Emulsions charged interfaces

Any conclusion that a low interfacial tension per se is an indication of enhanced emulsion stabiUty is not rehable. In fact (8), very low interfacial tensions lead to instabiUty. The stabiUty of an emulsion is influenced by the charge at the interface and by the packing of the emulsifier molecules, but the interfacial tension at the levels found in the common emulsion has no influence on stabiUty. 

The adsorption of ions at insulator surfaces or ionization of surface groups can lead to the formation of an electrical double layer with the diffuse layer present in solution. The ions contained in the diffuse layer are mobile while the layer of adsorbed ions is immobile. The presence of this mobile space charge is the source of the electrokinetic phenomena.t Electrokinetic phenomena are typical for insulator systems or for a poorly conductive electrolyte containing a suspension or an emulsion, but they can also occur at metal-electrolyte solution interfaces. 

Such an explanation cannot however account for the abnormally high values of the adsorption of simple salts at a water-oil interface, as found by Lewis. If we are to accept these results (though the emulsion method is certainly the least satisfactory of those used for the present purpose) some secondary effect must be looked for, such as the increased solubility of oil in water when emulsified and in the form of small particles. (The mean radius in one experiment was found to be about 4 x 10 cms., a size at which Hulett and Ostwald (see page 166) found a notable increase in the solubility of calcium and barium sulphates.) It is probable however that the chief error introduced was the disturbance brought about by the effect on the surface tension of the free charges of the salt ions (Lewis, Zeit. Physikal. Chem. Lxxiii. 129, 1910). (See Ch. VII.)... 

Therefore, two contributory factors may provide an explanation for more effective electrostatic / steric stabilization of the so-called mixed emulsions in comparison with the sequentially assembled biopolymer interfaces of the bilayer emulsions firstly, a greater hydrophilicity of the adsorbed protein-polysaccharide complexes, caused by the larger net negative charge, and, secondly, a more bulky architecture of the normal complexes as compared to the interface complexes. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

If the surfactant is ionic and imparts a charge to the interface, then the dispersed particle will be surrounded by an ion atmosphere. We see in Chapters 11 and 13 how an ion atmosphere surrounding a particle may slow down the rate at which such particles come together. This is one of the ways by which an emulsion may achieve some degree of kinetic stability. 

Some inorganic electrolytes stabilize oil-in-water emulsions. One example is potassium thiocyanide (KCNS), which dissociates in the aqueous phase. The anion CNS adsorbs at the interface, which becomes negatively charged. As a result the oil droplets repel each other electrostatically. 

One characteristic of phase boundaries, especially those involving an aqueous phase, is the probable existence of an electrical potential across the interface. Although such charge phenomena an not always present, when in existence, they have an enormous impact on system properties. Chargi effects are usually most important in aqueous suspensions, emulsions, foams, aerosols, and othe dispersions in which one phase is Lnely divided in another phase, creating a large interfacial area. 

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