Interfacial thermodynamics and local mechanical equilibria

Some aspects of interfacial thermodynamics will now be addressed. For simplicity we will neglect anisotropy effects (except for the consideration of equilibrium shape). The free interfacial enthalpy of the interface (of area a) can be charjicterized by defining an excess parameter G, which represents the excess of over the 

The parameter most relevant for the formal treatment is the interfaciaJ tension 51,53,54,55 describes the increase of free enthalpy (p, T, n constant) on increase of the interfacial area 

By subtraction of the G-contribution of the bulk the change of free surface enthalpy is (S, are defined analogously to G ) obtained as 

While Eq. (5.66) defines 7 as the change in free enthalpy of the whole system at constant mole numbers, Eq. (5.69) represents 7 as the change in excess Gibbs energy at constant excess mole number of the surface region. The possibility of integrating Eq. (5.68) at constant fx, 7 and T yields 

According to Eq. (5.70) 7 can be only identified with the surface-related excess Gibbs energy (G /a) if we ignore effects caused by changes in the mole number at the cost of the bulk composition. The validity of Eq. (5.70) is equivalent to the 

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