Closed miscibility loop

Fig. 2.2 Liquid immiscibility. The guaiacol (A) + glycerol (B) system happens to have a closed miscibility loop. The (phase) coexistence curves are shown on the left-hand side (a) for lower temperatures, at which a lower critical solution temperature (LCST), = 40°C, is seen, and on the right-hand side (b) for higher temperatures, where a UCST, Tcs = 82°C, is seen. The compositions of the A-rich phases" and the B-rich phases are shown at 50°C and 70°C, respectively.

There is a large body of experimental work on ternary systems of the type salt + water + organic cosolvent. In many cases the binary water + organic solvent subsystems show reentrant phase transitions, which means that there is more than one critical point. Well-known examples are closed miscibility loops that possess both a LCST and a UCST. Addition of salts may lead to an expansion or shrinking of these loops, or may even generate a loop in a completely miscible binary mixture. By judicious choice of the salt concentration, one can then achieve very special critical states, where two or even more critical points coincide [90, 160,161]. This leads to very peculiar critical behavior—for example, a doubling of the critical exponent y. We shall not discuss these aspects here in detail, but refer to a comprehensive review of reentrant phase transitions [90], We note, however, that for reentrant phase transitions one has to redefine the reduced temperature T, because near a given critical point the system s behavior is also affected by the existence of the second critical point. An improper treatment of these issues will obscure results on criticality. 

The case of UCST > LCST is observed with water-soluble polymers. Examples of these are poly(vinyl alcohol) (see Figure 6-14), poly(vinyl methyl ether), methyl cellulose, and poly(L-proline). The heating of aqueous solutions of these polymers causes a decreasing solvation of the polymer and thus a demixing. In some cases, closed miscibility loops can be observed. 

Figure 9.14 Examples of binary mixtures that have both a UCST and an LCST. Left Mixtures of nicotine (C10H14N2) and water have a closed solubility loop, with UCST = 233°C and LCST = 61.5°C [13]. Right Mixtures of 1-hexene (C5H12) and methane have a miscibility gap, with UCST = 133.8 K and LCST = 179.6 K [14], Pure hexene solidifies at 133.3 K, so the UCST occurs just above the melting curve of the mixtures.

The spinodal temperatures were obtained for a fixed blend ratio (PCL/SAN 60/ 40) and three different copolymer compositions by using three different methods, as discussed above. All obtained values of % were plotted as a function of the copolymer composition (Figure 5.46), creating a UCST border of PCL/SAN blends. It is reasonable that the curve has a minimum because, according to thermodynamic calculations, the miscibility window might be a closed area under some circumstances [34]. An experimentally obtained LCST border and a calculated miscibility loop are also shown in Figure 5.46 [29]. 

For the two-component, two-phase liquid system, the question arises as to how much of each of the pure liquid components dissolves in the other at equilibrium. Indeed, some pairs of liquids are so soluble in each other that they become completely miscible with each other when mixed at any proportions. Such pairs, for example, are water and 1-propanol or benzene and carbon tetrachloride. Other pairs of liquids are practically insoluble in each other, as, for example, water and carbon tetrachloride. Finally, there are pairs of liquids that are completely miscible at certain temperatures, but not at others. For example, water and triethylamine are miscible below 18°C, but not above. Such pairs of liquids are said to have a critical solution temperature, For some pairs of liquids, there is a lower (LOST), as in the water-tiiethylamine pair, but the more common behavior is for pairs of liquids to have an upper (UCST), (Fig. 2.2) and some may even have a closed mutual solubility loop [3]. Such instances are rare in solvent extraction practice, but have been exploited in some systems, where separations have been affected by changes in the temperature. 

The phase diagram of a nonionic amphiphile-water binary system is more complicated (see Figure 3.12). A classic upper critical point exists, but it is usually located below 0°C. At higher temperatures most nonionic amphiphiles show a miscibility gap, which is actually a closed loop with an upper as well as a lower critical point. The lower critical point CPp is often referred to as the cloud point temperature. The upper critical point often lies above the boiling temperature of the mixture (at 0.1 MPa). The position and the shape of the loop depend on... 

According to the type of T versus q> diagram (Fig. 25.4), the binary solution can exhibit an upper critical solution temperature (UCST), a lower critical solution temperature (LCST), or both (close-loop phase behavior). Above the UCST or below the LCST the system is completely miscible in all proportions [82], Below the UCST and above LCST a two-phase liquid can be observed between cp and cp". The two-phase liquid can be subdivided into unstable (spontaneous phase separation) and metastable (phase separation takes some time). These two kinds of mixtures are separated by a spinodal, which is outlined by joining the inflexion points (d AGIdcp ) of successive AG versus cp phase diagrams, obtained at different temperatures (Fig. 25.3b). Thus, the binodal and spinodal touch each other at the critical points cp and T. ... 

If two critical solution points are present in the system, the parameter 2 in Eq. (10-29) must be taken into account. An example is the system poly (ethylene glycol) + water [5], which shows a closed loop miscibility gap. The phase behavior of this system is plotted in Figure 10.8, where the molar mass of the polymer... 

Another interesting example is the application of the model to liquid-liquid equilibria in binary mixtures that exhibit closed-loop immiscibility curves. Some binary systems such as mixtures of 2,6-lutidine and water are completely miscible at high and low temperatures and are immiscible in an intermediate region. The temperature-like variable t = CfAT in the crossover model may now be defined in such a way that t remains always positive in the one-phase region [26, 33] ... 

The two-term crossover Landau model has been used by Edison and Sengers to represent the closed-loop miscibility of a number of liquid mixtures [26]. As an example we show in Fig. 4 the results obtained for the immiscibility curve of the mixture of 2,6-lutidine and water. 

Hydration in aqueous polymer solutions and closed-loop miscibility gaps... 

Fig. 6.10 Phase diagram of aqueous solutions of poly(ethylene oxide) showing the closed-loop miscibility gap. Theoretical curves (solid lines) are fitted to the experimental data of the cloud points (symbols) measured by Saeki et al. [45,46]. The number-average molecular weight in the experiment covers the range 2.17 x 10 -1.02 x 10. (Reprinted with permission from Ref. [29].)...

If we have a system with free-volume or specific interactions, an increase in temperature causes phase separation at LOST. In real systems, where several types of interactions are effective, phase behavior with two regions of partial miscibility of components with UCST and LOST (Fig. 2, binodals 1 and 2) or hourglass-shaped binodal and spinodal curves (Fig. 2, binodal 3) can be expected (5,6,9,13-16). In some cases, a closed loop of immiscibility with LOST and UCST (Fig. 2, binodal 4) or a closed loop and region of partial immiscibility at high temperatures with LOST (Fig. 2, binodals 2 and 4) are observed. This pattern of phase behavior is caused by a diminishing intensity of specific interactions with increasing temperature. 

Type of data cloud points (closed loop miscibility gap) ... 

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