92168.pdf

86 Liquid-gas and liquid-liquid interfaces

obtained from dynamic light scattering (page 61), ultracentrifugation
(page 31), viscosity and low-angle X-ray scattering.

Factors affecting critical micelle concentrations

1, Increasing the hydrophobic part of the surfactant molecules

favours micelle formation (see Table 4.3). In aqueous medium,

the c.m.c. of ionic surfactants is approximately halved by the

addition of each CH 2 group. For non-ionic surfactants this effect is

usually even more pronounced. This trend usually continues up to

about the C 16 member. Above the C 18 member the c.m.c. tends to

be approximately constant. This is probably the result of coiling of

the long hydrocarbon chains in the water phase^50.

Table 4.3 Critical micelle concentrations for a homologous series of sodium alkyl

sulphates in water at 40°C"

Number of carbon atoms

c.m.c./lCT^3 mol drrT^3

8

140

10

33

12

8,6

14

2.2

16

0.58

18

0.23

Micelle formation is opposed by thermal agitation and c.m.c.'s

would thus be expected to increase with increasing temperature.

This is usually, but not always, the case, as discussed on page 93.

With ionic micelles, the addition of simple electrolyte reduces the

repulsion between the charged groups at the surface of the micelle

by the screening action of the added ions (see Chapter 7). The

c.m.c. is, therefore, lowered, as illustrated in Table 4.4.

Table 4.4 Critical micelle concentrations of sodium dodecyl sulphate in aqueous

sodium chloride solutions at 25°C!1

c. (NaCl)/mol drrT^3

c.m.c,/10~^3 mol dm~^3

0

8.1

0.01

5.6

0.03

3.1

0.1

1.5

0.3

0.7

The addition of organic molecules can affect c.m.c.'s in a variety

of ways. The most pronounced changes are effected by those

molecules (e.g. medium chain-length alcohols, see page 89) which

can be incorporated into the outer regions of the micelle. There

they can reduce electrostatic repulsion and steric hindrance, thus