MICELLESMICELLESThermodynamically Stable ColloidsThermodynamically Stable Colloids

(Chapter 4, pp. 84-93 in Shaw)(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:

0 0.01 0.02Concentration, mol/L

C

c.m.c.

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 G for the formation of micelles.

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 G for the formation of micelles.

Solution:

1) Contact area ~ 4 x 0.45 x 2.5 = 4.5 nm2

2) So for 1 micelle A ~ -315 nm2

3) G = g x A = 0.07 N/m x –315 10-18 m2 = -2.21 10-17 J

or for one mole

G = -132 kJ/mol

This is a very large energy change! (rough calculation)

Two distinct contributions to the energetics of micellisation:

1. The intermolecular attractions between the hydrocarbon chains in the interior of the micelle (a small effect).

2. 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.

Factors affecting cmc’s:

1. Length of the hydrocarbon chain:

# C’s 8 10 1216

cmc, mmol/L 140 33 8.60.58

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 0.3cmc, mmol/L 8.1 3.1

0.7

4. Addition of organic molecules can go in a variety of ways.

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.

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:

Spherical micelles

Microemulsions (alcohol + soap + oil)

Vessicle bilayer micellar structures

Cylindrical micelles

Laminar micelles

Main reasons for accepting the fact that micelles are spherical:

1. Cmc’s 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.

2. Micelles are approximately monodispersed and size depends on length of hydrophobic tail.

3. For diffusion reasons, solubilization would not readily take place if the micelles were solid.

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:

1. Pharmaceuticals2. Detergency3. Emulsion polymerization (30% of

polymers are made through this process) and

4. Micellar catalysis of organic reactions.

Micelles can also carry a small amount of water and make electrostatic stabilization possible in non-aqueous media.

Inversed micelles:

These are formed in non-aqueous media and have an inversed structure:

Non-polar oilTailsHeads

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)

Energetics of Micellisation The Mass Action Model

mX (X)m

c(1-x) cx/m

Equilibrium constant:

)x1(clnRTmcx

lnmRT

G

orKlnmRT

G

)x1(c

m/cxK

m

Since at the critical micelle concentration m is large and x is small, this relationship can be reduced to:

x is fraction of monomer units aggregated and m is the number of monomer units/micelle.

G = RT ln(cmc)

Therefore:

And:

dT)cmcln(d

RTSGH

)cmcln(RdT

)cmcln(dRT

dTGd

S

2

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 H of micellisation is positive endothermic and the process is totally entropy driven. Possible causes for positive entropy of micellisation:

1.A decrease in the amount of water structure upon formation of micelle.2.Hydrocarbon chains gain freedom upon formation of the micelle.

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 18Kraft T, C 8 16 30 45 56