Page 103 - Introduction to Colloid and Surface Chemistry

Liquid-gas and liquid-liquid interfaces 93

In general, micellisation is an exothermic process and the c.m.c.
increases with increasing temperature (see page 86). This, however,
is not universally the case: for example, the c.m.c. of sodium dodecyl
sulphate in water shows a shallow minimum between about 20°C and
25°C. At lower temperature the enthalpy of micellisation given from
equation (4.28) is positive (endothermic), and micellisation is entirely
entropy-directed.
The cause of a positive entropy of micellisation is not entirely clear.
A decrease in the amount of water structure as a result of
micellisation may make some contribution. A more likely contribution,
however, involves the configuration of the hydrocarbon chains, which
probably have considerably more freedom of movement in the
interior of the micelle than when in contact with the aqueous
medium.

The Krafft phenomenon

Micelle-forming surfactants exhibit another unusual phenomenon in
that their solubilities show a rapid increase above a certain
temperature, known as the Krafft point. The explanation of this
behaviour arises from the fact that unassociated surfactant has a
limited solubility, whereas the micelles are highly soluble. Below the
Krafft temperature the solubility of the surfactant is insufficient for
micellisation. As the temperature is raised, the solubility slowly
increases until, at the Krafft temperature, the c.m.c. is reached. A
relatively large amount of surfactant can now be dispersed in the
form of micelles, so that a large increase in solubility is observed.
Table 4.5 Krafft temperatures for sodium alkyl sulphates in water

Number of carbon atoms 10 12 14 16 18
Krafft temperature/°C 8 16 30 45 56

Spreading

Adhesion and cohesion

The work of adhesion between two immiscible liquids is equal to the
work required to separate unit area of the liquid-liquid interface and