Mixing excess enthalpy, nonideal mixed

Nonideal solutions may be classified into two limiting cases. In one limiting case, called regular solutions,-AGe AHe, i.e. most of the deviation from ideality is due to the excess enthalpy of mixing. Since AGe = AI/e — TASe, it follows that for regular solutions ASe 0. Furthermore, since ASe = —(0AGe/0T), from (8.4.18) it follows that the activity coefficients are given by... 

Consideration of the thermodynamics of nonideal mixing provides a way to determine the appropriate form for the activity coefficients and establish a relationship between the measured enthalpies of mixing and the regular solution approximation. For example, the excess free energy of mixing for a binary mixture can be written as... 

In real systems, nonrandom mixing effects, potentially caused by local polymer architecture and interchain forces, can have profound consequences on how intermolecular attractive potentials influence miscibility. Such nonideal effects can lead to large corrections, of both excess entropic and enthalpic origin, to the mean-field Flory-Huggins theory. As discussed in Section IV, for flexible chain blends of prime experimental interest the excess entropic contribution seems very small. Thus, attractive interactions, or enthalpy of mixing effects, are expected to often play a dominant role in determining blend miscibility. In this section we examine these enthalpic effects within the context of thermodynamic pertubation theory for atomistic, semiflexible, and Gaussian thread models. In addition, the validity of a Hildebrand-like molecular solubility parameter approach based on pure component properties is examined. 

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