If you have an ACS member number, please enter it here so we can link this account to your membership. (optional)

ACS values your privacy. By submitting your information, you are gaining access to C&EN and subscribing to our weekly newsletter. We use the information you provide to make your reading experience better, and we will never sell your data to third party members.



Coffee-ring effect produces chemical patterns

Chemists take advantage of chemical equilibria in a network of reversible reactions to precipitate out rings

by Jyllian Kemsley
October 24, 2016 | A version of this story appeared in Volume 94, Issue 42

Fluorescence images of a single ring of imine AB and concentric rings of imines AB and AʹBʹ.
Credit: Angew. Chem. Int. Ed.
Combining two amines (A and Aʹ) and two aldehydes (B and Bʹ) yields a millimeter-sized coffee-ring fluorescent imine precipitate, first of red AB and then of green AʹBʹ.

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.


This article has been sent to the following recipient:

Chemistry matters. Join us to get the news you need.