Critical Micelle Concentration

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Under Construction by Rebecca Perry

Definition

The Critical Micelle Concentration (CMC) is the concentration of surfactants in a solution above which micelles form out of any additional surfactant added to the system. Below the CMC, surfactants adsorb to an interface, lowering the interfacial tension, or disperse within the bulk. There may be an occasional micelle formed below the CMC. Likewise, above the CMC, a very limited amount of added surfactants may enter the dispersed phase or adsorb onto an interface. However, above the CMC, the vast majority of added surfactant will form micelles.

The Critical Micelle Concentration is related to the law of mass action. In chemistry, we look at the law of mass action to see how much of a reactant (individual surfactants) will form a product (micelles). Below the CMC, almost no micelles form, while above it, lots of micelles form

Applications

The University of Wisconsin, Madison website has a lab experiment with movies of the steps exploring critical micelle concentration. The experiment requires adding SDS (a surfactant) to water while a strip containing water-insoluable dye sits in the container. At first, no dye leaves the strip. However, once micelles form localized pockets of tail groups, the dye moves from the paper into the centers of the micelles which are free to float about the container.

In industry, micelles are important in soaps, cosmetics, and even oil recovery. Knowing the CMC can help develop a product with certain characteristics. It is interesting that above the CMC, the surface tension is fairly constant. Using surfactant could be a way to control the surface tension even if you are not interested in the micelles in the bulk.

In the paper Colloid Surfactants for Emulsion Stabilization, the authors observe clusters of amphiphilic particles analogous to micelles of surfactant molecules.

References

[1] R. Jones, "Soft Condensed Matter," Oxford University Press Inc., New York (2002).

[2] T. Witten, "Structured Fluids: Polymers, Colloids, Surfactants," Oxford University Press Inc., New York (2004).

[3] http://soft-matter.seas.harvard.edu/index.php/Surfactant_phases

[4] http://www.mrsec.wisc.edu/Edetc/nanolab/micelle/index.html ed by George Lisensky, Beloit College. Last modified July 11, 2008 .

[5] Colloid Surfactants for Emulsion Stabilization.