At constant temperature for closed system,

Initial free energy

where is the initial number of moles, is the initial pressure of the vapor and is the chemical potential of vapor at . After formation of liquid nucleus final energy of the system is given by

where is called the energy penalty because of the formation of a new interface.

Thus the change in free energy of the system is

Substituting the values of we can write,

(5.1)

where which is independent of .