Page 273 - Introduction to Colloid and Surface Chemistry

10 Emulsions and foams

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Oil-in-water and water-in-oil emulsions "

An emulsion is a dispersed system in which the phases are immiscible
or partially miscible liquids. The globules of the dispersed liquid in
the usual type of emulsion (sometimes now called a macroemulsion)
are usually between 0.1 /*m and 10 /um in diameter, and so tend to be
larger than the particles found in sols.
The practical application of emulsions and emulsion technology is
considerable, and includes foodstuffs (especially dairy produce),
pharmaceutical preparations, cosmetics, agricultural sprays and
bituminous products. Emulsions enable the dilution of an expensive
or concentrated ingredient with an inexpensive, but immiscible,
diluent. For example, water-insoluble agrochemicals are generally
marketed in the form of oil in water emulsions which can be diluted
with water by the user in order to permit even, low-level application.
Emulsions also allow enhanced control over other factors, such as
rheology and the various manifestations of chemical reactivity
(including taste, odour and toxicity). A large volume of technological
information on emulsions exists, much of it in private files. In
general, empirical knowledge is well ahead of fundamental under-
standing.
Microemulsions, with droplet diameters of 0.01-0.1 /urn, can also
be prepared and these are currently the subject of much fundamental
investigation and new applications (e.g. oil recovery from porous
rocks). Whether microemuisions should be regarded as true emulsions
or as swollen micelles (see section on solubilisation, page 89) is a
matter of controversy.
The visual appearance of an emulsion reflects the influence of
droplet size on light scattering, and varies from milky-white-opaque,