Applications of Surfactants Emulsions

In contrast to microemulsions, ordinary emulsions are thermodynamieally unstable, but they can be stable in a practical sense if the energy barrier to flocculation is snfficiently high. As with other colloidal dispersions, this energy barrier may be electrical in nature if the drops are charged, if water is the continuous phase, and if the ionic strength is not too high (cf. the discussion of DLVO theory in Chapter 3). Typically, stabihty is provided by adsorbed surfactants and polymers. However, it can also stem from small solid particles that are not completely wet by either phase and thus accumulate at the drop surfaces (Aveyard et al., 2003). 

Ivanov and Kralehevsky (1997) agree that the final emulsion type depends on the relative coaleseence rates of oil-in-water and water-in-oil dispersions, but suggest that the rates are determined by the behavior of the thin liquid films formed when two drops approaeh. Consider, for example, a thin film of water between two oil drops. If the surfaetant is preferentially soluble in water and has a low solubihty in oil, it can remain in the film, slow film drainage, and stabilize the film at some equilibrium thiekness or at least hinder film breakup, whieh leads to eoaleseenee. In contrast, if the solubility of the surfactant in oil is high, it can diffuse into the oil, where it is not available to slow film drainage and hinder breakup. A similar argument ean be made for the oil film between two water drops in a water-in-oil emulsion. 

As we have seen, the optimal salinity or PIT concept based on the equilibrium phase behavior provides much useful insight in formulating emulsions and 

FIGURE 4.29 Sketch of the hole developing in the thin film between emulsion drops. 

The relative amounts of oil and water employed also influence phase continuity. Other factors being equal, oil-in-water emulsions are formed when only small amounts of oil are present and water-in-oil emulsions are formed when only small amounts of water are present. Indeed, stable dilute emulsions can be 

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