264 Emulsions and foams
determining droplet size distributions in O/W emulsions. In this
method a dilute dispersion is made to flow through a small orifice.
The passage of a non-conducting particle through the orifice causes a
momentary increase in the electric resistance between electrodes
placed either side of the orifice. The magnitude of this increase in
resistance depends on the size of the particle. The electronic circuitry
of the system is such that particles above a certain size are counted
every time the resistance rises above the corresponding preselected
cut-off value. By varying this cut-off value, a cumulative particle size
distribution can be determined. The lower particle diameter limit for
this technique is about 0.7 /urn, which makes it unsuitable for the
study of most colloidal dispersions, but suitable for studying the
relatively large droplets in O/W emulsions.
If an emulsion is prepared by homogenising two pure liquid
components, phase separation will usually be rapid, especially if the
concentration of the dispersed phase is at all high. To prepare
reasonably stable emulsions, a third component - an emulsifying
agent (or emulsifier) - must be present. The materials which are most
effective as emulsifying (and foaming) agents can be broadly
classified as:
- Surface-active materials.
- Naturally occurring materials.
- Finely divided solids.
The functions of the emulsifying agent are to facilitate emulsification
and promote emulsion stability. The emulsifying agent forms an
adsorbed film around the dispersed droplets which helps to prevent
coagulation and coalescence. The stabilising mechanism is usually
complex and may vary from system to system. In general, however,
the factors which control droplet coagulation are the same as those
which control the stability of sols (see Chapter 8), whereas stability
against droplet coalescence depends mainly on the mechanical
properties of the interfacial film.
The following factors (which depend on the nature of the
emulsifying agent and/or on a suitable choice of formulation and
manufacturing conditions) favour emulsion stability:
1, Low interfacial tension The adsorption of surfactant at oil-water
interfaces causes a lowering of interfacial energy, thus facilitating