Micelles

Comparison of liposome and micelle

Micelle in water solution

Micelles ( micelles from latin mica , lumps ' , smaller bites'), and association colloids mentioned are together mounted molecule complex ( aggregates ) of amphiphilic molecules or surface-active substances .

properties

In a dispersion medium (mostly water) the amphiphilic molecules bind to one another due to the hydrophobic effect . This process is called self-assembly . Micelles form in polar solvents such as water above a certain mass concentration , the critical micelle concentration  (CMC), due to the tendency of the surfactants to phase separate. This means that the hydrophilic parts (heads) of the surfactant molecules align themselves with the adjacent water molecules, whereas the hydrophobic parts (tails) coalesce and thus form their own phase (see graphic). Such a micelle is typically a few nanometers in size , e.g. B. approx. 4 nm for an SDS micelle in water.

Inverse micelles

Also in organic , i.e. H. In non-polar solvents there are micelles, only with a reverse orientation (inverse micelles, also called reverse micelles). In addition to spheres, there are many other geometric shapes (rods, plates, etc.), depending on the size and length ratio of head to tail.

Reverse micelles are used e.g. B. brew in the extraction of proteins from fermentation . The protein in the core of the reverse micelle is thereby dissolved.

Super micelles

Electron microscope image of a windmill-like super micelle (scale 500  nm ).

Supermicelles are hierarchical supramolecular structures made up of micelles.

Liquid crystals

If a solution contains very high concentrations of surface-active substances, the many micelles can form higher states of order, namely liquid crystals .

Educational energy

The micelle formation process occurs spontaneously; H. it is based on a thermodynamic equilibrium . The driving force here is the release of water molecules that were previously associated with the surfactant molecules, which increases entropy . The enthalpy of micelle formation can be described as follows: ${\displaystyle \Delta H_{m}}$

${\displaystyle \Delta H_{m}=-n\cdot \left({\frac {\delta \ln c_{k}}{\delta T}}\right)\cdot RT^{2}}$

With

• Constant , varies between 1 and 2 depending on the charge on the micelles ${\displaystyle n}$
• critical concentration for micelle formation (cmc, see above) ${\displaystyle c_{k}}$
• Temperature in Kelvin ${\displaystyle T}$
• universal gas constant . ${\displaystyle R}$

molar mass

The relationship between the molar mass  M and the CMC is described using the Debye equation :

${\displaystyle {\frac {K\left(c-CMC\right)}{\tau }}={\frac {1}{M}}+2A_{2}(c-CMC)}$

With

• the optical constant K (depending on the wavelength and refractive index of the solvent and the solution)
• the concentration  c of the amphiphilic molecule
• the critical micelle concentration CMC
• the turbidity  and ${\displaystyle \tau }$
• the second osmotic virial coefficient  A ₂ .

Further meaning

The agglomeration of asphaltenes in mineral oil products is also referred to as micelle formation, e.g. B. in heating oil EL .

literature

• William M. Gelbart: Micelles, Membranes, Microemulsions, and Monolayers. Springer Science & Business Media, 2012, ISBN 978-1-4613-8389-5 , p. 208

Web links

Wiktionary: Mizelle  - explanations of meanings, word origins, synonyms, translations

• Theoretical introduction to measuring surface tension and CMC

See also

Krafft temperature (Krafft point)

Individual evidence

1. ↑ Y. Moroi: Micelles. Springer Science & Business Media, 2013, ISBN 978-1-4899-0700-4 , p. 1.
2. ↑ Xiaoyu Li, Yang Gao, Charlotte E. Boott, Mitchell A. Winnik, Ian Manners: Non-covalent synthesis of supermicelles with complex architectures using spatially confined hydrogen-bonding interactions . In: Nature Communications . 6, 2015, p. 8127. bibcode : 2015NatCo ... 6E8127L . doi : 10.1038 / ncomms9127 . PMID 26337527 . PMC 4569713 (free full text).
3. ↑ William M. Gelbart: Micelles, Membranes, Microemulsions, and Monolayers. Springer Science & Business Media, 2012, ISBN 978-1-4613-8389-5 , p. 2.