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Encyclopedia > Micelle
Schematic of a micelle.
Schematic of a 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 an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle centre. This type of micelle is know 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. 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. The process of forming micellae is known as micellisation and forms part of the phase behaviour of many lipids according to their polymorphism. Image File history File links MicelleSchematic. ... Image File history File links MicelleSchematic. ... Surfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids. ... In general, a colloid or colloidal dispersion is a substance with components of one or two phases, a type of mixture intermediate between a homogeneous mixture (also called a solution) and a heterogeneous mixture with properties also intermediate between the two. ... Drinking water This article focuses on water as we experience it every day. ... The adjective hydrophilic describes something that likes water (from Greek hydros = water; philos = friend). ... A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ... In chemistry, hydrophobic or lipophilic species, or hydrophobes, tend to be electrically neutral and nonpolar, and thus prefer other neutral and nonpolar solvents or molecular environments. ... Polymorphism in biophysics is the aspect of the behaviour of lipids that influences their long-range order, i. ... A bilayer is a closely packed double layer of atoms or molecules. ... Fig. ... The correct title of this article is . ... The ionic strength of a solution is a function of the concentration of all ions present in a solution. ... Polymorphism in biophysics is the aspect of the behaviour of lipids that influences their long-range order, i. ... A polyunsaturated triglyceride. ... Polymorphism in biophysics is the aspect of the behaviour of lipids that influences their long-range order, i. ...

Contents

Solvation

Individual surfactant molecules that are in the system but are not part of a micelle are called "unimers." 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 are a class of solids in which gas molecules occupy cages made up of hydrogen-bonded water molecules. ... Snowflakes by Wilson Bentley, 1902 Ice is the name given to any one of the 14 known solid phases of water. ... Quartz crystal Synthetic bismuth crystal Insulin crystals Gallium, a metal that easily forms large single crystals A huge monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA. In chemistry and mineralogy, a crystal is a solid in which the constituent atoms, molecules...


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. 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 the charged parts surfactants. In chemistry, the critical micelle concentration (CMC) is defined as the concentration of surfactants in free solution in equilibrium with surfactants in aggregated form. ... The Krafft temperature, or critical micelle temperature, is the minimum temperature at which micelles form from surfactants. ... Thermodynamics (from the Greek θερμη, therme, meaning heat and δυναμις, dunamis, meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. ... A spontaneous process in chemical reaction terms is one which occurs with the system releasing free energy in some form (often, but not always, heat) and moving to a lower energy, hence more thermodynamically stable, state. ... Ice melting - classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ... In thermodynamics and molecular chemistry, the enthalpy or heat content (denoted as H or ΔH, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the useful work obtainable from a closed thermodynamic system under constant pressure. ... The hydrophobic effect is the property that nonpolar molecules like to self-associate in the presence of aqueous solution. ...


Inverse 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. 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. In chemistry, a nonpolar compound is one that does not have concentrations of positive or negative electric charge. ...


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. 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 also clean 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. In chemistry, the critical micelle concentration (CMC) is defined as the concentration of surfactants in free solution in equilibrium with surfactants in aggregated form. ... An emulsion is a mixture of two immiscible substances. ... Laundry detergents are just one of many possible uses for detergents Detergent is a compound, or a mixture of compounds, intended to assist cleaning. ... In physics, surface tension is an effect within the surface layer of a liquid that causes that layer to behave as an elastic sheet. ... Emulsion polymerization is a type of polymerization that takes place in an emulsion typically 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 (ie, lecithin) and lipid soluble vitamins (A, D, E and K) by the small intestine within the micelle.


See also

This article or section is in need of attention from an expert on the subject. ... A diagonal molecular slab from the DPPC lipid bilayer simulation1; color scheme: PO4 - green, N(CH3)3 - violet, water - blue, terminal CH3 - yellow, O - red, glycol C - brown, chain C - grey. ... Surfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids. ...

References

J. M. Seddon, R. H. Templer. The Polymorphism of Lipid-Water Systems. From the Handbook of Biological physics, Vol. 1, ed. R. Lipowsky, and E. Sackman. (c) MCMXCV Elsevier Science B.V. All rights reserved.


  Results from FactBites:
 
Spartanburg SC | GoUpstate.com | Spartanburg Herald-Journal (951 words)
A micelle (rarely micella, plural micellae) is an aggregate 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 tail regions in the micelle centre.
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.
Ultrasonic Method of Determining the Critical Micelle Concentration of Surfactants (204 words)
An examination of the models of the micelle structure shows that the major concept is that several micellar structures are possible and do exist.
The formation of a micelle and each different type of its structure is looked on as that of a new phase.
Hence we expected that an examination of ultrasonic velocity at different concentrations in the region of the critical micelle concentration should exhibit a discontinuity in the plot of velocity versus concentration.
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