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A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or
supramolecular assembly thumbnail, Circular helicate 9+,_where_L_stands_for_s_tris-bpy_ligand_strand;_the_central_gray_atom_is_Cl,_while_the_smaller_gray_spheres_are_Fe..html" ;"title="Fe5L5)Clsup>9+, where L stands for s tris-bpy ligand strand; the central gray atom is ...
) of
surfactant Surfactants are compounds that lower the (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as s, agents, , s, or s. The word "surfactant" is a of ''surface-active a ...

surfactant
phospholipid Phospholipids, also known as phosphatides, are a class of lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular bi ...

phospholipid
molecules dispersed in a liquid, forming a
colloidal suspension A colloid is a mixture In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior ...

colloidal suspension
(also known as associated colloidal system). A typical micelle in
water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , even though it provide ...
forms an aggregate with the
hydrophilic A hydrophile is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms ...
"head" regions in contact with surrounding
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

solvent
, sequestering the
hydrophobic In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of at ...
single-tail regions in the micelle centre. This phase is caused by the packing behavior of single-tail
lipids In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

lipids
in a
bilayer :''For bilayers in biology, see lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cell (biology), ce ...
. 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
phase Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
s, including shapes such as ellipsoids, cylinders, and
bilayer :''For bilayers in biology, see lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cell (biology), ce ...

bilayer
s, 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 Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
,
pH
pH
, and
ionic strength The ionic strength of a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the ...
. The process of forming micelles is known as micellisation and forms part of the phase behaviour of many lipids according to their polymorphism.


History

The ability of a soapy solution to act as a
detergent A detergent is a surfactant Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting ...
has been recognized 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 James William McBain FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resour ...
at the
University of Bristol The University of Bristol is a Red brick university, red brick Russell Group research university in Bristol, England. It received its royal charter in 1909, although it can trace its roots to a Society of Merchant Venturers, Merchant Venturers' sc ...

University of Bristol
. As early as 1913, he postulated the existence of "colloidal ions" to explain the good electrolytic conductivity of
sodium palmitate Palmitic acid, or hexadecanoic acid in IUPAC nomenclature The International Union of Pure and Applied Chemistry (IUPAC) has published four sets of rules to standardize chemical nomenclature. There are two main areas: * IUPAC nomenclature of in ...

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''. The term ''micelle'' was coined in nineteenth century scientific literature as the ''elle''
diminutive A diminutive is a root word A root (or root word) is the core of a word that is irreducible into more meaningful elements. In morphology, a root is a morphologically simple unit which can be left bare or to which a prefix A prefix is an aff ...
of the Latin word ''mica'' (particle), conveying a new word for "tiny particle".


Solvation

Individual surfactant molecules that are in the system but are not part of a micelle are called "
monomer In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

monomer
s". Micelles represent a
molecular assembly
molecular assembly
, in which the individual components are thermodynamically in equilibrium with monomers of the same species in the surrounding medium. 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 having no contact with water. By contrast, surfactant monomers are surrounded by water molecules that create a "cage" or solvation shell connected by
hydrogen bonds A hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of ...
. This water cage is similar to a
clathrate A clathrate is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched a ...
and has an
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

ice
-like
crystal A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformatio ...

crystal
structure and can be characterized according to the hydrophobic effect. The extent of lipid solubility is determined by the unfavorable entropy contribution due to the ordering of the water structure according to the hydrophobic effect. Micelles composed of ionic surfactants have an electrostatic attraction to the ions that surround them in solution, the latter known as
counterion image:Polystyrolsulfonat.svg, 160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with Na+ as the counterion. A counterion (pronounced as two words, i.e. "counter" "ion", and sometimes written as two words) is the ion that a ...
s. Although the closest counterions partially mask a charged micelle (by up to 92%), 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 charge in hydration of the system.


Energy of formation

Micelles form only when the concentration of surfactant is greater than the
critical micelle concentration In colloidal and surface chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and t ...
(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 Thermodynamics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in ot ...

thermodynamics
: Micelles can form
spontaneously
spontaneously
because of a balance between
entropy Entropy is a scientific concept as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamic ...

entropy
and
enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant p ...

enthalpy
. In water, the
hydrophobic effect The hydrophobic effect is the observed tendency of nonpolar In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make u ...
is the driving force for micelle formation, despite the fact that assembling surfactant molecules is unfavorable in terms of both enthalpy and entropy of the system. At very low concentrations of the surfactant, only monomers are present in solution. As the concentration of the surfactant is increased, a point is reached at which the unfavorable entropy contribution, from clustering the hydrophobic tails of the molecules, is overcome by a gain in entropy due to release of the solvation shells around the surfactant tails. At this point, the lipid tails of a part of the surfactants must be segregated from the water. Hence, they start to form micelles. In broad terms, above the CMC, the loss of entropy due to assembly of the surfactant molecules is less than the gain in entropy by setting free the water molecules that were "trapped" in the solvation shells of the surfactant monomers. Also important are enthalpic considerations, such as the electrostatic interactions that occur between the charged parts of surfactants.


Micelle packing parameter

The micelle packing parameter equation is utilized to help "predict molecular self-assembly in surfactant solutions": : \frac where v_o is the surfactant tail volume, \ell_o is the tail length, and a_e is the equilibrium area per molecule at the aggregate surface.


Block copolymer micelles

The concept of micelles was introduced to describe the core-corona aggregates of small
surfactant Surfactants are compounds that lower the (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as s, agents, , s, or s. The word "surfactant" is a of ''surface-active a ...

surfactant
molecules, however it has also extended to describe aggregates of
amphiphilic An amphiphile (a.k.a. amphipath; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. ...
block copolymers in selective solvents. It is important to know the difference between these two systems. The major difference between these two types of aggregates is in the size of their building blocks. Surfactant molecules have a
molecular weight A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...
which is generally of a few hundreds of grams per mole while block copolymers are generally one or two orders of magnitude larger. Moreover, thanks to the larger hydrophilic and hydrophobic parts, block copolymers can have a much more pronounced amphiphilic nature when compared to surfactant molecules. Because of these differences in the building blocks, some block copolymer micelles behave like surfactant ones, while others don't. It is necessary therefore to make a distinction between the two situations. The former ones will belong to the ''dynamic micelles'' while the latter will be called ''kinetically frozen micelles.''


Dynamic micelles

Certain amphiphilic block copolymer micelles display a similar behavior as surfactant micelles. These are generally called dynamic micelles and are characterized by the same relaxation processes assigned to surfactant exchange and micelle scission/recombination. Although the relaxation processes are the same between the two types of micelles, the kinetics of unimer exchange are very different. While in surfactant systems the unimers leave and join the micelles through a
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration In chemistry Chemistry is the study of the properties and behavior of . It is a that covers ...
-controlled process, for copolymers the entry rate constant is slower than a diffusion controlled process. The rate of this process was found to be a decreasing power-law of the
degree of polymerizationThe degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by DP ...

degree of polymerization
of the hydrophobic block to the power 2/3. This difference is due to the coiling of the hydrophobic block of a copolymer exiting the core of a micelle. Block copolymers which form dynamic micelles are some of the tri-block Poloxamers under the right conditions.


Kinetically frozen micelles

When block copolymer micelles don't display the characteristic relaxation processes of surfactant micelles, these are called ''kinetically frozen micelles''. These can be achieved in two ways: when the unimers forming the micelles are not soluble in the solvent of the micelle solution, or if the core forming blocks are glassy at the temperature in which the micelles are found. Kinetically frozen micelles are formed when either of these conditions is met. A special example in which both of these conditions are valid is that of polystyrene-b-poly(ethylene oxide). This block copolymer is characterized by the high hydrophobicity of the core forming block,
PS
PS
, which causes the unimers to be insoluble in water. Moreover, PS has a high
glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous In condensed matter physics Condensed matter physics is the field of that deals with the macroscopic and microscopic physical propert ...
which is, depending on the molecular weight, higher than room temperature. Thanks to these two characteristics, a water solution of PS-PEO micelles of sufficiently high molecular weight can be considered kinetically frozen. This means that none of the relaxation processes, which would drive the micelle solution towards thermodynamic equilibrium, are possible. Pioneering work on these micelles was done by Adi Eisenberg. It was also shown how the lack of relaxation processes allowed great freedom in the possible morphologies formed. Moreover, the stability against dilution and vast range of morphologies of kinetically frozen micelles make them particularly interesting, for example, for the development of long circulating drug delivery nanoparticles.


Inverse/reverse micelles

In a
non-polar In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
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 center. These inverse micelles are proportionally less likely to form on increasing headgroup charge, since hydrophilic sequestration would create highly unfavorable electrostatic interactions. It is well established that for many surfactant/solvent systems a small fraction of the inverse micelles spontaneously acquire a net charge of +qe or -qe. This charging takes place through a disproportionation/comproportionation mechanism rather than a dissociation/association mechanism and the equilibrium constant for this reaction is on the order of 10−4 to 10−11, which means about every 1 in 100 to 1 in 100 000 micelles will be charged.


Supermicelles

Supermicelle is a hierarchical micelle structure (
supramolecular assembly thumbnail, Circular helicate 9+,_where_L_stands_for_s_tris-bpy_ligand_strand;_the_central_gray_atom_is_Cl,_while_the_smaller_gray_spheres_are_Fe..html" ;"title="Fe5L5)Clsup>9+, where L stands for s tris-bpy ligand strand; the central gray atom is ...
) where individual components are also micelles. Supermicelles are formed via
bottom-up
bottom-up
chemical approaches, such as
self-assembly Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the c ...

self-assembly
of long cylindrical micelles into radial cross-, star- or
dandelion ''Taraxacum'' () is a large genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxono ...

dandelion
-like patterns in a specially selected solvent; solid
nanoparticle A nanoparticle or ultrafine particle is usually defined as a particle of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched ...

nanoparticle
s may be added to the solution to act as nucleation centers and form the central core of the supermicelle. The stems of the primary cylindrical micelles are composed of various block
copolymer In polymer chemistry Polymer chemistry is a sub-discipline of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structu ...
s connected by strong
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
s; within the supermicelle structure they are loosely held together by
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
s, electrostatic or solvophobic interactions.


Uses

When surfactants are present above the
critical micelle concentration In colloidal and surface chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and t ...
(CMC), they can act as
emulsifier An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter In classical phys ...
s 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
detergent A detergent is a surfactant Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting ...
s, 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 Surface tension is the tendency of liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physi ...

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 Emulsion polymerization is a type of radical polymerizationFree-radical polymerization (FRP) is a method of polymerization, by which a polymer forms by the successive addition of free-radical building blocks. Free radicals can be formed by a number ...
. Micelles may also have important roles in chemical reactions. Micellar chemistry uses the interior of micelles to harbor chemical reactions, which in some cases can make multi-step
chemical synthesis As a topic of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they unde ...
more feasible. Doing so can increase reaction yield, create conditions more favorable to specific reaction products (e.g. hydrophobic molecules), and reduce required solvents, side products, and required conditions (e.g. extreme pH). Because of these benefits, Micellular chemistry is thus considered a form of
Green chemistry Green chemistry, also called sustainable chemistry, is an area of chemistry and chemical engineering focused on the design of products and processes that minimize or eliminate the use and generation of hazardous substances. While environmental chem ...
. However, micelle formation may also inhibit chemical reactions, such as when reacting molecules form micelles that shield a molecular component vulnerable to oxidation. Micelle formation is essential for the absorption of fat-soluble vitamins and complicated lipids within the human body.
Bile salts Bile acids are steroid , hypothetical a steroid with 32 carbon atoms. Its core ring system (ABCD), composed of 17 carbon atoms, is shown with IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federatio ...
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. During the process of milk-clotting,
proteases A protease (also called a peptidase or proteinase) is an enzyme that catalysis, catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of n ...
act on the soluble portion of
casein Casein ( , from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in rel ...

casein
s, κ-casein, thus originating an unstable micellar state that results in clot formation. Micelles can also be used for
targeted drug delivery Targeted drug delivery, sometimes called smart drug delivery, is a method of delivering to a patient in a manner that increases the of the medication in some parts of the body relative to others. This means of delivery is largely founded on nanome ...
as gold nanoparticles.


See also

*
Critical micelle concentration In colloidal and surface chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and t ...
*
Micellar liquid chromatography Micellar liquid chromatography (MLC) is a form of reversed phase liquid chromatography In chemical analysis Analytical chemistry studies and uses instruments and methods used to separate, identify, and quantify matter. In practice, sep ...
*
Micellar solutionsImage:MICELLARPHASE.JPG, Schematic of a micellar solution showing spherical micelles distributed in water (solvent) and having no long-range positional order. A micellar solution consists of a dispersion of micelles in a solvent (most usually water). ...
*
Micellar solubilization Micellar solubilization (solubilization) is the process of incorporating the solubilizate (the component that undergoes solublization) into or onto micelles. Solublization may occur in a system consisting of a solvent, an association colloid (a coll ...
*
Lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive electrical charge on one side and a negative charge on another side, which produces the resting pot ...
*
Liposome s in an aqueous An aqueous solution is a solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the water the solvent. In chem ...

Liposome
*
Surfactant Surfactants are compounds that lower the surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than wate ...

Surfactant
*
Vesicle (biology) In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
*
Amphiphile An amphiphile (a.k.a. amphipath; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. ...


References

{{reflist, 30em Supramolecular chemistry Colloidal chemistry Membrane biology