David Biello explains Soda Fountains, or Diet Coke, Mentos and Surface Tension — or why dropping a few Mentos mints into a bottle of Diet Coke creates such a spectacular geyser:
At first glance, it might seem as if a chemical reaction is taking place. Perhaps the Mentos are alkaline and the soda (despite its name) is acidic. That would make the whole thing a variation on my all-time favorite science bit: the baking soda and vinegar volcano. But a quick read of Mentos ingredients reveals precious little alkali: sugar, glucose syrup, hydrogenated coconut oil, gelatin, dextrin, “natural flavor,” corn starch and gum arabic. While hydrogenating the coconut oil most likely involves some caustic soda or other alkaline agent, this would not be enough to drive the spectacular fountains.So the process must be physical rather than chemical. [...] Dropping a Mentos (or, as in the demonstrations above, a slew of Mentos) into the soda breaks the surface tension in two ways. First, the candy is uneven on a microscopic level. These tiny bumps and pits make it easier for bubbles to form because the surface tension is suddenly spread out over a wider area. Anywhere the surface tension is lowered by the Mentos, bubbles will appear at what are known as nucleation sites. “If there’s a way for that gas molecule to get more freedom, it will find it,” notes Joel Finegold, a detergent chemist. “It will actually break through that weakness in the surface tension.” This is also the reason that almost any substance–salt, ice cream, or even quarters in a glass of beer–will quickly be covered in a coating of tiny bubbles when dropped into a carbonated beverage.
But the makeup of the Mentos is also helping here. Gum arabic–an oily gum derived from the acacia tree–and the coconut oil contribute to the effect by paradoxically making the water molecules even more strongly attracted to one another than usual. Remember, oil and water don’t mix. This frees up space in surrounding areas for the carbon dioxide to bubble free again. “A simple experiment mixing soda and and gum arabic in a beaker results in fairly rapid evolution of gas and large bubble formation,” writes Michael Montague-Smith, a chemist at the University of Maryland. “In a container with a narrow neck, sufficient gas could cause quite the geyser.” Finegold speculates that balls of candlewax would work just as well, thanks to the oiliness of the wax.