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The critical micelle concentration (CMC) is defined as the concentration of 'surface active molecules' or surfactants above which micelles are spontaneously formed in water. (For someone who doesn't know what a micelle is, what is a micelle? What does it look like?)
Surfactants can arrange themselves at the surface of the water such that the one part interacts with the water and the other interacts with the air. However, the presence of these molecules on the surface lowers the surface tension. At low concentrations surfactants will favor arrangement on the surface because as the surface becomes crowded with surfactant more molecules will arrange into micelles. Based on this, measurements of surface tension can be used to determine the CMC.
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