Symmetric Ideal Solution as a Reference System

Suppose that the two components A and B are similar in the sense of Section 4.6, which, in this case, means [see Eq. (4.95)] 

in such a case, the solution will be SI i.e., from (4.168), we have 

Note that is the chemical potential of pure A at this particular T and P and therefore it includes the effect of A-A interactions through Baa If on the other hand, the system is not SI, then we define the activity coefficient as 

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