Model of a Two-Component Mixture

As in the model of the previous section, we have M sites each of which may be occupied by either an or a 5 molecule. We introduce the interaction energies Waa, Wbb, and Wba for each type of nearest neighbor. The energy levels of the system are determined by 

Each A—A bond contributes two s each A—B bond contributes one If we sum over 

From (4.2.53) and (4.2.54) we can compute all relevant thermodynamic quantities of the system. 

A simple case occurs when IF = 0, i.e., when Wab is the arithmetic average of Waa and Wbb or, equivalently, 

This is the symmetrical ideal solution. We shall discuss the thermodynamics of ideal solutions further in Chapter 6. The average number of As (or Bs) in the system is obtained from 

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