**3. Monotectic systems**

522 Advances in Crystallization Processes

but the shift is only modest as compared with the increase of fusion enthalpy to that of pure anthracene (see Figure 2). This indicates that the ability of anthracene to reach a lower energy crystalline configuration is significantly impeded by the presence of relatively small

Additionally, Powder X-ray diffraction patterns for the same anthracene (1) + pyrene (2) system were also obtained. Figure 4 shows that the crystal structure of the eutectic mixture is similar to that of pyrene because peaks at 10.6, 11.6, 14.9, 16.3, 18.2, 23.3, 24.7 and 28.0 degree are all retained in the mixture diffraction pattern. This is consistent with the DSC result that implies that the Δfus*H* of the eutectic is very close to that of pure pyrene, and indicates that the crystal structures of the eutectic mixture and pure pyrene are similar. Likewise, Figure 4 shows that the crystal structure of a mixture at *x*1 = 0.90 is comparable to

Fig. 4. X‐ray diffraction patters of pure components and mixtures of anthracene (1) + pyrene

amounts of pyrene.

that of pure anthracene.

(2) (Rice et al., 2010).

In contrast to eutectic systems, in which both components solidify below eutectic temperature, a monotectic reaction is characterized by the breakdown of a liquid into one solid and one liquid phase (Singh et al., 1985), i.e. one liquid phase decomposes into a solid phase and a liquid phase when the temperature is below the monotectic temperature. Figure 1(C) shows the phase diagram of a typical monotectic system. The monotectic composition is determined by the intersection of a liquidus line and a liquid miscibility gap (Singh et al., 1985). Generally, monotectic systems are less studied than eutectic systems.

Binary organic mixtures with PAHs can form monotectic systems. Table 2 lists the monotectic and eutectic point of a few monotectic forming PAH systems. Monotectic systems are characterized by monotectic, eutectic and upper consolute temperatures, though the upper consolute temperature is often not reported. The monotectic temperature, tM, is the temperature at monotectic composition and the upper consolute temperature is the highest melting temperature of the mixture system, i.e. the critical point where the two liquid phases having identical composition become indistinguishable.


Table 2. Melting temperatures of previously reported binary PAH monotectic systems

Rai and Pandey studied the phase behavior of succinonitrile (1) + pyrene (2) mixture system (Rai and Pandey, 2002), which is a typical monotectic system (Figure 5). The enthalpy of fusion of pyrene, 17.65 kJ·mole-1 (Chickos and Acree, 1999), is much higher than that of succinonitrile, 3.7 kJ·mole-1 (Rai and Pandey, 2002). The monotectic point is 416.5 K (143.3°C) at *x*1=0.025. The eutectic temperature is 328.5 K (55.4°C) at *x*1=0.744 and the upper consolute temperature, tC (465.2 K, 192.0°C), is 48.7 K above the monotectic point. When *x*1 is between monotectic and eutectic composition, the two liquids, L1 (rich in pyrene) and L2 (rich in succinonitrile) are mutually immiscible. However, if the temperature is above the consolute temperature, there is complete miscibility in liquid state, i.e. only one liquid phase exists.

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 525

mixture M2 becomes a mixture of liquid B and solid D. If M2 is further cooled to temperature T3, the liquid composition changes continuously from B to E along the liquidus curve, while the solid composition changes from D to F along the solidus curve. Additionally, the Hume-Rothery rules, named after William Hume-Rothery, are used to describe the conditions under which an element can dissolve in a metal and form a solid

Szczepanik and Skalmowski (Szczepanik et al., 1963; Szczepanik and Ryszard, 1963) studied the phase behavior of over 60 PAH binary mixture systems, and demonstrate that PAH mixture systems also form solid solution, as shown by naphthalene + 1-methynaphthanlene, naphthalene + anthracene, phenanthrene + anthracene, phenanthrene + carbazole, anthracene + acridine, anthracene + fluoranthene, and chrysene + 1,2-benzanthracene systems. It is not known whether the the Hume-Rothery rules still work for PAH mixtures. However, it is worth noting that the number of such systems is small, compared with the

Due to the large molecular mass and complexity of the crystal structure of PAHs, i.e. polymorphism and racemate, the phase behavior of some of the PAH binary mixtures may be different from the above described three phase behaviors. Three complicated PAH binary mixture systems, i.e. anthracene + benzo[a]pyrene system (Rice and Suuberg, 2010), pyrene + 9,10-dibromoanthracene system (Fu et al., 2010), and anthracene + 2-bromoanthracene are

Benzo[a]pyrene has a much larger molecular mass compared to pyrene, which leads to phase behavior in the anthracene (1) + benzo[a]pyrene (2) system (Rice and Suuberg, 2010) that is different from that of the anthracene + pyrene system. The phase diagram of anthracene (1) + benzo[a]pyrene (2) system ( see Figure 6) indicates an eutectic-like mixture behavior. A eutectic-like phase is formed near *x*1 = 0.26 between 414 and 420 K. There is however always a gap between the thaw curve and the lowest liquidus temperature, which is distinct from true eutectic behavior such as in Figure 1(A) or Figure 2. Therefore, mixtures of anthracene and benzo[a]pyrene form a single, amorphous, solid eutectic-like phase at *x*1 = 0.26 that lacks any organized crystal structure and which melts throughout the 414 to 420 K temperature range. This region of phase transition, represented by the shaded region of Figure 6, is not rate dependant and is observed in both the DSC and melting temperature analysis for all combinations of anthracene + benzo[a]pyrene, providing evidence that this region represents the melting temperature range of a single, amorphous, solid phase. This conclusion is also supported by the X-ray

Powder X‐ray diffraction studies were conducted to study the crystal structures of the anthracene (1) + benzo[*a*]pyrene (2) system (see Figure 7). The eutectic-like mixture lacks any organized crystal structure because the few peaks that exist in the X‐ray pattern are not well defined and do not rise much above the baseline. Additionally, there is no real

solution.

introduced here.

diffraction results.

number of eutectic-forming systems.

**5. Systems with complex phase behavior** 

**5.1 Anthracene + benzo[a]pyrene system** 

Fig. 5. Phase diagram of succinonitrile (1) + pyrene (2) mixture system (Rai and Pandey, 2002).

#### **4. Solid solution**

A solid solution is a solid mixture in which one or more atoms and/or molecules of one of the components occupies sites in the crystal lattice of the other component without significantly changing its crystal structure, even though the lattice parameter may vary. So this kind of system has a homogenous crystalline structure and is also called isomorphic system, because the components are completely miscible in both the liquid and solid phases. Figure 1(B) shows the phase behavior of a binary mixture system that forms a solid solution. In the diagram, the curve ABC and ADC are the liquidus and solidus curves, respectively. The area above ABC curve represents the region of homogeneous liquid solutions and the area below ADC curve represents the region of homogeneous solid solution. The area enclosed by ABCD is the region of liquid + solid solution. For instance, a mixture M1 at temperature T1is cooled to temperature T2, the mixture M2 becomes a mixture of liquid B and solid D. If M2 is further cooled to temperature T3, the liquid composition changes continuously from B to E along the liquidus curve, while the solid composition changes from D to F along the solidus curve. Additionally, the Hume-Rothery rules, named after William Hume-Rothery, are used to describe the conditions under which an element can dissolve in a metal and form a solid solution.

Szczepanik and Skalmowski (Szczepanik et al., 1963; Szczepanik and Ryszard, 1963) studied the phase behavior of over 60 PAH binary mixture systems, and demonstrate that PAH mixture systems also form solid solution, as shown by naphthalene + 1-methynaphthanlene, naphthalene + anthracene, phenanthrene + anthracene, phenanthrene + carbazole, anthracene + acridine, anthracene + fluoranthene, and chrysene + 1,2-benzanthracene systems. It is not known whether the the Hume-Rothery rules still work for PAH mixtures. However, it is worth noting that the number of such systems is small, compared with the number of eutectic-forming systems.
