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Micelles (singular "micelle"), or micellae (singular "micella"), are spherical clusters of hydrocarbon molecules that act as emulsifying agents. 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 type of micelle is known as a normal-phase micelle (oil-in-water micelle). An Inverse micelle has a hyprophobic and hydrophilic side, with the hyrodphilic side at the center and the hydrophobic side facing the solvent. 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.
Micelles form when the polar head and the non polar tails arrange in a special way. They are usually driven to arrange either with the polar heads out (oil in water) or with the polar head in (water in oil). Micelles only form when the concentration of surfactant is greater than the critical micelle concentration. The surfactant is any surface active material that can part the surface upon entering. The higher the critical micelle concentration, the more micelles there are. Micelle formation also depend on the Krafft temperature. If the temperature is below the Krafft temperature, then there is no spontaneous formation of micelles. As the temperature increases, the surfactant will turn into a soluble form and be able to form micelles from a crystalline state. The hydrophobic effect is also a driving force that needs to be taken into account. This effect is characterized by the fact that like to form intermolecular aggregates in aqueous substances and in intramolecular molecules. Micelle formation can be summed up by thermodynamics, driven by entropy and enthalpy.
The micelle packing parameter equation is utilized to help "predict molecular self-assembly in surfactant solutions":
Role in oil emulsion
When surfactants are present above the critical micelle concentration , they can act as emulsifiers that will allow a compound that is normally insoluble (in the solvent being used) to dissolve. 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 lipophilic material (such as oils and waxes) that cannot be removed by water alone. Detergents clean also by lowering the surface tension of water, making it easier to remove material from a surface. The emulsifying property of surfactants is also the basis for emulsion polymerization.
- Micelle. 2015. In Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/w/index.php?title=Micelle&oldid=694423373
- Friedrich Krafft. 2015. In Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/w/index.php?title=Friedrich_Krafft&oldid=668396294
- Nagarajan, R. 2002. "Molecular Packing Parameter and Surfactant Self-Assembly: The Neglected Role of the Surfactant Tail†". Langmuir 18: 31. http://pubs.acs.org/doi/abs/10.1021/la010831y
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