Micelles

Micelle: Particle of colloidal dimensions that exists in equilibrium with the
molecules or ions in solution from which it is formed.

Micelle (polymers): Organized auto-assembly formed in a liquid and
composed of amphiphilic macromolecules, in general amphiphilic di-
or tri-block copolymers made of solvophilic and solvophobic blocks.

Note 1: An amphiphilic behavior can be observed for water and an organic
solvent or between two organic solvents.

Note 2: Polymeric micelles have a much lower critical micellar concentration
(CMC) than soap or surfactant micelles, but are nevertheless at equilibrium
with isolated macromolecules called unimers. Therefore, micelle formation
and stability are concentration-dependent.

A micelle (/maɪˈsɛl/) or micella (/maɪˈsɛlə/) (plural micelles or micellae, respectively) 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 phases, including shapes such as ellipsoids, cylinders, and bilayers, are also possible. The shape and size of a micelle are 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 micellisation and forms part of the phase behaviour of many lipids according to their polymorphism.

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