Behavior of Excess Properties

As suggested by Fig. 11.4, the principal excess properties (G, , and S ) canexliibit a variety 

Excess properties for liquid mixtures depend primarily on temperature and composition therefore comparison of data for different mixtures is best done at fixed T and x. Since many data are available at near-ambient temperatures, T is chosen as 298.15 K (25°C). Because extreme values for often occur near equimolar composition we fix a i = X2 = 0.5. 

The six possible combinations of sign for the ttoee excess properties are enumerated in Table 16.6. Each combination defines a region on the diagram of Fig. 16.4, wliich shows /RT vs. H /RT in skeletonform. 

NP/NP mixtures (the open circles on Fig. 16.5) tend to concentrate in Regions I and VI for such mixtures, H and are normally positive. When is positive (enthalpy domination), G /RT rarely exceeds about 0.2. If is negative (entropy domination), G / RT is rarely less than —0.2. 

NA/NP mixtures (the filled circles) usually fall in Region I, with occasional significant excursions into Region II. Thus, G and are positive, as is (normally) S. Both G and can be large. 

---