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1.4. Emulsions
When two liquids are immiscible but do not separate immediately they are said to form an emulsion. Some emulsions are quite stable and will take a long time to separate. For example milk is an emulsion of water and fat but is fairly stable. Other emulsions may separate quite quickly, for example a simple salad dressing of oil and vinegar will separate almost immediately (note that vinegar is water based).
The emulsion itself consists of small droplets of one liquid within the body of a second liquid. An emulsion containing droplets of oil in water is called an oil-in-water emulsion and the oil is called the dispersed phase while the water is called the continuous phase. An emulsion with droplets of water in oil is called a water-in-oil emulsion. A good oil-in-water emulsion will consist of very fine oil droplets homogeneously dispersed throughout the body of water.
Since colas are oil-in-water emulsions this discussion of emulsions assumes that an oil phase is being dispersed within a water phase. In practice neither the water phase nor the oil phase of an emulsion are likely to be pure substances. For example in colas the continuous water phase is an acidic solution of citric or phosphoric acid together with other ingredients such as caramel and sugar, the oil phase is a complex mixture of organic molecules from the essential oil flavourings. In general the water soluble molecules all stay in the water phase and the oil phase will be a mixture of all the liquid molecules that are not soluble in water (i.e. a mixture of oils).
1.4.2. Surface Energy
All surfaces have a surface energy, this energy is responsible for phenomena such as surface tension. If you place a drop of oil into a glass of water a new surface at the interface between the oil and water is created and this surface will have an energy (Figure 1.5). This energy must be provided from somewhere, thus to create an emulsion from oil and water you must supply energy. For example you must shake salad dressing to create an emulsion from the oil and vinegar, if you do not shake it the two liquids stay in separate layers (Figure 1.6).
Figure 1.5: A drop of oil in water. Figure 1.6: Making new surfaces in salad dressing emulsion.
Crudely speaking all systems try to reduce their energy to a minimum, thus our drop-of-oil-in-water system wants to reduce the area of the oil-water interface; less surface area equal less surface energy. For a given volume of oil the minimum surface area possible is obtained by forming a sphere; therefore oil drops in water and bubbles of gas in a liquid are always spherical.
1.4.3. Emulsion Failure
Emulsions can fail in four basic ways, each of which causes the homogenous dispersion of oil droplets to be lost (Figure 1.7):
1. Coalescence - Two small oil spheres have a combined surface area (and therefore surface energy) that is larger than a single big sphere containing the same volume of oil. Thus if salad dressing is left to stand the small bubbles will coalesce to form bigger and bigger spheres until the oils has completely separated from the water.
2. Flocculation - The small spheres of oil stick together to form clumps or flocs which act as if they are larger drops. Therefore the oil is no longer evenly distributed through the water.
3. Creaming - Most oils are less dense than water and will therefore float to the top. However, the drops will not necessarily coalesce.
4. iv. Breaking - Due to Coalescence and creaming combined, the oil separates completely from the water so that it floats at the top in a single, continuous layer.
Figure 1.7: The failure of emulsions.
Figure 1.7 clearly shows that flocculation and creaming leave the fine oil droplets intact while making them less well distributed throughout the water. Therefore, these two processes can be reversed by putting in a small amount of energy (i.e. moderate stringing or shaking). Brownian motion, within the water phase, can provide enough energy to keep exceptionally small droplets agitated and hence creaming is less likely to happen with fine emulations. If the emulsion contains larger oil droplets, they will soon rise to the top.
Coalescence and breaking lead to large bodies of oil separating from the water and essentially result in the emulsion separating completely. To reverse this process the emulsion must be remade and this will require a lot of energy. Emulsions are all thermodynamically unstable, meaning that they will eventually separate however, they can be stabilised and in some cases they can remain intact indefinitely. Both coalescence and flocculation are more likely if the surface energy between the two phases is high and if the surface area to volume ratio is high (i.e. the oil droplets are very small). However, emulsions of small droplets are easier to stabilise because creaming is less likely.
1.4.4. The Origins Of Charged Droplets In Emulsions
In many emulsions the droplets of the dispersed phase have an electrostatic charge. In colas the droplets have a negative charge, this arises due to the dissociation of acidic groups at the surface of the droplets (Figure 1.8, also see Section 1.3). These acidic groups form part of some of the oxygen containing essential oil molecules.
The magnitude of the surface charge depends on the strength of the acid groups in the oil phase (for colas the carboxylic groups are quite weak acids and alcohols are very weak acids) and on the pH of the surrounding water phase (for coca-cola this is about 2.5). The more acidic a cola's water phase is, the less surface charge there will be on the droplets and as the pH is lowered a point is reached where the droplets no longer have a surface charge at all, this point is known as the isoelectric point.
Figure 1.8: Carboxylic acid groups at an oil/water surface producing a surface charge by dissociation.