October, ’23] 
griffin: constitution of oil emulsions 
431 
fatty acids are very soluble in mineral oils and so are taken up by the oil 
droplets. This leaves the alkali in the water. The process of hydrolysis 
goes on until the concentration of the alkali is so great that the tendency 
to hold the fatty acids in the water in the form of soap is as great as 
their tendency to go into solution in the oil. 
The soap which is thus broken down does not appear to serve any useful 
purpose and is apparently a total loss. It has been shown experimentally 
that this loss may be prevented by adding enough alkali to keep the soap 
from hydrolyzing and freeing fatty acid which would go into solution 
in the oil. This is in agreement with the work of Briggsand Schmidt 2 
who found that small quantities of alkali tended to assist in the forma¬ 
tion of emulsions of benzene in soap solution. The fish-oil soap, which 
is recommended by the Bureau of Entomology for use in making oil 
emulsions, 3 contains a considerable excess of alkali and is particularly 
suited to the preparation of oil emulsions. Of course, in the addition 
of alkali the possibility of injury to vegetation by excessive quantities 
of it must be borne in mind. 
Films Surrounding the Droplets of Oil 
If an oil is broken into droplets in water, the droplets readily coalesce 
and the oil separates out. However, if a soluble soap is present in the 
water, soap immediately attaches itself to the oil droplets to form films 
around them. This film prevents the droplets of oil from coalescing and 
is the essential feature in stabilizing emulsions of this type. It is ex¬ 
ceedingly thin; in fact, its thickness is the length of a single molecule 
of the soap and is the same whether the soap solution from which the 
film is formed is dilute or concentrated. The quantity of soap necessary 
to form it is comparatively small. One gram of soap will cover an area 
of 500 to 1000 square meters, depending on the kind of soap. One very 
good commercial emulsion contained 66% of oil and had droplets which 
averaged about 0.0003 centimeters in diameter. These have an area 
of 2 square meters for each cubic centimeter of oil dispersed. One gram 
of soap would therefore form the film necessary for from 375 to 750 
cubic centimeters of this emulsion. With coarser emulsions (larger oil 
droplets) less soap is needed. Thus, if the emulsion is 66% oil and has 
droplets averaging 0.001 centimeter in diameter, 1 gram of the soaps 
would be used to form the film for 1,250 to 2,500 cubic centimeters 
of the emulsion. 
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2 Briggs and Schmidt, J. Physical Chem., 19:478-99 (1915) 
3 Quaintance and Siegler, Farmers’ Bulletin 908, U. S. D. A. (1918). 
