Preparation of Emulsions.
Emulsions are stable suspensions of one liquid in another, the liquids being immiscible. Stability of the emulsion is obtained by dispersion of very fine droplets of one liquid, called the disperse phase, through the other liquid, which is called the continuous phase. The emulsion is stable when it can persist without change, for long periods of time, without the droplets of the disperse phase coalescing with each other, or rising or settling. The stability of an emulsion is controlled by
interfacial surface forces,
size of the disperse phase droplets,
viscous properties of the continuous phase and
density difference between the two phases.
The dispersed particles in the emulsion have a very large surface area, which is created in the process of emulsification. Surface effects depend upon the properties of the materials of the two phases, but very often a third component is added which is absorbed at the interface and which helps to prevent the droplets from coalescing.
These added materials are called emulsifying agents and examples are phosphates and glycerol monostearate.
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):
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.
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.
Size Reduction > SUMMARY, PROBLEMS
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