Critical Micelle Concentration

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Definition

The Critical Micelle Concentration (CMC) is the concentration of surfactants in a solution above which micelles form. Below the CMC, surfactants adsorb to a surface, lowering the surface 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 [2]. In chemistry, we look at the law of mass action to see how much of a reactant (in this case, individual surfactants) will form a product (in this case, micelles). Below the CMC, almost no micelles form, while above it, lots of micelles form.

Applications

A website from the Materials Research Science and Engineering Center at the University of Wisconsin describes an experiment exploring critical micelle concentration. Movie clips on the website show the addition of 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 hydrophobic tails, 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 in 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. Adding enough surfactant to get above the CMC could be a way to control the surface tension of a system by lessening the effects of a bit of surfactant added or taken out.

In the paper Colloid Surfactants for Emulsion Stabilization, the authors observe clusters of amphiphilic particles analogous to micelles of surfactant molecules. Do amphiphilic particles also obey a critical aggregation concentration?

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] I. Morrison, "Surfactant Phases," http://soft-matter.seas.harvard.edu/index.php/Surfactant_phases.

[4] J. Breitzer, M. Lye, & G. Lisensky, "Critical Micelle Concentration," University of Wisconsin, Madision, Materials Research Science and Engineering Center (July 11, 2008). http://www.mrsec.wisc.edu/Edetc/nanolab/micelle

[5] J. Kim, D. Lee, H. Shum, & D. Weitz, "Colloid Surfactants for Emulsion Stabilization," Adv. Mater. 20, 3239-3243 (2008).