Micelle

A micelle (rarely micella, plural micellae) is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms a roughly spherical or globular aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle center. Micelles are often globular and roughly spherical in shape, but ellipsoids, cylinders, and bilayers are also possible. The shape of a micelle is a function of the molecular geometry of its surfactant molecules and solution conditions such as surfactant concentration, temperature, pH, and ionic strength. The process of forming micelles is known as micellization.

Solvation

Individual surfactant molecules that are in the colloid but are not part of a micelle are called "monomers." In water, the hydrophilic "heads" of surfactant molecules are always in contact with bulk solvent, regardless of whether the surfactants exist as monomers or as part of a micelle. However, the hydrophobic "tails" of surfactant molecules have less contact with water when they are part of a micelle. In a micelle, the hydrophobic tails of several surfactant molecules assemble into an oil-like core that has less contact with water. In contrast, surfactant monomers are surrounded by water molecules that create a "cage" of molecules connected by hydrogen bonds. This water cage is similar to a clathrate and has an ice-like crystal structure.

Micelles composed of ionic surfactants are surrounded by a "cloud" of tightly-bound ions. Because these ions have a charge opposite or counter to the charge of the ionic surfactant, they are called counterions. Although the bound counterions partially neutralize a charged micelle (by up to 90%), the effects of micelle charge may affect the structure of the surrounding solvent at appreciable distances from the micelle. Ionic micelles can influence many properties of the mixture, including its electrical conductivity. Adding salt to a colloid containing micelles can decrease the strength of electrostatic interactions and lead to the formation of larger ionic micelles.

Energy of formation

Micelles only form when the concentration of surfactant is greater than the critical micelle concentration (CMC), and the temperature of the system is greater than the critical micelle temperature, or Krafft temperature. The formation of micelles can be understood using thermodynamics: micelles can form spontaneously because of a balance between entropy and enthalpy. In water, the hydrophobic effect is the driving force for micelle formation, despite the fact that assembling surfactant molecules together reduces their entropy. Broadly speaking, above the CMC, the entropic penalty of assembling the surfactant molecules is less than the entropic penalty of the caging water molecules. Also important are enthalpic considerations, such as the electrostatic interactions that occur between charged (or ionic) surfactants.

Reverse Micelles

In a nonpolar solvent, it is the exposure of the hydrophilic head groups to the surrounding bulk solvent that is energetically unfavorable. In this case the hydrophilic groups are sequestered in the micelle core and the hydrophobic groups remain solvent-exposed on the surface of the micelle. These reverse micelles are extremely difficult to form from surfactants with charged head groups, since hydrophilic sequestration would create highly unfavorable electrostatic interactions.

Uses

When surfactants are present above the CMC, they can act as emulsifiers that will solubilize a compound normally insoluble in the solvent being used. This occurs because the insoluble species can be incorporated into the micelle core, which is itself solubilized in the bulk solvent by virtue of the head groups' favorable interactions with solvent species. The most common example of this phenomenon is detergents, which clean poorly soluble hydrophobic material (such as oil, grease, or dirt) that cannot be cleaned by water alone. Detergent also helps clean by lowering the surface tension of water, making it easier to remove dirt from a surface. The emulsifying ability of surfactants is also the basis for emulsion polymerization.

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Solvation

Energy of formation

Reverse Micelles

Uses

See also