Page 96 - Introduction to Colloid and Surface Chemistry

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
50
the long hydrocarbon chains in the water phase .

Table 4.3 Critical micelle concentrations for a homologous series of sodium alkyl
sulphates in water at 40°C"

Number of carbon atoms 8 10 12 14 16 18
3 3
c.m.c./lCT mol drrT 140 33 8,6 2.2 0.58 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 0 0.01 0.03 0.1 0.3
3
c.m.c,/10~ mol dm~ 3 8.1 5.6 3.1 1.5 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