Stabilization Coalescence

Figure 2. States of emulsion formation and breakdown. In the upper figures the dark area represents oil, the clear areas water. In the lower figures, shaded circles represent oil surrounded by water.

used may be efficient at reducing y even at a low interfacial concentration but does not prevent the oil droplets from touching and coalescing during creaming.

A true emulsion stabilizer prevents coalescence (Fig. 2). In essence, the thin layer of water between the oil droplets is stabilized by various mechanisms. If the surfactant is anionic, then the surfaces of both oil droplets carry a negative charge and they are mutually repelled by electrostatic effects. This sort of stabilization is sensitive to ionic strength, and a high salt concentration will suppress the electrostatic repulsion, promote contact and coalescence, and lead to rapid emulsion breakdown.

A second kind of stabilization is shown by surfactants in which the hydrophilic portion is quite large; for example, the polyoxyethylene chain of the Tweens or ethoxylated monoglyceride. In this case the chain is anchored at the surface of the oil droplet by the lipophilic tail, but it is strongly hydrated and generates a layer of bound water around the droplet, preventing contact and coalescence. This functionality is relatively insensitive to salt concentration.

A third kind of stabilization is due to simple steric hindrance of contact. The alpha-tending emulsifiers such as propylene glycol monostearate (PGMS) form an actual solid layer at the oil-water interface (1,2). This film physically prevents the contents of oil droplets from coalescing even though their surfaces may be touching. Gums such as gum arabic and gum ghatti stabilize oil-in-water emulsions by a similar mechanism, forming a film of adsorbed polymer around the oil droplet, while some water-soluble proteins perform the same function in mixtures of ground meat and fat for making sausage.

It should be noted that emulsion stabilization is not directly related to the ability to lower interfacial tension. For example, 1% GMS in the oil phase lowers y to 3 X 10"7 J/cm2 but has little effect on coalescence rate, whereas 12% PGMS yields a y of 9 X 10" 7 J/cm2 but gives an emulsion with excellent long-term stability.

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