A micelle or micella 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 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. Inverse micelles have the head groups at the centre with the tails
extending out. 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.
History The ability of a soapy solution to act as
a detergent has been recognized for centuries. However, it is 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". Lipid micelles represent a 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" of
molecules connected by hydrogen bonds. This water cage is similar to a clathrate and has
an ice-like 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 counterions. Although the closest counterions
partially mask a charged micelle, 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, 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. At very low concentrations
of the lipid, only monomers are present in true solution. As the concentration of the
lipid is increased, a point is reached at which the unfavorable entropy considerations,
derived from the hydrophobic end of the molecule, become dominant. At this point, the lipid
hydrocarbon chains of a portion of the lipids must be sequestered away from the water. Therefore,
the lipid starts to form micelles. In broad terms, 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 of surfactants.
Micelle Packing Parameter The micelle packing parameter equation is
utilized to help "predict molecular self-assembly in surfactant solutions":
where is the surfactant tail volume, is the tail length, and is the equilibrium area per
molecule at the aggregate surface. 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. 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, they can act as emulsifiers that will allow
a compound that is normally insoluble 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 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 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 and lipid-soluble vitamins within the micelle
by the small intestine. Micelles are used for targeted drug delivery.
See also Liposome
Vesicle Lipid bilayer
Surfactant Micellar liquid chromatography
Notes
References J. M. Seddon, R. H. Templer. Polymorphism
of Lipid-Water Systems, from the Handbook of Biological Physics, Vol. 1, ed. R. Lipowsky,
and E. Sackmann. 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.
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. Makoto Takeo. Disperse Systems, WILEY-VCH,
ISBN 3-527-29458-9