Thursday, September 11, 2008

Micelles

These are aggregates of surfactant molecules or ions in a solution or liquid colloid. These aggregates are made up of the water loving surface and an inner fatty core.

Micelle formation is done at a well defined concentration that is the critical micelle concentration.

Usually surface active agents behave as normal compounds in dilute solution but at certain reasonably well defined concentrations relatively sharp changes occur in the physical properties of these solutions.


These changes are attributed to the association of the amphipathic molecules or iond into aggregates of colloidal dimensions that are known as micelles.

Ionic and non-ionic substances that exhibit this type of behavior are referred to collectively as association colloids.

Critical Micelle concentration and micellar structure:
The minimum concentration at which physical properties of solutions of association colloids marked changes is known as critical micelle concentration. It is abbreviated as CMC.

Determination of critical micelle concentration:

(H. B. Klevens et al.)Critical micelle concentration (CMC) can be determined:
1. By Conductivity and transport number, which require the application of external electric forces.
2. Spectral dye method.
3. By solubilization studies which require the use of dyes or hydrocarbons.
4. By viscosity which involves the application of a shearing force.

Concept of formation of micelles is introduced to explain the apparent changes in osmotic properties and electrical conductivity with concentration in solutions of ionic associated colloids.

The conductivity indicated that a considerable degree of electrolytic dissociation was occurring in solution whereas the osmotic properties indicated that considerable aggregation of ions into single colligative units was also occurring above CMC.

Types of Micelles:
There are two basic types of micelles:

a. a small approximately spherical charged micelle which existed in all concentration i.e. above and below the CMC and which was largely responsible for the appreciable electrical conductivity and

b. a large undissociated lamellar micelle which only existed above the CMC and was responsible for the low osmotic properties at such concentrations.

Model of micelle:


Hartley’s model of spherical micelle

This model consists of a spherical charged micelle with a radius approximately equal to the chain length of the amphipathic ion.

The spherical type of micelle is now accepted as existing in all solutions of associated colloids at and just above the CMC.

However, in more concentrated solutions physical measurement for example X-ray diffraction, viscosity, light scattering indicate the existence of large asymmetric micelles.

How, large micelles are formed?
The rearrangement from spherical to larger and more widely separated asymmetric micelles has been ascribed to a reaction of the system in an effort to reduce the intermicellar repulsive forces that arise from the closer and closer approach of spherical micelles as the concentration of amphipathic material increases.

Different micellar shapes like rods and lamellae are also formed in different systems.

Stability and size of spherical micelles:
The cohesive force between water molecules is much stronger than either the attraction between the lipophilic parts of the surface active agents or the attraction between water and the lipophilic chains.

Therefore, the surface active agent tends to be squeezed out of solution in order to reduce the large degree of separation of water molecules that would be caused by the presence of many monomeric amphipathic molecules.

This effort which tends to cause a phase separation is counterbalanced to some extent by the hydrophilic nature of the polar groups.

In addition, the attractive forces between water molecules decay very rapidly with distance of separation since they are inversely proportional to somewhere between the fourth and seventh power of the distance.

Thus, the weak of separating water molecules by a relatively large distance of on the formation of a micelle is little different from that involved in the introduction of an amphipathic monomer.

Ionic surface active agent:
The electrical repulsion between adjacent similarly charged ions tends to disrupt the micelles of an ionic surface active agent.

In such a case, micelle formation is therefore dependent on the balance between this disruptive effect and the constructive “squeezing out of solution” effect.

Nonionic micelles:
Since, the electrical repulsive effect is absent in non-ionic micelles. Various suggestions have been made regarding the existence of a factor that would tend to oppose micelle formation for example cross sectional area and solvation of the hydrophilic group.

However, the precise nature of such a factor is still in doubt.

Association of ionic and non-ionic surface active agents:
The association of ionic and nonionic surface active agents is also aided by the “hydrophobic bonding” between the hydrocarbons chains.

This type of bonding involves vander waals forces of attraction, the effect of which is therefore of less significance than those mentioned previously.

In addition, an increase in temperature will have a disruptive effect on the formation of micelles since their rate of deaggregation will be increased.

The size of a spherical micelle depends on the structure of the surface active agent. In the model of the micelle, the radius is approximately equal to the length of the hydrocarbon chain.

If the diameter were to increase beyond this point then the unlikely structure would either include a space in the centre into which the hydrocarbon chains could not reach or the presence of some of the ionic groups between the hydrocarbon chains.
References:
H. B. Klevens. Critical micelle concentration as determined by refraction. The journal of physical chemistry, 1948.

Further Reading:
Dynamics of Surfactant Self-Assemblies: Micelles, Microemulsions, Vesicles and Lyotropic Phases (Surfactant Science) by Raoul Zana

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