|MadSci Network: Chemistry|
Interesting question! I don't have any data to share to confirm my thoughts on this topic but I am sure your local library will be able to help you find some physical chemistry texts with explanations of surfactant chemistry or foaming... The different types of sweeteners used in the two products is causing the different foaming behaviors. High fructose corn syrup (HFCS), used in regular Coke, is rather syrupy or soapy and increases the viscosity of the liquid. It also impacts the surface tension of the beverage but I can't remember if it is higher or lower (lower, I think, since soaps do the same thing). In any case, foam is collapsed by causing drainage of the liquid from the lamellar region (thin liquid film between bubbles). The aspartame in the diet Coke is making this drainage happen more slowly than the HFCS in the regular version. The diet's foam lasts longer and it looks like it fizzes more than the regular. And the two usually have the same amount of carbonation (assuming you opened them at the same time and they were at the same temperature). You could try doing a quick comparison of surface tensions between the two at home: pour a glass of each soda and let them stand out overnight until they are flat. Try this surface tension experiment using the two sodas. It should be easier to "float" the blade on one surface (the one with higher surface tension). You can also try comparing your results with glasses of regular and soapy water of the same temperature... I hope this information helps! Kieran
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