Oriented wedge theory

A.S. Kabalnov and H. Wennerstrom Macroemulsion Stability The Oriented Wedge Theory Revisited. Langmuir 12, 276 (1996). 

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

Optical Brighteners Optimum Salinity Oriented-Wedge Theory... 

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... 

One might perhaps retain a part of the oriented-wedge theory to explain the different solubility of the monovalent and the polyvalent soaps. If the molecules are wedge-shaped, with the hydrocarbon ends the thicker, then they will naturally pack into a micelle, or aggregate, with the hydrocarbon ends outwards and the water-soluble groups in the centre. Then the soap should be soluble in oils, not in water, as is actually the case with the polyvalent soaps. But if the water-soluble end is the wider, the micelle should have the water-soluble ends of the molecules outwards, and be itself soluble in water, as is the case with the monovalent soaps. 

This simple and attractive theory is certainly, however, insufficient to cover the whole of the facts, just as the oriented wedge theory of soluble... 

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. 

Figure 16. The oriented-wedge theory of emukion type.

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) ... 

Kabalnov A, Wennerstrom H. Macroemulsion stability the oriented wedge theory revisited. Langmuir 1996 12 276-292. 

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. 

A. Kabalnov and H. Wennerstrom, Macroemulsion stability The oriented wedge theory revis-... 

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. 

This concept received the name of the oriented wedge theory and, in modem terms, can be reformulated as a relationship between the macroemulsion type and... 

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)...

---