Whether you’re buying ingredients for an at home “Coke and Mentos” demonstration, asking a flight attendant for a beverage, or just trying to pour a can of soda into a glass before hockey comes back on, you may have noticed something: Diet sugar-free sodas fizz more than regular sugar-rich sodas when opened.
The degree of carbonation or “fizziness” of a soda is partly a function of how easily carbon dioxide bubbles can form in the sugary flavour water we call pop. When it’s easier for bubbles to form, you get more of them and therefore an increased “fizziness”.
When a liquid has a high surface tension, it means that the bonds between the liquid’s molecules are very strong. Surface tension is why some spiders can walk on water—the spider’s weight isn’t enough to break apart the water molecules! In a substance with high surface tension, bubbles will not form very easily.
Surfactants are chemicals that decrease the surface tension of a liquid. They will therefore make it a bit easier for bubbles to form. Regarding Diet Coke, aspartame, and potassium benzoate (a preservative) are surfactants! Caffeine as well, but it has much less of an effect due to its low concentration.
Bubbles of gas will struggle to form in very viscous liquids, like maple syrup or waffle batter. Diet soda actually has a slightly higher viscosity than sugary soda, which slightly diminishes its fizzing potential. However, a slightly higher viscosity means that when bubbles do form, they’re a bit more stable. This explains why Diet Coke not only fizzes more than classic Coke, but the foam also lasts longer!