92168.pdf

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