Emulsions and foams 269Breaking of emulsionsIn many instances it is the breaking of an emulsion (demulsification)
which is of practical importance. Examples are the creaming,
breaking and inversion of milk to obtain butter, and the breaking of
W/O oil-field emulsions. Small amounts of water often get emulsified
in lubricating oils, hydraulic oils and heat-exchange systems, and it is
necessary to remove this water to prevent corrosion and other
undesirable effects.
A number of techniques are used commercially to accelerate
emulsion breakdown. Mechanical methods include centrifugal separa-
tion, freezing, distillation and filtration. Another method is based on
the principle of antagonistic action - i.e. the addition of O/W-
promoting emulsifiers tends to break W/O emulsions, and vice-versa.
Emulsions can also be broken by the application of intense electrical
fields, the principal factors involved being electrophoresis in the case
of O/W emulsions and droplet deformation in the case of W/O
emulsions.Microemulsions^127 "^128Microemulsions are emulsions with droplet diameters in the range of
about 0.01 to 0.1 pm. They are, consequently, of low turbidity.
The formation of a microemulsion involves the creation of a
situation in which the oil-water interfacial tension approaches zero
(or even becomes transiently negative). With all single ionic
surfactants and most single non-ionic surfactants this is not possible,
since yow is still sizeable when the c.m.c. or the limit of solubility is
reached. To achieve the required lowering of yOw a co-surfactant
must be included. For example, O/W microemulsions can be formed
using a mixture of potassium oleate and pentanol as the emulsifying
agent. In general, less co-surfactant is required for O/W than for W/O
microemulsions. Electrolytes promote the formation of W/O micro-
emulsions.
In view of the high oil-water interfacial area which must be
created, the fraction of emulsifying agent in microemulsion formula-
tions tends to be significantly higher than that in ordinary emulsions.
A typical microemulsion formulation would be 10-70 per cent oil,
10-70 per cent water and 5-40 per cent emulsifying agent^48.