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Name:  Malvika
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: 4/2/2005


Question:
Why does the dye spread out and swirl when you put a few drops of dye to some full cream milk and add a little detergent solution?


Replies:
Dyes are organic compounds. Although they contain groups that tend to make them water soluble -- food coloring and Easter egg dyes for example -- they are also hydrophobic. Both milk and various detergents contain certain types of chemicals called surfactants (an acronym for "surface active agents"). Surfactants have a molecular structure that has both a hydrophobic (water-hating) "end" and a hydrophylic (water-loving) "end". In water and/or milk they form bodies, called micelles, in which the hydrophobic ends "clump together" inside the micelle and the hydrophylic ends are pointed toward the water phase. Hydrophobic compounds such as dyes (and other 'oily' substances) migrate to the inside of the micelle. This increases their "solubility" in the detergent/water solution. In fact this is the mechanism for the way that soap removes dirt from your hands or clothes.

Milk all by itself contains some components that have this surface active behavior too, so in general you would find that an oily substance, a dye or otherwise, would tend to disperse more readily in milk than in pure water alone. You would find for example that a couple of drops of olive oil will disperse much more readily in milk than in water without surfactant.

Vince Calder



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