Gibbs energy and definition of the surface tension

In this chapter we introduce a more useful equation for the surface tension. This we do in two steps. First, we seek an equation for the change in the Gibbs free energy. The Gibbs free energy G is usually more important than F because its natural variables, T and P, are constant in most applications. Second, we have just learned that, for curved surfaces, the surface tension is not uniquely defined and depends on where precisely we choose to position the interface. Therefore we concentrate on planar surfaces from now on. 

Assuming that the interface is flat (planar) we have the same pressure in both phases / = P13 = P and we get 

With the help of this equation it is also possible to give a definition of the interfacial tension, which is equivalent to the previous definition  

The surface tension is the increase in the Gibbs free energy per increase in surface area at constant T, P, and JV.  

5 Free surface energy, interfacial enthalpy and Gibbs surface energy 

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