Immiscibility and Consolute Behavior

As positive deviations (7.56) increase (as in oil-water mixtures), the tendency toward immiscibility and phase separation increases. In this case, the simple liquid region of (7.61) may separate into two distinct liquid phases one A-rich and one B-rich in composition. Such liquid-liquid partial miscibility is a ubiquitous feature of aqueous solutions of hydrophobic solutes, particularly at lower temperatures. 

In contrast, a heterogeneous solution of noncritical composition (e.g., v xc, as shown by the arrow and dashed line in Fig. 7.11) shows a qualitatively different behavior as it is rises through the coexistence boundary and into the homogeneous region near and above Tc. For each increase in temperature along the dashed line in Fig. 7.11, a horizontal tie-line yields both the compositions of the A-rich and B-rich liquids (from the two ends of the tie-line), as well as the relative amounts of each phase (from the lever rule). Clearly, the critical composition xc remains near the middle of the tie-line as T increases toward Tc, whereas a noncritical composition x xc moves toward one or other terminus of the tie-line as the temperature is raised. 

The upper portion of Fig. 7.12 depicts a succession of snapshots of the solution along the dashed line of Fig. 7.11, showing, for example, the greater ratio of A-rich (lighter color, 

We can also envision the limit in which positive deviations are still more extreme (i.e., A-B interactions are still less favorable compared with A-A, B-B interactions), leading finally to closing of the miscibility gap, as follows  

Lower temperature tends to reduce miscibility, so the liquid-liquid coexistence region tends to spread to a wider composition range at lower T, as shown in Fig. 7.11 or (7.63). However, exceptional cases are known in which the liquid-liquid coexistence region terminates in a lower consolute point, so that complete miscibility can be achieved by cooling below this point. Perhaps most remarkable in this respect is the famous nicotine/water 

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