Detergents contain surfactant molecules that give soaps and detergents some interesting properties which make soap films possible. They have one end that likes water and one end that rejects it. These two ends of the molecule also do not like each other. So, surfactant molecules become evenly distributed with the water-hating ends sticking out of the solution.
When there are too many surfactant molecules for the molecules to be comfortably spaced, they collect in balls called micelles (illustration: left panel) with their water-hating end in the center and the water-loving end on the outside. As more water is added to a solution that contains micelles, surfactant molecule leave the micelles if they can maintain their preferred spacing. As more water is added, there will be a point where there is just enough water so that there will be no more micelles and the surfactant molecules are tightly packed. This concentration is called the Critical Micelle Concentration or CMC.At concentrations above the CMC, the surface tension is relatively stable regardless of the actual concentration. When the concentration approaches and exceeds the CMC, the surface tension of the solution increases.
For more information, see the Wikipedia article.
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