EMULSIONS 



131 



If the metal of a soap has a higher valence than one and holds 

 more than one hydrocarbon chain, the oil end of the soap mole- 

 cule will be larger than the metal end, and the membrane will 

 bend in the reverse direction ; the emulsion then becomes a water- 

 in-oil one (Fig. 87). Again, theory demands and experiment 

 proves that calcium soaps, with a bivalent metal, form water-in- 

 oil emulsions. If this hypothesis holds generally, then all soaps 

 of monovalent cations, such as sodium, potassium, silver, and 

 caesium, should form oil-in-water emulsions, and all soaps of 

 bivalent cations, such as calcium, magnesium, and zinc, should 

 form water-in-oil emulsions, and this they do. 



Hildebrand carried this theory further and gave it very con- 

 vincing experimental support. He argued that soaps with 

 trivalent cations should produce water-in-oil emulsions as do 

 bivalent ones but that these emulsions should have smaller 

 droplets and therefore should be more stable systems, because 

 three hydrocarbon chains on one metal atom will give a broader 

 wedge and therefore a sharper curve, a smaller globule, and a 

 more stable emulsion. Furthermore, in any one group, of mono-, 

 di-, or trivalent metals, the stability of the emulsion should be 

 determined by the size of the metal atom, because it determines 

 the slope of the wedge. It would seem, therefore, that the direc- 

 tion and degree of curvature of the membrane and therefore 

 the type and stability of the emulsion are determined by the 

 size and the valence of the metal. The following table ^ substanti- 

 ates this: 



Metal 



Na. 



K.. 



Cs. 



Ca. 



Mg 



Zn. 



Al. 



Fe. 



Life of 

 emulsion 



6 weeks 

 8 weeks 

 8 weeks 



1 hour 



2 days 

 24 days 



7 days 

 10 days 



The theory is but weakly supported by the monovalent metals, 

 although affirmatively so, for the large metal caesium forms a 

 1 Data from Hildebrand. 



