Coacervation of soap solutions

Isotropic solution (nigre lye). With these it must be noted that these solutions can be transformed into gels by addition of electrolytes, after which a separation into two layers can follow — the region of the coacervation of soap solutions studied later by Bungenberg de Jong and his pupils 2°. Middle soap, an anisotropic (polarising microscope) viscous solution which contains still rather a lot of water (about 50%). 

An important question arises from these experiments on the coacervation of soap solutions how can we connect up these experiments with the ideas on the micelles which must occur in soap solutions and how must we picture the structure of these remarkable soap-rich layers ... 

Many salts reduce the viscosity of aqueous acacia solutions, while trivalent salts may initiate coagulation. Aqueous solutions carry a negative charge and will form coacervates with gelatin and other substances. In the preparation of emulsions, solutions of acacia are incompatible with soaps. 

Addition of acid to a soap solution causes the KCl concentration required for the coacervation to fall sharply (Fig, 29). This is a logical consequence of the fact that the number of uncharged spots on the micelle — and therewith the extension in the plane of the particle — increases. The large and flat micelles required for coacervation are formed more rapidly. 

In our opinion these examples demonstrate the value of our way of looking at the problem. Emphasis must finally be laid on one thing. In spite of the fact that we consider the phenomena in soap solutions throughout as equilibrium phenomena, we use terms as micelle , coacervate, and so on, which on account of their colloid chemical past call forth ideas of strictly determined boundary surfaces (Freundlich s Kapillarchemie). We wish however to retain these terms without crediting the boundary surface of micelle-equilibrium liquid with a separate significance. We thus look upon a micelle in a soap solution as a formation which is in equilibrium with the rest of the solution but which through its large dimensions and its structure has properties which the soap molecule as such does not possess. It is only with this restriction that we wish to continue to speak of micelles, coacervates, etc. 

If one adds KCl to a solution of Na-oleate, the viscosity rises rapidly at a particular concentration so that the system can even gelatinise. If more KCl is added a separation into two layers begins, the upper one of which contains practically all the soap. With rising KCl-concentration this coacervate layer becomes smaller and smaller. 

At room temperature no coacervates can be made with many soaps (stearate, palmitate etc.) for the simple reason that these soaps crystallise. This must be attributable to the magnitude of force B (Fig. 36). When now it is possible for us to unloosen the paraffin chains, coacervation will in fact be able to occur. Thus Bdngenberg de Jong (unpublished communication) was able to make a turbid, crystallised Na-stearate solution perfectly clear by adding menthol. We must imagine that the crystals are slowly transformed into micelles. Afterwards coacervation could be obtained with the aid of KCl. 

Aqueous solutions of complex soaps (1-3) are drag reducers, as are certain conventional soaps (4-6) and nonionic surfactants (7-11), and they do not have some of these deficiencies. They have the advantage of regaining their drag reducing effectiveness after subjection to high shear fields. The latter two are effective near their coacervation temperature or cloud point (upper consolute temperature). The addition of electrolyte lowers the cloud point and therefore the temperature at which effective drag reduction occurs. Cloud points can be adjusted to convenient temperatures in this manner. 

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