EXIULSIFICATION BY SOLID POWDERS* BY A . d . SCARLETT, W. L. MORGAN AND J . H. HILDEBRAND
Emulsification by means of fine insoluble powders was reported by Ramsden1 in 1903, and further studied by Pickering2 in 1907, who worked upon the problem of plant sprays. Although the emulsifying power of fine solids had doubtless been operative in the preparation of various plant sprays for some time previous, this use, even today, has apparently occurred without clear realization, since soaps are usuglly also added, that the necessarily finely divided insecticide itself may act as an emulsifier. I n 1898 Elniore3 developed the first ore flotation process which took advantage of the ability of some finely powdered ores to act as emulsifiers in certain oil-water systems. Pickering carried on a large amount of work with powders as emulsifying agents and stated among his conclusions that solid particles, in order to have emulsifying power, must display but a small tendency toivards coalescence, must be wet more easily by the aqueous phase than by the oil, and must not be crystalline. Pickering apparently knew only oil-in-water type emulsions, and it was not generally realized a t that time that emulsions may exist of water in oil. B a n ~ r o f tin , ~ 1912, in reviewing Pickering’s work added “Pickering does not point out that a corollary of his hypothesis is that insoluble particles, which are more readily wetted by oil than by water, should tend to promote the emulsification of water in oil.“ Schlaepfer,s following this suggestion, found that lampblack goes into the kerosene layer when shaken with it and water, and that it accordingly gives emulsions of water in oil. Briggs,6 also working in the Cornell laboratory, made the following statement : “It’is generally agreed that the liquid which wets the solid emulsifier the more strongly under the conditions of the experiment tends t o become the outside phase, the less strongly wetting liquid being broken up into drops. Just why this is so, is difficult to explain satisfactoril2; for the monient.” I n 1923 Finkle, Draper and Hildebrand’ suggested the following mechanism for the explanation of emulsion type, “LIany powdered solids have been shown to be capable f stabilizing emulsions. I t seems to be possible to account for the type of emulsion produced by the following hypothesis. It is obvious, first, that the powder must collect a t the interface in order to be effective. Now, this will occur only when the solid is wetted by both liquids, *Contribution from the Chemical Laboratory of the L-niversity of California. Ramsden: Proc. Roy. SOC.,72, I j 6 (1903). Pickering: J. Chem. S o c . , P l , z o ~ o(190;); I
