MICELLES

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MICELLES Thermodynamically Stable
Colloids (Chapter 4, pp. 84-93 in Shaw)
In dilute solutions surfactants act as normal
solutes. At well defined concentrations, however,
abrupt changes in osmotic pressure, turbidity,
electrical conductance and surface tension take
place
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• The behavior can be explained in terms of
  organized aggregates or micelles in which the
  hydrophobic chains are oriented towards the
  inside of the particles.
• Micellisation is an alternative to adsorption by
  which the interfacial energy of a surfactant
  solution might decrease.

Example Sodium Stearate C17H35COONa forms
micelles containing approximately 70 molecules
(on average). The length of a molecule is 2.5 nm
and the cross section is 0.45 nm x 0.45 nm. If
the interfacial tension between the hydrocarbon
tail and the water is 70 mN/m calculate DG for
the formation of micelles.
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• Example Sodium Stearate C17H35COONa forms
  micelles containing approximately 70 molecules
  (on average). The length of a molecule is 2.5 nm
  and the cross section is 0.45 nm x 0.45 nm. If
  the interfacial tension between the hydrocarbon
  tail and the water is 70 mN/m calculate DG for
  the formation of micelles.
• Solution
• Contact area 4 x 0.45 x 2.5 4.5 nm2
• So for 1 micelle DA -315 nm2
• DG g x DA 0.07 N/m x 315 10-18 m2 -2.21
  10-17 J
• or for one mole
• DG -132 kJ/mol
• This is a very large energy change!
• (rough calculation)

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• Two distinct contributions to the energetics of
  micellisation
• The intermolecular attractions between the
  hydrocarbon chains in the interior of the micelle
  (a small effect).
• Micellisation allows strong water-water
  interactions which would otherwise be prevented
  if the surfactant was in solution as single
  molecules wedged between the solvent water
  molecules. This is often referred to as the
  hydrophobic effect and gives a very large
  contribution to micellisation.

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• Factors affecting cmcs
• Length of the hydrocarbon chain
• Cs 8 10 12 16
• cmc, mmol/L 140 33 8.6 0.58
• 2. Higher T raises the cmc.
• 3. With ionic micelles, the addition of simple
  electrolyte lowers the cmc. Why?
• For SDS in water
• C(NaCl mol/L) 0 0.03 0.3
• cmc, mmol/L 8.1 3.1 0.7
• 4. Addition of organic molecules can go in a
  variety of ways.

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• Medium chain length alcohols can be incorporated
  into the outer layers of the micelle and reduce
  the electrostatic repulsion and steric
  hinderance thus lowering the cmc (microemulsions
  are effectively formed with octanol and a soap).
• Sugars structure water and lower the cmc. Why?
• Urea and formamide break structures and their
  addition causes an increase in the cmc.

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STRUCTURE OF MICELLES Micelles tend to be fairly
spherical over a wide range of concentrations. At
high concentrations there are marked transitions
to liquid crystal like structures. These are
still thermodynamically stable colloids Spherica
l micelles Microemulsions (alcohol soap
oil) Vessicle bilayer micellar
structures Cylindrical micelles Laminar micelles
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• Main reasons for accepting the fact that micelles
  are spherical
• Cmcs depend almost entirely on the nature of the
  lyophobic part of the surfactant. If micelles
  form some sort of a lattice structure then the
  hydrophilic head would also be expected to play
  some role.
• Micelles are approximately monodispersed and size
  depends on length of hydrophobic tail.
• For diffusion reasons, solubilization would not
  readily take place if the micelles were solid.

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• What is solubilization?
• Above the cmc, surfactant solutions can
  solubilize (i.e. disperse on a colloidal scale) a
  large amount of otherwise insoluble material in
  their lyophobic centres. Micelles are the
  vehicles for detergent systems.
• Solubilization is of great importance in
• Pharmaceuticals
• Detergency
• Emulsion polymerization (30 of polymers are made
  through this process) and
• Micellar catalysis of organic reactions.

Micelles can also carry a small amount of water
and make electrostatic stabilization possible in
non-aqueous media.
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Inversed micelles These are formed in
non-aqueous media and have an inversed structure
Non-polar oil
Tails Heads
These systems can be used to carry polar material
in a non-polar medium (e.g. CaO or MgO in burner
fuel to prevent oxidation by SOx)
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Energetics of Micellisation The Mass Action Model
c(1-x) cx/m
x is fraction of monomer units aggregated and m
is the number of monomer units/micelle.
Equilibrium constant
Since at the critical micelle concentration m is
large and x is small, this relationship can be
reduced to
DG RT ln(cmc)
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Therefore And

• In general, micellisation is an exothermic
  process and the cmc increases with increasing
  temperature. However, for SDS, there is a shallow
  minimum in the cmc between 20 and 25 Celsius. At
  lower temperatures the DH of micellisation is
  positive endothermic and the process is totally
  entropy driven. Possible causes for positive
  entropy of micellisation
• A decrease in the amount of water structure upon
  formation of micelle.
• Hydrocarbon chains gain freedom upon formation of
  the micelle.

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The Krafft Phenomenon
Above a certain temperature known as the Krafft
Point, certain surfactants show a marked increase
in solubilizing power. At the low temperature
limited solubility of the surfactant is
insufficint for micellisation C
atoms 10 12 14 16 18 Kraft T, C 8 16 30 45 56