Emulsions and foams 26.5
the development and enhancing the stability of the large interfacial
areas associated with emulsions.
- A mechanically strong and elastic interfacial film This is particu-
larly important when the volume fraction of the dispersed phase is
high,
The stability of emulsions stabilised by proteins arises from the
mechanical protection given by the adsorbed films around the
droplets rather than from a reduction of interfacial tension.
Finely divided solids for which the contact angle is between 0°
and 180° have a tendency to collect at the oil-water interface (cf,
flotation; page 161), where they impart stability to the emulsion.
Surfactants can also stabilise in the mechanical sense. Coalescence
involves droplet coagulation followed by a squeezing of film
material from the region of droplet contact, and the latter is more
favoured with an expanded film than with a close-packed film. For
example, very stable hydrocarbon oil in water emulsions can be
prepared with sodium cetyl sulphate (dissolved in the water) plus
cetyl alcohol (dissolved in the oil) as emulsifier (a condensed
mixed film being formed at the interface), whereas hydrocarbon
oil in water emulsions prepared with sodium cetyl sulphate plus
oleyl alcohol (which gives an expanded mixed film) are much less
stable^219. The most effective interfacial films are the mixed films
which are formed as a result of the combined use of water-soluble
and oil-soluble emulsifying agents.
It is also important that the emulsifier films have sufficient
elasticity to enable recovery from local disturbances (see Gibbs-
Marangoni effect; page 274).
- Electrical double layer repulsions (see page 212) Interparticle
repulsion due to the overlap of similarly charged electric double
layers is an important stabilising mechanism in O/W emulsions.
When ionic emulsifying agents are used, lateral electric double
layer repulsion may prevent the formation of a close-packed film.
This film-expanding effect can be minimised by using a mixed
ionic plus non-ionic film^220 (see above) and/or by increasing the
electrolyte concentration in the aqueous phase^221.
- Relatively small volume of dispersed phase (see below).
- Narrow droplet size distribution Larger droplets are less unstable
than smaller droplets on account of their smaller area-to-volume
ratio, and so will tend to grow at the expense of the smaller
droplets (see page 68). If this process continues, the emulsion will
