A Constructive Chemical Conversation, by Alison Grinthal, Wim L. Noorduin, and Joanna Aizenberg

The beautiful photographs in this publication just make you itch to go into the lab and try to reproduce them.  The structures appear to be ideal for lawn ornaments - delicate, colorful and very attractive.  Furthermore, they can be made with easily available materials, just sodium metasilicate, barium chloride, a flat piece of metal-coated glass, and carbon dioxide from the air. By controlling the pH, the temperature and the amount of CO2 admitted, the authors were able to grow structures of precipitated SiO2 or BaCO3 on top of one another. Some of the complex structures they can grow strongly resemble green plants with red or purple flowers.

The shapes of the structures precipitated are the result of an intricate interplay of thermodynamics and kinetics.  Silicon dioxide and barium carbonate coprecipitate in aqueous solutions of barium chloride and sodium silicate in the pH range 8-12. In the experiment, precipitation is triggered by the diffusion of carbon dioxide into the solution to produce barium carbonate, and a gradient in pH is produced by the diffusion of CO2 down from the surface of the solution.  The reaction that precipitates barium carbonate also lowers the pH of the solution near the crystals. This slows the formation of barium carbonate and triggers deposition of silica (SiO2), which consumes acid and revives the barium carbonate reaction. The interplay of these reciprocal reactions forms various curved shapes, as the reagents are depleted regionally and hydrogen ions diffuse around the growing structures.  The higher the pH, the more carbonate is favored, but silica is most stable within a limited pH range. 

But wait a minute! How big are these structures?  The extended title of the article says that these are microscale, but the beautiful figures in American Scientist have no scale on them, so it is not possible to tell whether these can be seen with a magnifier or a microscope.  What about the colors? The solids precipitated in these reactions are SiO2 or BaCO3, both of which are colorless.  Where do the colors come from? What concentrations of reactants and what temperatures are used?  Unfortunately, none of the answers to these questions is found in American Scientist, and the published article also includes no references. The answers to the experimental questions can be found in Science 2013 340, p. 832-836 and its Supplementary Materials. There, one learns that a typical “plant” is about 50 micrometers in size. The colors are all produced in the computer rendering, so they can be adjusted arbitrarily, and have no reality. After the solutions are degassed, carbon dioxide is introduced by partially uncovering the beaker in which the reaction occurs, and gas diffuses into the solution over a period of minutes, sometimes as small as two minutes.  Most of the reactions were run at room temperature (which was unspecified), but some structures were grown at 4 degrees Celsius.  Typical concentrations were 19.1 mM BaCl2, 8.2 mM Na2SiO3, and a pH adjusted to 11.8. All of these growing conditions are accessible to even an amateur chemist, but an electron microscope is needed to see what you have grown.

Publication information
Pick Attribution: 

Alison Grinthal, Wim L. Noorduin, Joanna Alzenberg

Publication Date: 
Thursday, July 14, 2016