THE SURFACE FILMS OF INSOLUBLE MATERIALS

Introduction. Conditions of spreading of non-volatile substances on liquids. 

If a drop of oil be placed upon the surface of a liquid in which it is insoluble there will exist at the moment of contact an unstable system in which three surface forces are operative at three interfaces as indicated in the diagram. 

We will define by o-i, ar the two liquid-air and o-ia the liquid-liquid interfacial surface tensions. If cr + o-ia= Ca, it is evident that the system will be in equilibrium, as no decrease in the free energy of the system can take place by spreading or contraction of the drop. On the other hand if 

The conditions of equilibrium finally attained by the lens of the non-spreading liquid on the surface will be expressed by the equation Ti cos a + Via = Cz, an expression first obtained by Neumann, 

A third possibility evidently exists when the magnitudes of the three surface tensions involved are as follows  

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