EMULSIONS 129 



when the oil film thickened or thinned, the bar moved, and the 

 motion was recorded by levers and a scale. The substances with 

 which Langmuir worked were the higher fatty acids, the molecules 

 of which are long chains. The simpler ones in the series, such 

 as formic and acetic acids, are water-soluble and therefore do 

 not form oily films. The higher ones, such as stearic acid and 

 the closely related oleic acid, resemble oils and are insoluble in 

 water. Their molecules are long, with a carboxyl group at 

 one end. The structural formula of one of the simpler of these 

 fatty acids, butyric (CH3-CH2-CH2-COOH), is 



H H H 



I I I ^O 

 H— C— C— C— Cf 



H H H 



It is evident that the molecule is polar, i.e., its two ends are 

 different, and it is further known that the carboxyl (COOH) 

 group at the active polar end is more readily soluble in water, 

 because of its OH radical, than is the oily organic CH3 radical 

 at the other end. In the case of higher fatty acids, the chain is 

 much longer, as in oleic acid, with its 17 carbon atoms 

 (C17H33COOH). Such linear molecules, with one, the COOH, 

 end water-soluble and the other, CH3, end not, will very naturally 

 stand on end when on the surface of water. The union between 

 the carboxyl end of the chain molecule and the water is one of 

 residual valence, which is merely the chemist's way of saying 

 that he knows only that the bond is weaker than a primary- 

 valence one. The molecules of such organic substances when 

 at the interface between oil and water, as in emulsions, will orient 

 themselves with the COOH ends protruding into the water on 

 the one side and the CH3 ends protruding into the oil on the other. 

 With these facts in mind, Hildebrand considered the case of 

 soap films as stabilizers of emulsions. 



Soap molecules are polar. In a soap, such as sodium oleate, 

 the polar end is the metal sodium, and the nonpolar end the 

 organic radical. Such molecules will orient at the interface 

 between oil and water so that the metal end is in the water and 

 the organic end in the oil (Fig. 85). If the molecules are of the 

 same size at both ends, they will, when packed together in a row, 

 assemble in a straight line; but if the ends are not alike, i.e., if 



