Page 99 - Introduction to Colloid and Surface Chemistry

Liquid-gas and liquid-liquid interfaces 89

3. For diffusion reasons, solubilisation (see next section) would not
take place readily if the micelle were solid.

As mentioned above, the length of the surfactant's hydrocarbon
chain will dictate the radius of a spherical micelle. This in turn
determines the spacing of the outer polar groups. On this basis, for
example, a dodecyl sulphate micelle surface would be expected to be
approximately one-third sulphate groups and two-thirds hydrocarbon.
The results of neutron scattering studies are consistent with this
expectation. In an ionic micelle, the tendency of this hydrocarbon-
water interfacial area to contract is balanced by head-group repulsion.
Addition of electrolyte reduces this head-group repulsion, thus
favouring an area per head-group that is smaller than the geometric
optimum for a spherical micelle. Under such conditions, the micelle
is likely to distort to a non-spherical shape.
There is evidence from nuclear magnetic resonance spectroscopy
"
and partial molar volume measurements 153 154 which points to the
possible existence of bound water in the micelle interior in the region
of the first few CH 2 groups in from the polar head groups. The
hydrocarbon interior of the micelle may, therefore, be considered in
terms of an outer region which may be penetrated by water and an
inner region from which water is excluded.
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Solubilisation

Surfactant solutions above the c.m.c. can solubilise otherwise
insoluble organic material by incorporating it into the interior of the
micelles; for example, the dye xylenol orange dissolves only sparingly
in pure water but gives a deep red solution with sodium dodecyl
sulphate present above its c.m.c.
The balance of electrostatic and hydrophobic interactions can be
such as to cause the locus of solubilisation to be anywhere in the
micelle from close to the surface to the inner core.
Solubilisation is of practical importance in the formulation of
pharmaceutical and other products containing water-insoluble ingredi-
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ents , detergency, where it plays a major role in the removal of oily
soil (pages 166-176), emulsion polymerisation (page 17) and micellar
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catalysis of organic reactions .
In micellar catalysis, reactant must be solubilised at a location near
to the micelle surface where it is accessible to reagent in the aqueous