B Factors Determining Stability

The term stability, when applied to macroemulsions used for practical applications, usually refers to the resistence of emulsions to the coalescence of their dispersed droplets. The mere rising or settling of the droplets (creaming) because of a difference in density between them and the continuous phase is usually not considered instability. Flocculation or coagulation of the dispersed particles, without coalescence of the liquid interior of the particles, although a form of instability, is not considered as serious a sign of instability as coalescence or breaking of the 

The rate of coalescence of the droplets in a macroemulsion is stated to be the only quantitative measure of its stability (Boyd, 1972). It can be measured by counting the number of droplets per unit volume of the emulsion as a function of time in a haemocytometer cell under a microscope (Sherman, 1968) or by means of a Coulter centrifugal photosedimentometer (Groves, 1964 Freshwater, 1966). 

The rate at which the droplets of a macroemulsion coalesce to form larger droplets and eventually break the emulsion has been found to depend on a number of factors (1) the physical nature of the interfacial film, (2) the existence of an electrical or steric barrier on the droplets, (3) the viscosity of the continuous phase, (4) the size distribution of the droplets, (5) the phase volume ratio, and (6) the temperature. 

Physical Nature of the Interfacial Film The droplets of dispersed liquid in an emulsion are in constant motion, and therefore there are frequent collisions between them. If, on collision, the interfacial film surrounding the two colliding droplets in a macroemulsion ruptures, the two droplets will coalesce to form a larger one, since this results in a decrease in the free energy of the system. If this process continues, the dispersed phase will separate from the emulsion, and it will break. The mechanical strength of the interfacial film is therefore one of the prime factors determining macroemulsion stability. 

For maximum mechanical stability, the interfacial film resulting from the adsorbed surfactants should be condensed, with strong lateral intermolecular forces, and should exhibit high film elasticity. The liquid film between two colliding droplets in an emulsion is similar to the liquid lamella between two adjacent air sacs in a foam (Chapter 7) and shows film elasticity for the same reasons (Gibbs and Marangoni effects). 

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