The
size of the disperse phase droplets is important and these are commonly
of the order 1-10 mm
diameter. Below 0.1 mm
droplet diameter, the dispersion is often spoken of as colloidal. Coalescence
of the disperse phase droplets is hindered
by increased viscosity in the continuous liquid phase. The nearer the
densities of the components are to each other, the less will be the
separating
effect of gravitational forces. Stokes' Law gives a qualitative indication
of the physical factors that influence the stability of an emulsion.
This
is because the relative flow of the particles under gravitational forces
may break the emulsion, so stability is enhanced by small settling
velocities.
From eqn. (10.2):
v = D2g(rp
- rf)/18m
the critical importance
of particle size, occurring as a squared term, can be seen. Also it shows
why emulsions are more stable when density differences are small and when
the viscosity of the continuous phase is high.
Preparation of Emulsions
The essential feature
of an emulsion is the small size of the disperse phase droplets. This
can be achieved by imposing very high shearing stresses upon the liquid
that is to be dispersed and the shearing forces break the material into
the multitude of fine particles.
Shearing
is, generally, attained by passing the liquid through a high pressure
pump, to bring it up to pressures of the order of 7 x 103 kPa,
and then discharging this pressure suddenly by expansion of the liquid
through a small gap or nozzle; the equipment is often called a homogenizer.
In passing through the nozzle, very large shear forces are exerted on
the liquid, disrupting cohesion and dispersing it into the very small
particles.
Centrifugal
forces may also be used to obtain the shearing action. Discs spinning
at high velocities give rise to high shearing forces in liquids flowing
over them. Flow between contra-rotating discs, which may have pegs on
the disc faces, can be used to produce emulsions. Designs in which small
clearances are used between a stationary disc and a high speed flat or
conical rotating disc are called colloid mills. Another source of energy
for shearing is from ultrasonic vibrations induced in the liquid.
Existing
emulsions can be given increased stability by decreasing the size of
the droplets
either by impact or shearing the emulsion still further; the process
is called homogenization. Homogenizing results
in smaller and more uniform droplet sizes and a practical example is the
homogenizing of milk.
Examples
of emulsions met with frequently in the food industry are - milk (fat
dispersed in water), butter (water dispersed in fat), mayonnaise (oil
in water) and ice cream (fat in water which is then frozen).
Milk
is an emulsion of fat in water, which is not stable indefinitely as it
separates on standing, into skim milk and cream. This is caused by the
density differences between the fat and the water, the fat globules rising
as predicted by Stokes' Law and coalescing at the surface to form a layer
of cream. After homogenizing, this separation does not occur as the globules
are much reduced in size. Homogenizing is also used with ice cream mixes,
which are dispersions of fat and air in sugar solutions, and in the manufacture
of margarine.
The
same surface effects that govern liquid emulsions also apply to
dispersions of solids in liquids
and of liquids or solids in gases. Colloidal solutions of solids can
be produced if the particle size is of the necessary order, below
about 0.1 mm,
and again stability depends upon the surface properties of the materials.
Aerosols, for example, fine mists in the atmosphere, can also be
quite stable.
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Preparation
of Emulsions. 