble component is put on top. The purpose of the gel is to ensure that the NaOH can invade the gel region only by diffusion -- the small pores of the gel inhibit convection or other mechanical mixing. In this case, the Mg2+ ions in the gel solution will react with OH- to form Mg(OH)2 (solubility product ~10-12). As the concentration of NaOH is much high than that of the MgCl2, the precipitation zone rapidly moves down the test tube. However, as shown in the figure at the bottom left, the final pattern of precipitated Mg(OH)2 embedded in the gel is not uniform, instead showing "Liesegang" bands, named for the German chemist who first studied them carefully around the turn of the century.
Many of the qualitative details of this simple pattern formation experiment can be understood with careful modeling of the coupled diffusion equations. However, the frequent occurence of low-symmetry patterns in systems with very low solubility products or confined diffusion geometries has recently led to discussion of the importance of local thermodynamic concentration fluctuations in the invading salt front. Hence, precipitation reactions in gels may become an important testing ground for understanding the role of fluctuations in diffusion processes.
In the microstructural kinetics lab, we are investigating diffusion- limited precipitation reactions in confined geometries -- both with and without gels -- with several precipitation reactions spanning a wide range of solubility products. Pictures of low-symmetry or broken symmetry patterns will be posted here shortly.
Go to Microstructural Kinetics Lab homepage.