7.5. Physic-chemical fundamentals of emulsion technique
After a general introduction into the structure and performance of emulsions, this article deals with the performance of water/perchloroethylene emulsions used in dyeing. In particular, two conditions must be fulfilled by a water/perchloroethylene emulsion: First of all, its boiling point must be higher than the dyeing temperature; secondly, its life must be significantly longer than the dyeing time. Useful criteria are described showing under which circumstances these two conditions are fulfilled. To determine these criteria, the vapour pressure of the emulsion is measured as a function of temperature. In the same way, the life of the emulsion is determined as a function of pressure and temperature. As a result, the vapour pressure of water/perchloroethylene emulsions is made up additively of the partial pressures of the water and of the perchloroethylene. The partial pressure of the emulsifying additives is not important. A measuring cell is described for measuring the life of the emulsion by examining the emulsion stability as a function of pressure and temperature. The results show that while the emulsion tends to break rapidly in the boiling range, it will be stable to a large extent outside the boiling range. The life of the emulsion depends on the rate of vaporization. The experiments show that both the boiling point and the stability limit of the emulsion can be shifted to elevated temperatures without applying any super-pressure.
The experiments also show that a disintegrated water/perchloroethylene emulsion can be largely re-emulsified if condensation losses are avoided. The interplay between disintegration and re-emulsification is explained. The consequences for dyeing from water/perchloroethylene emulsions are discussed.
General Fundamentals on the Origin, the Structure and the Performance of Emulsions Principles.
We have an emulsion when two liquids are present in such a manner that one liquid is dispersed in the other in the form of droplets. The liquid which is present in the form of droplets is known as the dispersed, or inner, phase, while the other liquid is the dispersing, or outer, phase. Most of the emulsions which occur in practice, contain water and oil or, in the place of oil, another organic liquid which cannot be mixed with water. Each one of these liquids can act as the dispersed or as the dispersing phase. When the water is dispersed in oil or in an organic liquid which is not miscible with water, we have a water-in-oil emulsion (w/o type); while in the reverse case we have an oil-in-water emulsion (o/w type). The following conditions must be fulfilled to produce an emulsion from two liquids:
-the liquids must be immiscible;
-energy must be added to the liquids by mechanical action (e.g. by shaking, beating, stirring or vibrating).
It is easy to understand the reason for the first condition. If the two liquids were miscible (for example, like alcohol and water), an absolutely uniform mixture would be obtained by shaking or stirring. No formation of droplets would be possible.
Energy required producing an emulsion
The need to add energy from outside can be explained in the following manner: Let us assume we have to disperse a liquid finely in air. This can be done, for example, by spraying. To this end, the surface of the liquid will have to be extended considerably, i.e. after spraying there will be more molecules than before in the areas which are near the surface. As long as a molecule stays inside a liquid, it is attracted from all sides by adjacent molecules of -the same kind, which means the resultant force is zero. But if the distance of a molecule from the surface is less than the range rw of the molecular forces (rw<1*10~7 cm), it will be subjected to a resultant force of attraction directed to the inside of the liquid (see fig.7.3.). Hence, if a liquid molecule is to be transported to the surface from the inside of the liquid, a force must be overcome which is directed to the inside, i.e. energy must be expended. The smaller the droplets of the spray, i.e. the finer the dispersion, the greater is the surface which must be newly created, and the more molecules must be transferred to the energized "surface position". Hence the energy E which must be expended to bring about this transfer is proportional to the surface /IS to be newly created:
E ~ ΔS (1)
When we introduce proportionality constant a, we get:
E ~ a·ΔS (2)
If the process of surface enlargement is conducted isothermally and reversibly, E represents the free surface energy. In that case о is called the specific free surface energy or surface tension. Its magnitude depends on the extent of the interaction of the molecules, and hence it is different for every liquid. The stronger the interaction, the larger will be.
Fig.7.3. Diagram to explain the surface energy
Force must be overcome which is directed to the inside, i.e. energy must be expended. The smaller the droplets of the spray, i.e. the finer the dispersion, the greater is the surface which must be newly created, and the more molecules must be transferred to the energized "surface position". Hence the energy E which must be expended to bring about this transfer is proportional to the surface /IS to be newly created.
In an emulsion, however, there is no interface liquid/air, but an interface liquid/liquid. The interfacial tension replaces the surface tension. Let aD be the surface tension of the disperse phase, and σG that of the dispersing phase; then we have for the interfacial tension ctdG' according to Antonoff:
aDG = | aD- σG | (3)
According to equation (2), this means, however, that the energy needed for emulsification depends on the surface tension of both phases. The surface tension of water at 20 °C is 72.75 dyn/cm. At the same temperature, perchloro-ethylene has a surface tension of 31.74 dyn/cm. According to Antonoff's rule, we must therefore expect the interfacial tension water/perchloroethylene to be about 41 dyn/cm. Hence for 1 cm2 of interface to be newly created, an energy expenditure of about 41 ergs is needed.
To influence the energy required for the creation of new interfaces, the surface tension of one or both phases must be changed. This can be done by adding foreign substances: If foreign molecules are introduced into a liquid, which interact more strongly with the molecules of the liquid than the molecules of the liquid itself will act on each other, the surface tension must increase in accordance with the explanations we have just given. With less interaction, the surface tension would drop. But there will also be another effect. In case of stronger interaction, the forces directed to the inside, acting on the foreign molecules in the surface, will be greater than in the case of the molecules of the liquid itself. As a result, the foreign substance will accumulate in the- inside of the liquid (negative absorption); if the interaction is weaker, the foreign substance will accumulate on the surface (positive absorption).
Mechanical addition of energy when an emulsion is produced
Emulsions are generally prepared by adding the necessary energy to the liquids by mechanical means, either by shaking, beating, stirring, turbulent mixing or vibrating. This will produce strong frictional and shear forces at the interfaces of both liquids. These forces will transform the majority of the liquid which is to be dispersed, into films and threads, which will eventually fall apart to form individual droplets.
Stabilization of emulsions
An emulsion is an unstable system because of the large interface between the dispersed and the dispersing phases: All the molecules in the interface have an elevated energy potential. Hence, the larger the interface, i.e. the finer the dispersion of the dispersed phase in the dispersing phase, the greater the free energy in the emulsion. But every system conducted under isothermal conditions tends towards a state of minimum free energy. The emulsion can approach this by reducing the interface between the dispersed and the dispersing phase. Hence the droplets will tend to unite, forming large units. In course of time, the dispersed phase and the dispersing phase will separate, and the initial state is restored.
To enable the droplets to form large units, they must be brought together. This is effected by the Brownian movement of the molecules or by thermal convection. When the droplets attain a certain size, they will settle out, or separate by flotation, depending on whether the specific gravity of the dispersed phase is higher or lower than that of the dispersing phase. While this takes place, they unite to form still larger units.
To give the emulsion sufficient stability, the droplets must live long. This can be achieved, for example, by giving them an electric charge of the same sign so that they repel one another. Another possibility is to provide the droplets with a protective cover which prevents them from coalescing. This possibility is almost exclusively preferred. The protective covers are formed by adsorption of foreign substances, i.e. the actual emulsifying additives, in the interface of the droplets. Whether and in what way the surface tension also changes, depends on the nature of the adsorption process.
Emulsifying additives
Most emulsifiers used in the laboratory and in the field, are surface-active agents. They are usually of linear structure, and they consist of two entirely different structural elements. One of these is non-polar, i.e. the focal points of the positive and negative electric charges coincide. In the second element, the focal points of the negative and positive charges are separate, i.e. the unit is an electric dipole, i.e. polar. Non-polar groups tend to non-polar compounds, polar groups to polar compounds. Water is highly polar, while oil-like compounds are non-polar. As a result, the non-polar part of an emulsifier is repelled by water (hydrophobic effect) and attracted to oil (lipophilic effect). The polar part of the substance, on the other hand, is attracted to water (hydrophilic effect) and is incompatible with oil (lipophobic effect). In other words: The emulsifier molecules are orientated in such a manner that the polar part points to the water, while the non-polar part points to the oil. In this manner, a protective cover forms around the droplets.
If the polar component predominates, the emulsifier molecule is pulled too far into the water. If the non-polar component is prominent, the emulsifier molecule is pushed into the oil. The molecules will accumulate in the interface oil/water only if the ratio of hydrophilic and hydrophobic components in the emulsifier is properly balanced out. It is only then that a well-stabilizing inter-facial film is formed.
Complete balancing out of non-polar and polar components is usually not possible in emulsifiers. One of the components wilt always predominates to some extent. That is why we must differentiate between oil-soluble and water-soluble emulsifiers.
Type of emulsion
Whether an emulsion of the w/o or o/w type is formed, depends mainly on the emulsifier. The ratio of polar to non-polar components is important. Oil-soluble emulsifiers will preferably form w/o emulsions; water-soluble emulsifiers prefer o/w emulsions. But this does not mean an emulsifier will only produce one kind of emulsion. The tendency to prefer one of them can be influenced in the opposite direction, so that it will no longer predominate. For example, if there is much less of one liquid than of the other, the liquid with the smaller volume will generally form the dispersed phase. Let us assume we have an emulsion prepared from 1,000 cm3 perchloroethylene and 10 cm3 water: In that case, for purely geometrical reasons, the water will probably form the dispersed phase.