Issue Date: July 14, 2008
It was only a matter of time. The Mentos-Diet Coke reaction has finally exploded itself into a peer-reviewed journal. Readers may recall a previous Newscripts item briefly describing the reaction way back in 2006 (C&EN, Jan. 16, page 48). Since then, countless videos have flooded YouTube (a quick search returned more than 7,000) and e-mail inboxes. Mentos fizzed. Diet Coke sprayed. Hilarity ensued.
Although others have informally explained the science of the fizzy reaction, including a 2006 episode of “MythBusters” on the Discovery Channel, none have systematically studied the intrinsic details of it—until now. In the June issue of the American Journal of Physics, Tonya S. Coffey, an assistant professor of physics and astronomy at Appalachian State University, in Boone, N.C., and her team of fearless physics students reported on the ingredients, temperature dependence, duration, and other parameters of the suddenly famous Mentos-Diet Coke reaction.
Coffey and her students found that the geyserlike behavior of the SODA-CANDY EXPLOSION is not affected by either the type of Mentos (fruit or mint is fine) or the presence of caffeine in the soda. Coffey’s experiments also determined that a simple acid-base reaction does not a fountain create; pH levels of the soda were the same before and after the reaction (not to mention that none of the ingredients in Mentos is basic). Coffey’s group was able to confirm that the surfactant gum arabic, which is found in the candy’s outer coating, is a key component of the reaction: It reduces surface tension, thereby allowing the soda’s carbon dioxide to make an especially rapid escape from the bottle.
The diet beverage’s aspartame and potassium benzoate provide a one-two punch to further reduce surface tension and ease bubble formation. This dynamic duo is more effective than sugar and creates a bigger explosion.
Coffey’s team also used a scanning electron microscope to measure the surface roughness of Mentos. The images revealed that the candy’s “microscape” provides multiple nucleation sites for carbon dioxide bubbles to form. Other materials—such as sand, dishwashing detergent, and salt—also triggered explosions, but these fell short of the Mentos’ over-the-top productions. The other key to creating a fantastic fizz is the rate of the reaction. Mentos are dense and sink to the bottom of the bottle quickly. Slower falling objects yielded comparatively piddly explosions.
So the Mentos-Diet Coke reaction owes its pizzazz to a slew of factors that create the perfect storm for fizz. And one more factor to keep in mind for your next fountain fest: Warmer soda leads to more volume loss (thank Henry’s law).
Molecular Formulas Turned Art
On a less messy note, John L. Meisenheimer Sr. of Orlando, Fla., alerted Newscripts to a SCULPTURE currently on display at Hannibal Square in Winter Park, Fla., entitled “Molecular Dog/C3H8.” The bronze sculpture was created by Miami artist Robert Chambers.
The propane dog is not Chambers’ first foray into blending science and art. According to the artist’s website, his father was a cell research scientist, and his mother was a painter and sculptor. When he was a child in chemistry class, Chambers found that he was able to remember molecular formulas by associating the structures with animals both real and mythological. To him, ethanol became a dog and sugar became Cerberus, the three-headed hound that guards Hades. With his current sculptures, Chambers ushers those mental associations into the material world.
Chambers’ “Molecular Dog/C3H8” sculpture will be on display in Winter Park through Aug. 31.
- Chemical & Engineering News
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