Emulsions oriented-wedge theory

Figure 7.3 Illustration of the oriented-wedge theory of emulsion type.

At one time a very attractive theory of the inversion of emulsions, called the oriented-wedge theory, was rather generally accepted. Starting from the fact that the soaps of divalent metals usually form water-in-oil emulsions, while those of monovalent metals form oil-in-water emulsions, it was suggested7 that, since in the divalent soaps there are two hydrocarbon chains attached to one metal atom, while in the monovalent there is only one chain, the molecules of these soaps are wedge-shaped, being wider at the water-soluble end in the case of the monovalent soaps, and at the oil-soluble end with the di- and trivalent soaps. A closely packed layer of these molecules would therefore naturally curve with the concave side towards the oil in the case of the monovalent soaps, with the convex side towards the oil in the case of the divalent soaps thus the... 

The oriented wedge theory states that the emulsifier at the interface is wedge shaped. The ionized end of a sodium soap has a wider (effective) radius than the hydrocarbon chain, hence the oil-water interface should be curved with the convex side oriented toward the water phase. This favors oil droplet formation, hence gives an o/w emulsion. The polar end of zinc distearate, on the other hand, is smaller than the two hydrocarbon chains, the interface is convex toward the water phase, and a w/o emulsion is formed. 

Other special rules are (1) when surfactants are easily soluble in one phase, this is a continuous phase and (2) surfactants made from monovalent metal cations tend to produce 0/W emulsion, whereas those made from polyvalent metal cations produce W/0. This is called the oriented wedge theory (Bryan and Kantzas, 2007). Another related theory is the phase volume theory, proposed by Wilhelm Ostwald (winner of the Nobel Prize in chemistry, 1909) ... 

The oriented-wedge theory is based on the concept that, for anionic surfactant emulsifiers, polyvalent metal ions will tend to form each coordinate to the polar groups of two surfactant molecules, forcing the hydrocarbon tails into a wedge-like orientation. In this case, the hydrocarbon tails in a close-packed interfacial layer are most easily accommodated if the oil phase is the continuous phase. As a result, aruonic surfactants associated with monovalent metal cations should tend to produce O/W emulsions, while those of polyvalent metal cations should tend to produce W/O emulsions. This works best for carboxylate surfactants. 

The surface of O/W emulsions, which are stabilized by surface active salts of polyelectrolytes, usually carries a negative charge. O/W emulsions are formed if the interfacial film consists of a molecular complex with an oil-soluble component and an ionizable water-soluble component (25). The simple oriented-wedge theory of... 

Oriented-Wedge Theory. An empirical generalization used to predict which phase in an emulsion will be continuous and which dispersed. It is based on a physical picture in which emulsifiers are considered to have a wedge shape and will favor adsorbing at an interface such that most efficient packing is obtained, that is, with the narrow ends pointed toward the centers of the droplets. A useful starting point, but there are many exceptions. See also Bancroft s Rule, Hydrophile-Lipophfle Balance. 

Figure 7.8 (a and b) Cartoon representing the oriented wedge theory, as presented by Harkins et and Langmuird The monolayers covering emulsion droplets have different frustration energies, which favor one emulsion type over another. Note that the picture shows the macroscopic emulsion droplets and not the surfactant micelles. The theory is wrong because the monolayers are essentially planar on the molecular scale (c and d)... 

In spite of the fact that the mechanistic interpretation of emulsion stability by Bancroft was essentially the oriented wedge theory, this paper became acclaimed because of another empirical correlation. Bancroft states ... 

Figure 7.16 By contrast to the classical oriented wedge theory, in the Kabalnov-Wenner-strdm theory it is assumed that the spontaneous curvature affects not the free energy of the emulsion droplets but the free energy of the coalescence transition state the hole in the film. If the spontaneous curvature of the surfactant molecule fits the neck, the hole propagates without a significant barrier (a). In the opposite case, the nucleation is suppressed and the emulsions are stable (b)...

An analogous rule to the oriented-wedge and Bancroft theories states that the liquid that preferentially wets the solid particles will tend to form the continuous phase. Thus if there is a low contact angle (measured through the water phase), then an O/W emulsion should form. Exceptions occur for each of these rules, and sometimes one will work where the others do not. They do remain useful for making initial predictions. 

A simple geometric theory for the stabilization of emulsions is that of the oriented wedge, in which the adsorbed surfactant molecules are assumed to form a uniform structure of wedges around the emulsion droplet. If an emulsion of 1000-nm-diameter droplets is stabilized by a surfactant whose head group occupies a surface area of 0.45 nm, what must be the cross-sectional area of the hydrophobic tail for maximum effectiveness ... 

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