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Micelles are agglomerates of surface-active molecules (surfactants) in a solvent, usually water. Many of the properties associated with surfactants, such as their cleaning effect, are based on micelles. The ability to form micelles is a characteristic of all soluble surfactants.

How are micelles formed?

Surfactants are amphiphilic molecules, i.e., they consist of a hydrophilic (water-affinity) and a hydrophobic, lipophilic (water-repellent) part. As individual molecules (monomers) in solution, they accumulate on the surface and arrange themselves in such a way that the hydrophobic, long-chain component ("tail") faces outwards, while the hydrophilic "head" faces inwards into the bulk phase. As the surface becomes increasingly occupied, the surface tension decreases.

Once the surface is completely occupied and there is no room for further surfactant molecules, the surface tension does not decrease any further. If the concentration increases further, the additional surfactant molecules arrange themselves into micelles. These are mostly spherical structures in which the hydrophobic chains are directed inwards and the hydrophilic heads are directed outwards.

The common term "head" for the hydrophilic group and its representation as a sphere in most illustrations suggests that this group is always small and short-chain. In fact, however, the hydrophilic group can also be long-chain, so this popular explanation is somewhat misleading.

What is the critical micelle concentration?

The concentration above which micelles form is called the critical micelle concentration (CMC). The CMC is a substance-dependent value and an important parameter for surfactants.

▶ For more details, see the glossary article on critical micelle concentration (CMC).

What factors influence the formation of micelles?

In addition to the chemical structure of the surfactant and its concentration, the type of solvent, its salt content and pH value, and the temperature play an important role in the formation of micelles. 

How is the formation of micelles related to temperature?

The formation of micelles is a thermodynamic process and, as such, depends on temperature. In order for micelles to form, surfactant molecules must be present in solution as monomers. This solubility is only possible under suitable temperature conditions. There is a significant difference between ionic and non-ionic surfactants:

• In ionic surfactants (e.g., salts of fatty acids or long-chain sulfonic acids, such as sodium dodecyl sulfate = SDS), solubility and, accordingly, the ability to form micelles increases with temperature. The minimum temperature for micelle formation is called the Krafft temperature. This value differs between surfactants.
• Non-ionic surfactants (e.g., long-chain ethoxylates) become less soluble as the temperature increases. Above a substance-specific temperature known as the cloud point, micellar structures disintegrate or form larger aggregates. The solution separates into a surfactant-enriched phase and a surfactant-poor phase. 

What is the basis for the washing and cleaning effect of micelles?

First, surfactants lower the surface tension of the water, ensuring better wetting of the surface to be cleaned and more intensive penetration of textiles during the washing process. However, micelles are responsible for dissolving and removing hydrophobic contaminants (grease, oil, soot, etc.). The hydrophobic molecules are stored inside the micelles and mobilized.

The cosmetics industry expects micelles, for example in so-called micellar water, to remove impurities from the skin in a gentle manner. These formulations are usually free of oils or organic solvents and can be left on the skin after application or simply washed off with water if necessary. Solutions for industrial solvent-free degreasing are also based on the effect of micelles.

How are micelles used in pharmacy?

The ability of micelles to dissolve and mobilize fats and oils in an aqueous phase is used for hydrophobic pharmaceutical active ingredients. Embedded in micelles, these substances can be taken in a water-based liquid medicine and transported in the body.

Can surfactants form other types of molecular agglomerates?

If the concentration is significantly increased above the CMC, larger molecular assemblies can build up. The number of monomers bound in a micelle is largely constant, so larger micelles do not form, but their number initially increases. At very high concentrations, more complex structures such as rod-shaped agglomerates or double-layered lamellae are formed. Flowable structures with a long-range order assemble. These are referred to as liquid crystalline systems. 

What are reverse micelles?

Due to their amphiphilic nature, many surfactants are soluble in organic, hydrophobic solvents as well as in water. In this case, the orientation of the surfactant groups is reversed. In the agglomerate known as a reverse or inverse micelle, the hydrophobic groups face outwards and the hydrophilic groups face inwards.

Reverse micelles are used, for example, in extraction with supercritical carbon dioxide. With the addition of surfactants, the process can also be used for polar substances that are poorly soluble in CO₂.

How can the formation of micelles be explained physically?

Under certain conditions (concentration, temperature), micelle formation occurs as a spontaneous equilibrium process. The process is exothermic, i.e., energy is released and the micelle formation enthalpy ΔH_mic has a negative sign. 

The micelle formation enthalpy can be calculated from the change in CMC with temperature according to this formula. The equation reflects the fact that the chemical equilibrium of micelle formation shifts toward the starting product (surfactant monomer) as the temperature increases, so that micelles only form at higher concentrations. The CMC therefore increases with increasing temperature.

Literature

• J. Eastoe, A. Dupont, D. C Steytler, M. Thorpe, A. Gurgel, R. K. Heenan: Micellization of Economically Viable Surfactants in CO2 . Journal of Colloid and Interface Science 258 (2), 2003, pp. 367-373.
• H. Høiland, A. M. Blokhus: Solubilization in Aqueous Surfactant Systems. Introduction. In: K. S. Birdi (ed.): Surface and Colloid Chemistry. Bota Racon, New York, 1997, p. 239f.
• F. Krafft, A. Stern: On the Behavior of Fatty Acid Alkalis and Soaps in the Presence of Water. Parts I and II: Reports of the German Chemical Society, 27 (1894), pp. 1747–1761.
• G. S. Hartley: Aqueous Solutions of Paraffin-Chain Salts. A Study in Micelle Formation. Paris 1936.
• F.J. Mooney: The Formation of Micelles. Journal of Physical Chemistry, 48(5), 1944. pp. 329-334.