Micelle

A micelle is an aggregate (or "supramolecular assembly") of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre. This phase is caused by the packing behavior of single-tail lipids in a bilayer. The difficulty filling all the volume of the interior of a bilayer, while accommodating the area per head group forced on the molecule by the hydration of the lipid head group, leads to the formation of the micelle. This type of micelle is known as a normal-phase micelle (oil-in-water micelle). Inverse micelles have the head groups at the centre with the tails extending out (water-in-oil micelle).

Micelles are approximately spherical in shape. Other shapes can also occur depending on the size and shape of the surfactant molecule itself, and the forces of repulsion and attraction between the surfactant molecules, the solvent molecules, and the solute particles or droplets.

Structure and formation

Micelles form when the concentration of surfactant is greater than the critical micelle concentration (CMC) and is temperature-dependent. At concentrations above the CMC, surfactants form micelles. When surfactants are present in low concentrations, they act as normal solute molecules. Each surfactant molecule is surrounded by solvent molecules. With increasing surfactant concentration, the number of "free" surfactant molecules decreases and aggregates of surfactant molecules, or micelles, start to form. The formation of micelles can be understood using thermodynamics: Micelles can form spontaneously because of a balance between entropy and enthalpy.

Uses

Micelles are used in a variety of applications, from targeting drugs to the site of disease to enabling chemical reactions in the lab. In the case of drug delivery, they can be loaded with poorly soluble drugs, allowing these drugs to be dissolved in water and delivered directly to the disease site. This has the advantage of increasing the concentration of the drug at the site of action and reducing the concentration of the drug in the rest of the body, thereby reducing side effects.

See also

• Liposome
• Bilayer
• Surfactant

External links

• Micelle at Britannica
• Micelle at Chemguide

Micelle

• 
  Phospholipids aqueous solution structures
• 
  Micelle scheme
• Supramolecular assembly of micelles
  Supramolecular assembly of micelles
• Action of soap on oil
  Action of soap on oil