Emulsification by solid powders

Emulsification by solid powders. It has long been known that solid powders can sometimes emulsify one liquid in another that is, can stabilize one liquid in the form of small drops in the other. This can occur in all cases where the surface relationships between the solid and the two 

The mechanism of this emulsification has been explained in detail by Finkle, Draper, and Hildebrand,6 and by Ramsden.6 It was seen in 17 that if the contact angle is acute in liquid A, the greater part of the bulk of the particles, including the widest cross-section, is in that fluid. Fig. 41 shows a number of particles in contact in an interface, their greatest cross-section being in A. Any contraction of the interface must bend the surface with its load of solid particles so as to become concave to fluid B. Thus the contraction of the interface, after bringing the particles into contact, curves the interface towards that fluid which wets the powder least, in which the contact angle is obtuse. 

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

6 Clayton s Emulsions and their Technical Treatment, 2nd ed., Appendix I (1928). 

Bhatnagar,2 following Donnan,3 thinks that electrical effects at the interfaces also are important, and finds that the formation of a water-in-oil or an oil-in-water emulsion depends on the ions present in the solution, which are no doubt adsorbed on the surface of the solid. The rule that monovalent ions promote oil-in-water emulsions, which are reversed by polyvalent ions, appears to hold good with solid emulsifiers as well as with soluble soaps. Developments of the theory in this direction will be of great interest. 

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