Why are bubbles blue?

Well, if you haven’t tried Mountain Dew Pitch Black, please don’t on my account. It is a disgusting grape flavor. But there is one cool thing about it: It has blue foam. This is kind of interesting because the soda itself appears to be black. One can’t just let strange things like this abide untested.
If you look closely at the drink, it is not actually black — it is a dark purple. However, this is clearly a different hue than the fizzing bubbles. What gives? Your’s truly investigates!
My first hypothesis is that the dye used in the beverage is sensitive to pH, and also that the bubbles in the beverage (which are in close proximity to the off gassing CO₂) may be at a different pH than the rest of the beverage. I know that carbon dioxide is a weak acid, but I’m only guessing that the water in the foam is a different pH. To test this hypothesis, I poured a small amount of Mountain Dew Pitch Black into two clear glass containers — one with some baking soda ( a base ) and one with vinegar ( an acid ). The color of the liquid remained the same in both. This shoots down my first hypothesis.
However, when I mixed the two together, the vinegar and baking soda reacted most pleasantly. And the foam turned blue, too…
Next, I hypothesized that there might be two separate dyes in the drink — one of them blue. Somehow the act of foaming up separates these two ingredients. Or, it could be that the blue dye is in much greater concentration, so that when it is diluted in the foam it still appears blue when the other dye no longer is effective. To test this hypothesis, we used paper chromatography. We placed a thin line of soda on a paper towel, and then put a few ice cubes next to the line. With the help of a few drops of water from a wet hand once in a while, the water melted off the ice cubes and wicked through the paper towel. As the water moved through the paper, the dye molecules were dragged along with it. But due to their different molecular weight and affinity for paper, they moved at different rates. The result was a separate band of color for each dye in the soda.

Furthermore, I discovered on The mountain dew website that sure enough, it contains two dyes: Blue 1 and Red 40. With the help of the National Library of Medicine’s Household Products Index, I was able to locate the molecular structures of the two dyes in the Chemical Information Database. Here they are:

FD&C Blue 1 (left) and FD&C Red 40 (right)
There are a few simple things that can be noted by looking at the structures. First, Blue 1 is a bigger molecule. Bigger molecules tend to diffuse more slowly. Interestingly, the blue strip on my paper chromatography experiment moved faster than the red stripe — so the red dye although smaller must interact more strongly with paper.
So why is the foam blue? My hypothesis that the dyes are getting separated in the foam seems highly improbable — purifying mixtures is a very hard thing to do. Try desalinating water, or filtering out a dye sometime. I’m pretty confident that the foam appears blue simply because the blue dye is in the soda at a higher concentration, so that when diluted you see mostly the blue color. The reason the hue can change during dilution is only if the blue dye is over saturated enough so that it remains blue long after the red dye has been diluted away.
We know from experience that soda pop foam is diluted — Coke has creamy white foam even though the drink is caramel colored. Regular mountain dew foam is barely a butter yellow color even though the drink is bright lemon yellow.
I’m waiting for the chance to test this hypothesis, by performing a serial dilution of the beverage. I think that the color should change to blue if you mix the right amount of water with it. Stay tuned.

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(Later…) I just had another idea. It might be possible that the blue dye is actually preferentially associated with the foam. I’m thinking of soap as an example. Soap is a long molecule that has different hydrophobicity at each end. One end dissolves very well in water (like sugar or salt) — this end is hydrophillic (water loving). The other end is very oily and would rather be out of the water — that end is hydrophobic (water hating). The result is that the oily end of soap molecules surround greasy dirt and allow water to wash them away. Another side effect of soap is soap bubbles — which are tiny molecular sandwiches with the oily parts of the soap molecules sticking into the air. It never occurred to me before, but soap bubbles may have a higher concentration of soap in them than the rest of the solution.
So, if the blue dye in the Mountain Dew is a soapy molecule (soapier than the red), then it might preferentially stay in bubbles because the oily part of the molecule likes to stick in the air pockets. I can think of two more experiments to test this hypothesis. First of all, we could try to harvest the blue foam off the soda,then let the bubbles go away and compare the color of that liquid with the original. If the liquid is bluer, then we have proof that there was molecular separation.
The second experiment is to add some dish detergent to the Mountain Dew. The dish detergent is most likely a much better soap than the blue dye. It might fill up all the space in the bubbles, and it will also surround the oily parts of the dye — so it might cause the foam to no longer be blue. I’m not really sure this will work, because the soap will also cause there to be a lot more bubbles. That might nullify the first effects.
One other observation supports this latest hypothesis. But FD&C Red 1 is a flat, rigid mostly oily molecule with two water loving sulfates on either end. This probably isn’t a very good soap because the oily part is surrounded by the water loving part (remember soaps need to be polar to work well). FD&C Blue 40, on the other hand has three water loving sulfates, two of which are on floppy molecular connections that allow them to swivel around to the same side. That allows Blue 40 to have an oily side and a water loving side — just the recipie for soap. That oily side will like to associate with the surface of the water — which foam has a *lot* of.