Thermodynamics of mixing and assimilation in ideal-gas systems

In this appendix, we discuss briefly the concept of free energy of mixing and entropy of mixing, and the relatively newly defined concept of assimilation. A more detailed discussion of this topic may be found elsewhere (Ben-Naim 1987a,b). 

Consider a general mixture of c components at some given temperature T and pressure P, and composition Nx. Nc. The Gibbs energy of this system is 

Let fi be the chemical potential of the pure i th species at the same P and T. The total Gibbs energy of the combined pure species before the mixing, is then 

A particular case of equation (H.3), which is discussed in almost all textbooks of thermodynamics, is the case of mixing ideal gases. In this case, the first sum 

This quantity is commonly referred to as the free energy of mixing energy of mixing). Similarly, the quantity (or Gibbs 

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