An emulsion is formed when two nonsoluble liquids (e.g., an oil and water) are agitated together to disperse one liquid into the other, in the form of drops. Emulsions can either be oil-in-water (O/W) or water-in-oil (W/O), depending on whether the continuous phase is the water or the oil, respectively. Drop sizes normally vary from 1 μm to 50 μm. When the agitation stops, if the drops coalesce and the two phases separate under gravity, the emulsion has been temporary. To form a stable emulsion, an emulsifying agent must be added to the system.
Sometimes, the formation of an emulsion is the deliberate outcome of a manufacturing process. This is the case, for example, in the production of mayonnaise, where ground mustard seeds are normally added to act as an emulsifying agent. Other times, the formation of an emulsion is totally undesirable. An example is the case of the oil industry where emulsification of oil and brine is common. It may occur in the oil reservoir itself or while flowing through pipelines, mechanical devices, such as pumps, and gas separators.
Controlling factors in the formation of an emulsion are: mechanical energy, agitation time, temperature, volumetric ratio between the two phases, degree of dispersion of the internal phase and presence of impurities or surfactants. The material of the shearing plates for the homogenizer used in the emulsification process also influences the type of emulsion formed, e.g., oil-wetted plates strongly favor W/O emulsions [J. T. Davies (1964)].
There are many ways of producing an emulsion and it is usually achieved by applying mechanical energy through agitation, normally by using an homogenizer. Initially, the interface between the two phases is deformed and large droplets are formed. These droplets are subsequently broken up into smaller droplets by the continuing agitation. Impurities or surfactants present in the system adsorb at the interfaces of the droplets, lower the interfacial tension, and thereby facilitate coalescence. However, the surfactant film formed at the interface of the droplet also tends to resist coalescence. A detailed review of the principles of emulsion formation has been published recently by P. Walstra (1993), where droplet break-up in laminar and turbulent flow is discussed and quantitative relations are presented.
The stability of an emulsion is dependent on the magnitudes of the previously-mentioned opposing effects and is affected by: interfacial viscosity, electric charge on drops, droplet size and concentration, and viscosity of the continuous phase. Aging of an emulsion may also affect its stability as the nature of the interfacial film, which helps to keep it stable, can change with time.
The choice of surfactants for a particular process depends on the restrictions established for that particular application, e.g., in the food industry, emulsifying agents must be edible. Another factor to be considered in the choice of a stabilizing agent is whether the desired type of emulsion is an O/W or W/O, as the stabilizing agent largely determines which phase is the continuous one [Bancroft (1913)]. The phase in which the surfactant is more soluble will become the continuous phase.
Emulsions are often used in the most diverse fields, e.g., food industry, pharmaceutical products and manufacture of lubricants.
Some examples of emulsifiers are soaps, proteins, starch and gelatine.
Methods normally used to break emulsions are:
Gravity settling—Settling of emulsions is more rapid when the drop size is larger and when the continuous phase viscosity is lower. For faster separation, heat can be applied to reduce the viscosity of the continuous phase and sometimes to reduce the effectiveness of the surfactant.
Centrifugation—Faster separation by increasing the centripetal acceleration force.
Electrical coalescence—The application of an electrical current (direct or alternating) causes the internal phase droplets to coalesce.
Chemical methods—Coalescence can be achieved by the addition of suitable chemicals. For instance, by adding electrolytes the charge at the droplets’ interfaces may be neutralized and coalescence can result.
A combination of the above methods may also be chosen: heat to modify the continuous phase, chemistry to modify the emulsion, and electricity to finalize the separation.
Bancroft, W. D. (1913) J. Phys. Chem. 17: 514.
Davies, J. T. (1964) in Recent Progress in Surface Science (Danielli, J. F., Pankhurst, K. G. A. and Riddiford, A. C. Eds.). 2, Academic Press, New York and London.
Walstra, P. (1993), Principles of emulsion formation, Chem. Engng. Science 48, (2) 333–349. DOI: 10.1016/0009-2509(93)80021-H