Solution to the M&M Mystery?

Last year I wrote a blog about a very simple experiment can be done using water, a plate and M&M’s candies. The experiment can be seen in the video below:

Since posting about this experiment, several ChemEd X readers¹ and I have been actively discussing this experiment. Specifically, we have been trying to figure out why the colors stay in very distinct regions with sharp, well defined lines (Figure 1). 

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Figure 1:  Formation of distinct regions of color when water is carefully poured onto a plate of M&M’s candies.

Osmosis, polarity, density, emulsion formation and colloid formation were all discussed as possible processes involved in the formation of these clear-cut areas of color.  However, there is one idea we have not yet discussed, and I think it may be what is responsible for keeping the colors apart:  electric charge.

If all the dyes have the same charge, then certainly they should repel each other to some extent.  The repulsion of the like charges on these dyes would provide a very simple explanation for why the dyes stay in their own distinct region of space.  After all, like charges repel. 

After stumbling upon this idea, I checked the ingredients list of M&M’s milk chocolate candies.  Upon doing so, I noticed that they contain several different food dyes (Blue 1, Blue 2, Yellow 5, Yellow 6 and Red 40).  Upon looking up the structure of these dyes online, I discovered that all of these dyes are indeed negatively charged ions!

I decided to see if I could try a simple experiment to demonstrate the fact that food dyes are negatively charged.  I formed a gel using water and the polymer found in baby diapers (you can learn more about how to make such gels here; fast forward the video to about 1:10).  Side note:  I also mixed in a pinch of baking soda.  I poured the gel inside a small Tupperware container, and placed a piece of aluminum foil at opposite sides of the container.  I placed one drop of red food dye (Red 3 and/or Red 40, according to the manufacturer) in the middle of the Tupperware container.  Next, I connected six 9V batteries in series and used aluminum foil to connect the terminals of the battery assembly to the foil pieces on the opposite sides of the container (Figure 2):

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Figure 2:  Experimental set up used to demonstrate that red food dye is negatively charged.

Over the course of a half an hour, the food dye had migrated to the positive pole of the battery assembly (Figure 3).

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Figure 3:  Migration of red food dye

I next decided to try the same thing, but use M&M's candies instead.  This time I formed the gel in a large glass dish, placed aluminum foil on the ends of the dish and added M&M’s in a single line down the middle (Figure 4):

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Figure 4:  Initial placement of M&M’s in gel

The battery assembly was attached (Figure 5).

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Figure 5:  Experimental set up used to demonstrate that food dyes in M&M's candies are negatively charged.

And over the course of an hour, the dyes in the M&M’s had migrated to the positive pole of the battery assembly!

So it looks as though the dyes in M&M’s are all negatively charged, and this characteristic of these dyes seems to me to be a good explanation as to why they avoid mixing (Figure 1).  What do you all think? 

Reference: 

1.  Ben Barth, Erica Jacobsen, William Farmer, Lowell Thompson, and Mark Langella.  Both Lowell Thompson and I have had our students investigate what might be causing this non-mixing effect.  You can read about the explorations of Lowell’s students here.