Cross section view of the structures that can be formed by phospholipids in aqueous solutions

Scheme of a micelle formed by phospholipids in an aqueous solution. Phospholipids are a class of Lipids and are a major component of all Biological membranes All phospholipids contain a Diglyceride, a Phosphate

A micelle (rarely micella, plural micelles) is an aggregate of surfactant molecules dispersed in a liquid colloid. Surfactants are wetting agents that lower the Surface tension of a liquid allowing easier spreading and lower the Interfacial tension between two liquids A colloid is a type of mechanical Mixture where one substance is dispersed evenly throughout another A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle centre. Hydrophile, from the Greek (hydros "water" and φιλια (philia "friendship" refers to a physical property of a Molecule A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. In Chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of This type of micelle is known as a normal phase micelle (oil-in-water micelle). Inverse micelles have the headgroups 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. Polymorphism in biophysics is the aspect of the behaviour of Lipids that influences their long-range order i A bilayer is a double layer of closely packed atoms or molecules See also Monolayer Lipid bilayer The shape and size 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. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature pH is the measure of the acidity or alkalinity of a Solution. The ionic strength, I, of a solution is a function of the Concentration of all Ions present in a Solution. The process of forming micellae is known as micellisation and forms part of the phase behaviour of many lipids according to their polymorphism. Polymorphism in biophysics is the aspect of the behaviour of Lipids that influences their long-range order i Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble Polymorphism in biophysics is the aspect of the behaviour of Lipids that influences their long-range order i

History

The ability of a soapy solution to act as a detergent has been recognised for centuries. However it was only at the beginning of the twentieth century that the constitution of such solutions was scientifically studied. Pioneering work in this area was carried out by James William McBain at the University of Bristol. As early as 1913 he postulated the existence of “colloidal ions” to explain the good electrolytic conductivity of sodium palmitate solutions. ^* These highly mobile, spontaneously formed clusters came to be called micelles, a term borrowed from biology and popularized by G. S. Hartley in his classic book “Paraffin Chain Salts, A Study in Micelle Formation”. ^*

Solvation

Individual surfactant molecules that are in the system but are not part of a micelle are called "monomers. " In water, the hydrophilic "heads" of surfactant molecules are always in contact with the solvent, regardless of whether the surfactants exist as monomers or as part of a micelle. However, the lipophilic "tails" of surfactant molecules have less contact with water when they are part of a micelle -- this being the basis for the energetic drive for micelle formation. In a micelle, the hydrophobic tails of several surfactant molecules assemble into an oil-like core the most stable form of which has no contact with water. By 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. Clathrate hydrates (or gas clathrates gas hydrates clathrates hydrates etc were first documented in 1810 by Sir Humphrey Davy; they are crystalline Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating

Micelles composed of ionic surfactants have an electrostatic attraction to the ions that surround them in solution, the latter known as counterions. Although the closest counterions partially mask a charged micelle (by up to 90%), the effects of micelle charge affect the structure of the surrounding solvent at appreciable distances from the micelle. Ionic micelles influence many properties of the mixture, including its electrical conductivity. Adding salts to a colloid containing micelles can decrease the strength of electrostatic interactions and lead to the formation of larger ionic micelles. This is more accurately seen from the point of view of an effective change in hydration of the system.

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. In Chemistry, the critical micelle concentration ( CMC) is defined as the concentration of Surfactants above which Micelles are spontaneously The Krafft temperature, or critical micelle temperature is the minimum Temperature at which Surfactants form Micelles Below the Krafft temperature there The formation of micelles can be understood using thermodynamics: micelles can form spontaneously because of a balance between entropy and enthalpy. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " A spontaneous process is the time-evolution of a system in which it releases free energy (most often as heat and moves to a lower more thermodynamically stable energy state In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy In Thermodynamics and molecular chemistry, the enthalpy (denoted as H, h, or rarely as χ) is a quotient or description of In water, the hydrophobic effect is the driving force for micelle formation, despite the fact that assembling surfactant molecules together reduces their entropy. The hydrophobic effect is the property that non-polar molecules tend to form intermolecular aggregates in an aqueous medium and analogous intramolecular interactions Broadly speaking, above the CMC, the entropic penalty of assembling the surfactant molecules is less than the entropic penalty of caging the surfactant monomers with water molecules. Also important are enthalpic considerations, such as the electrostatic interactions that occur between the charged parts surfactants.

Scheme of an inverse micelle formed by phospholipids in an organic solvent. Phospholipids are a class of Lipids and are a major component of all Biological membranes All phospholipids contain a Diglyceride, a Phosphate A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution.

Inverse/Reverse Micelles

In a non-polar solvent, it is the exposure of the hydrophilic head groups to the surrounding solvent that is energetically unfavourable, giving rise to a water-in-oil system. "Polar molecule" and "Non-polar" redirect here In this case the hydrophilic groups are sequestered in the micelle core and the hydrophobic groups extend away from the centre. These inverse micelles are proportionally less likely to form on increasing headgroup charge, since hydrophilic sequestration would create highly unfavorable electrostatic interactions.

Uses

When surfactants are present above the CMC (Critical micelle concentration), they can act as emulsifiers that will allow a compound normally insoluble (in the solvent being used) to dissolve. In Chemistry, the critical micelle concentration ( CMC) is defined as the concentration of Surfactants above which Micelles are spontaneously An emulsion ( IPA: /ɪˈmʌlʃən/ is a mixture of two Immiscible (unblendable liquids 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. A detergent (as a noun is a material intended to assist Cleaning. Detergents also clean by lowering the surface tension of water, making it easier to remove material from a surface. For the work of fiction see Surface Tension (short story. Surface tension is a property of the surface of a Liquid that causes it to The emulsifying property of surfactants is also the basis for emulsion polymerization. Emulsion polymerization is a type of Radical polymerization that usually starts with an Emulsion incorporating water Monomer, and Surfactant

Micelle formation is essential for the absorption of fat-soluble vitamins and complicated lipids within the human body. Bile salts formed in the liver and secreted by the gall bladder allow micelles of fatty acids to form. This allows the absorption of complicated lipids (e. g. , lecithin) and lipid soluble vitamins (A, D, E and K) within the micelle by the small intestine.

See also

• Vesicle (biology)
• Lipid bilayer
• Surfactant

References

• J. A vesicle is a small bubble of liquid within a cell A more formal definition in Cell biology, would be that a vesicle is a relatively small intracellular membrane-enclosed A lipid bilayer or bilayer lipid membrane ( BLM) is a membrane composed of Lipid molecules (usually Phospholipids. Surfactants are wetting agents that lower the Surface tension of a liquid allowing easier spreading and lower the Interfacial tension between two liquids M. Seddon, R. H. Templer. Polymorphism of Lipid-Water Systems, from the Handbook of Biological Physics, Vol. 1, ed. R. Lipowsky, and E. Sackmann. (c) 1995, Elsevier Science B. V. ISBN 0-444-81975-4
• S. A. Baeurle, J. Kroener, Modeling effective interactions of micellar aggregates of ionic surfactants with the Gauss-Core potential, J. Math. Chem. 36, 409-421 (2004).
• McBain, J. W. , Trans. Faraday Soc. 1913, 9, 99.
• Hartley, G. S. , Aqueous Solutions of Paraffin Chain Salts, A Study in Micelle Formation, 1936, Hermann et Cie, Paris.