The formation of an oil-in-water emulsion is outlined in Figure 2. If 100 mL of pure vegetable oil plus 500 mL of water are mixed vigorously to obtain an emulsion in which the average diameter of the oil globules is 1 /im, 600 m2 (slightly more than 6,400 ft2) of oil-water interface is generated. Using a purified vegetable oil, the interfacial tension (y) versus water is about 3 x 10~6 J/cm2. To form the emulsion, 18 J of the energy input is converted into interfacial energy. The addition of 1% glycerol monostearate (GMS) to the oil phase will lower y to about 3 x 10"7 J/cm2 so that the excess interfacial energy is only 1.8 J. This excess interfacial energy is the driving force behind coalescence of the oil globules and also the force resisting subdivision of oil droplets during mixing (each division increases the amount of interface present in the system). In a series of emulsification experiments in which the amount of mixing energy was constant and y was changed by adding emulsifier, it was found that the average oil droplet diameter paralleled y\ that is, as more emulsifier was added, y decreased and so did the average droplet size. The total excess interfacial energy was roughly constant.

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