Coffee-ring effect produces chemical patterns

The characteristic darker edges of a dried splotch of coffee come from how water evaporates: The liquid disappears faster from the edge of a drop than the center, and liquid flowing outward to replenish the loss carries along suspended particles that accumulate at the edges. Chemists have now combined small-molecule chemical reactions with the coffee-ring effect to selectively deposit compounds from an evaporating drop (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201606546). The University of Strasbourg’s Joseph J. Armao IV and Jean-Marie Lehn worked with amine and aldehyde compounds that react reversibly to yield imine products. When they combined two amines (A and A´) and one aldehyde (B), the less soluble imine product, AB, precipitated at the edge of the drop. The precipitation caused the equilibrium to shift to produce more AB and less A´B, leaving behind a solution enriched in A´. When they combined two amines (A and A´) and two aldehydes (B and B´), the four products in equilibrium solution reverted to two as AB precipitated first and reaction equilibria shifted to produce A´B´. Such controlled deposition of solutes could have many applications, the researchers say, including in ink-jet printing and medical diagnostics.