Charged interfaces, stabilizing emulsions

Emulsifiers stabilize emulsions in various ways. They reduce interfacial tension and may form an interfacial film that prevents coalescence of droplets. In addition, ionic emulsifiers provide charged groups on the surface of the emulsion droplets and thus increase repulsive forces between droplets. Emulsifiers can also form liquid crystalline microstructures such as micelles at the interface of emulsion droplets. These are formed only at emulsifier concentrations larger than the critical micelle-forming concentration. These microstructures have a stabilizing effect. 

Some foods require an adjustment of pH to enhance, control, or mask taste or flavor. pH can affect emulsification since protein solubility and charge repulsion can be reduced at isoelectric pH. However, the cohesiveness of protein films tends to be maximal near the isoelectric pH (Mangino, 1994). Near the isoelectric pH, proteins are able to form close-packed dense films, thus increasing the protein concentration at an interface. Waniska and Kinsella (1988) demonstrated that at isoelectric pH, electrostatic repulsion was minimized, allowing hydrophobic residues to stabilize a compact tertiary structure of 3-lg. Klemaszewski et al. (1992) reported that the coalescence stability of emulsion stabilized by P-lg was dependent on pH. However, the coalescence stabilities of a-la- or SCN-stabilized emulsions were unaffected by pH. 

In foods, phospholipids are often transformed by processing so that they are often present at the interface of emulsions or cooperate in forming films on the surface of solid particles. The best emulsion stability is achieved when neutral phospholipids (such as phosphatidylcholine) are added to negatively charged lipids, which is sometimes difficult because the major lipidic fraction is neutral. In practice, it is suitable to select common phospholipid concentrates in which a certain amount of negatively charged phospholipid is present (Rydhag, 1979). 

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. 

Any conclusion that a low intcrfacial tension per sc is an indication of enhanced emulsion stability is nut reliable. In fact, very low interfacial tensions lead to instability. The stability of an emulsion is inlluenced 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 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. 

Nonionic surfactants produce the same interfacial films in the interface in a similar fashion as that mentioned above. As expected, there is no charge repulsion contribution to the stability of the emulsion. However, the polar groups of the surfactants (i.e., polyoxyethylene) are hydrated and bulky, causing steric hindrance among droplets and preventing coalescence. 

It was discussed that the structure created by the ternary system oil/water/ nanoparticle follows the laws of spreading thermodynamics, as they hold for ternary immiscible emulsions (oil 1 /oil 2/water) [114,116,117]. The only difference is that the interfacial area and the curvature of the solid nanoparticle has to stay constant, i.e., an additional boundary condition is added. When the inorganic nanoparticles possess, beside charges, also a certain hydrophobic character, they become enriched at the oil-water interface, which is the physical base of the stabilizing power of special inorganic nanostructures, the so-called Picker-... 

Foam stability is governed by similar factors as emulsion stability. Thus in a soap foam the negative charges located at the air-water interface lead to repulsion as the... 

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