**1. Introduction**

514 Advances in Crystallization Processes

Woodward, P.M.; Vogt, T.; Cox, D.E.; Arulraj, A.; Rao, C.N.R.; Karen, P. & Cheetham, A. K.

1998), pp. 3652-3665, ISSN: 0897-4756

(1998). Influence of Cation Size on the Structural Features of Ln1/2A1/2MnO3 Perovskites at Room Temperature. *Chemmistry of Materials*, Vol. 10, No. 11, (October

> Polycyclic aromatic hydrocarbons (PAHs) are a class of compounds that consist of multiple fused aromatic rings. Concerns have been raised regarding PAHs due to their known health effects(Luthy et al., 1994; Sun et al., 2003). In addition PAHs, chlorinated and brominated polycyclic aromatic hydrocarbons (ClPAHs and BrPAHs) are of interest commercially and of concern for their environmental effects (Shiraishi et al., 1985; Haglund et al., 1987; Nilsson and Ostman, 1993; Koistinen et al., 1994a, b; Ishaq et al., 2003; Kitazawa et al., 2006; Horii et al., 2008; Horii et al., 2009; Ohura et al., 2009; Ni et al., 2010; Ohura et al., 2010). The thermodynamic properties of pure PAHs have been widely studied for more than 50 years (Szczepanik et al., 1963; Wakayama and Inokuchi, 1967; Murray and Pottie, 1974; De Kruif, 1980; Mackay et al., 1982; Bender et al., 1983; Sonnefeld et al., 1983; Hansen and Eckert, 1986; Sato et al., 1986; Hinckley et al., 1990; Nass et al., 1995; Oja and Suuberg, 1997; Ruzicka et al., 1998; Chickos and Acree, 1999; Shiu and Ma, 2000; Burks and Harmon, 2001; Lei et al., 2002; Mackay et al., 2006; Odabasi et al., 2006; Goldfarb and Suuberg, 2008b, a, c; Ma et al., 2010). However, PAHs and halogenated polycyclic aromatic hydrocarbons (HPAHs) often exist as solid and/or liquid mixtures. Therefore it is also important to understand the phase behavior and crystal structures of these PAH and HPAH mixtures.

> Phase behavior involving solid-liquid equilibrium is the basis for crystallization in chemical and materials engineering. Binary mixture systems can have up to three degrees of freedom according to the Gibbs phase rule,

$$\mathbf{F} = \mathbf{C} \cdot \mathbf{P} + \mathbf{2} \tag{1}$$

where F is the degrees of freedom, C is the number of components, and P is the number of phases. Therefore, the equilibrium of binary systems is determined by three variables such as temperature, pressure, and composition, and this is of course increased by one compositional variable for each additional component.

More than half of the true binary organic mixture systems in the literature exhibit simple eutectic behavior (Matsuoka, 1991) (see Figure 1(A)), while about 10% of binary solid systems form solid solutions (Matsuoka, 1991) (see Figure 1(B)), in which the atoms or molecules of one of the components occupy sites in the crystal lattice of the other component

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 517

System *T*fus1/K *T*fus2/K *x*<sup>1</sup> *T*E/K

353.5 323.2 0.360 301.3

353.5 368.2 0.487 327.7

353.5 373.2 0.558 321.3

353.5 307.6 0.362 298.7

353.5 305.2 0.063 302.4

353.5 343.7 0.442 312.4

353.5 383.2 0.665 333.7

353.5 377.2 0.666 327.4

353.5 368.5 0.564 324.6

353.5 388.2 0.613 330.2

353.5 373.2 0.552 323.2

353.5 383.2 0.612 331

353.5 423.2 0.746 339.2

353.5 528.2 0.971 351.4

343.7 388.2 0.909 340.8

343.7 368.5 0.641 319.3

Naphthalene(1) + α-Naphthylamine(2) (Rastogi and Rama Varma, 1956)

Naphthalene(1) + α-Naphthol(2) (Rastogi and Rama Varma, 1956)

Naphthalene(1) + Phenanthrene(2) (Rastogi and Rama Varma, 1956;

Naphthalene(1) + Thionaphthene(2) (Szczepanik et al., 1963; Szczepanik

Naphthalene(1) + Biphenyl(2) (Szczepanik et al., 1963; Szczepanik

Naphthalene(1) + 2-methylnaphthalene(2)

dimethylnaphthalene(2)(Szczepanik and

dimethylnaphthalene(2)(Szczepanik and

Naphthalene(1) + Acenaphthene(2) (Szczepanik et al., 1963; Szczepanik

Naphthalene(1) + Phenanthrene (2) (Szczepanik et al., 1963; Szczepanik

Naphthalene(1) + Fluoranthene (2) (Szczepanik et al., 1963; Szczepanik

Naphthalene(1) + Pyrene (2) (Szczepanik et al., 1963; Szczepanik

Naphthalene(1) + Chrysene(2) (Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

Biphenyl(1) + Acenaphthene(2)

Naphthalene(1) + Fluorene(2) (Szczepanik et al., 1963; Szczepanik

Rastogi and Bassi, 1964)

(Szczepanik et al., 1963)

and Ryszard, 1963)

and Ryszard, 1963)

Ryszard, 1963)

Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

(Szczepanik et al., 1963; Szczepanik and Ryszard, 1963)

Biphenyl(1) + Fluorene(2)

Naphthalene(1) + 2,6-

Naphthalene(1) + 2,3-

without modifying its crystal structure. Additionally, about a quarter of these systems form intermolecular compounds (Matsuoka, 1991), such as monotectics (see Figure 1(C)). However, only limited research has been done on binary organic mixture systems, especially PAH binary mixture systems. Moreover, crystal morphology, i.e., polymorphs, racemates, and structural isomers, also affect the phase diagram and may induce non-ideal solid-liquid equilibrium.

Fig. 1. Phase diagram of eutectic (A), solid solution (B), and monotectic (C) systems.

### **2. Eutectic systems**

Figure 1(A) shows a phase diagram of a typical eutectic mixture system, which has a minimum melting temperature, i.e. a eutectic point. The eutectic point of a binary condensed mixture is defined as the temperature at which a solid mixture phase is in equilibrium with the liquid phase and a eutectic is generally considered to be a simple mechanical mixture of the solid and liquid (Rastogi and Bassi, 1964).

As in many other binary alloy mixtures, most PAH binary mixture systems exhibit eutectic behavior. Table 1 lists the eutectic point and eutectic concentration for about 50 binary PAHcontaining mixture systems, in which at least one of the components is a PAH. The shape of the phase diagram for most of these binary mixture systems is similar to the phase diagram of anthrancene + pyrene mixture system (see Figure 2), except for a few systems, whose eutectic concentration is quite close to a pure component, such as in the naphthalene + chrysene system and phenanthrene + chrysene system.

For the studies preformed by this group on the anthracene + pyrene system (Rice et al., 2010), mixtures were prepared at various compositions by using a melt and quench-cool technique (Fu et al., 2010). Generally, the melting points and enthalpies of fusion of these PAH binary mixtures were found to often actually be independent of mixture preparation techniques. The liquidus and thaw points were determined according to the method proposed by Pounder an Masson (Pounder and Masson, 1934). The thaw temperature is the temperature at which the first droplet of liquid appears in a mixture-containing capillary. The liquidus temperature is the maximum temperature at which both solid crystals and liquid are observed to coexist. Above this temperature, there is only liquid phase present.


without modifying its crystal structure. Additionally, about a quarter of these systems form intermolecular compounds (Matsuoka, 1991), such as monotectics (see Figure 1(C)). However, only limited research has been done on binary organic mixture systems, especially PAH binary mixture systems. Moreover, crystal morphology, i.e., polymorphs, racemates, and structural isomers, also affect the phase diagram and may induce non-ideal

Fig. 1. Phase diagram of eutectic (A), solid solution (B), and monotectic (C) systems.

mechanical mixture of the solid and liquid (Rastogi and Bassi, 1964).

chrysene system and phenanthrene + chrysene system.

Figure 1(A) shows a phase diagram of a typical eutectic mixture system, which has a minimum melting temperature, i.e. a eutectic point. The eutectic point of a binary condensed mixture is defined as the temperature at which a solid mixture phase is in equilibrium with the liquid phase and a eutectic is generally considered to be a simple

As in many other binary alloy mixtures, most PAH binary mixture systems exhibit eutectic behavior. Table 1 lists the eutectic point and eutectic concentration for about 50 binary PAHcontaining mixture systems, in which at least one of the components is a PAH. The shape of the phase diagram for most of these binary mixture systems is similar to the phase diagram of anthrancene + pyrene mixture system (see Figure 2), except for a few systems, whose eutectic concentration is quite close to a pure component, such as in the naphthalene +

For the studies preformed by this group on the anthracene + pyrene system (Rice et al., 2010), mixtures were prepared at various compositions by using a melt and quench-cool technique (Fu et al., 2010). Generally, the melting points and enthalpies of fusion of these PAH binary mixtures were found to often actually be independent of mixture preparation techniques. The liquidus and thaw points were determined according to the method proposed by Pounder an Masson (Pounder and Masson, 1934). The thaw temperature is the temperature at which the first droplet of liquid appears in a mixture-containing capillary. The liquidus temperature is the maximum temperature at which both solid crystals and liquid are observed to coexist. Above this temperature, there is only liquid

solid-liquid equilibrium.

**2. Eutectic systems** 

phase present.



Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 519

489.8 423.2 0.221 404.6

518 383.2 0.119 377.3

518 423.2 0.154 409.1

518 528.2 0.578 480.6

383.2 368.5 0.433 336.9

383.2 388.2 0.516 342.7

383.2 472.2 0.794 368.7

383.2 423.2 0.800 368.3

383.2 528.2 0.952 379.6

423.2 528.2 0.855 405.7

368.5 298.7 0.055 295

368.5 352.3 0.423 323.6

368.5 307.5 0.212 290.9

368.5 383.2 0.598 339.7

368.5 370.2 0.531 333.9

Anthracene(1) + Pyrene(2)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

Pyrene(1) + Chrysene(2)

Carbazole(1) + Pyrene(2)

Carbazole(1) + Chrysene(2)

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

Fluoranthene(1) + Acenaphthene(2) (Szczepanik et al., 1963; Szczepanik

Fluoranthene(1) + 2-methylanthracene (2) (Szczepanik et al., 1963; Szczepanik

Fluoranthene(1) + Fluorene(2) (Szczepanik et al., 1963; Szczepanik

Fluoranthene(1) + Pyrene(2) (Szczepanik et al., 1963; Szczepanik

Fluoranthene(1) + Chrysene(2) (Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

Acenaphthene (1) + 1,2-dimethylbenzene(2)

Acenaphthene (1) + 2-methylnaphthalene(2)

Acenaphthene (1) + 2,6-dimethylnaphthalene (2)

Acenaphthene (1) + 2,7-dimethylnaphthalene (2)

Acenaphthene (1) + 1,2,4,5-tetramethylbenzene(2)

Carbazole(1) + Fluoranthene(2) (Szczepanik et al., 1963; Szczepanik


359.2 368.5 0.578 326

388.2 368.5 0.431 338.6

388.2 375.2 0.658 361.6

373.2 343.7 0.691 324.8

373.2 368.5 0.495 327.5

373.2 375.2 0.704 331.5

373.2 388.2 0.637 368.7

373.2 423.2 0.747 354.7

373.2 332 0.318 309.7

373.2 388.2 0.621 342.6

373.2 383.2 0.532 347.7

373.2 528.2 0.957 369.2

489.8 518 0.943 488.4

489.8 472.2 0.108 471.5

489.8 528.2 0.662 464.6

Diphenylene oxide(1) + Acenaphthene(2) (Szczepanik et al., 1963; Szczepanik

Fluorene(1) + 2,3,6-trimethylnaphthalene(2)

Phenanthrene(1) + 2,3,6-trimethylnaphthalene(2)

Phenanthrene(1) + 3-methylphenanthrene(2)

Phenanthrene(1) + 4,5-dimethylphenanthrene(2)

(Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

Phenanthrene(1) + Fluoranthene (2) (Szczepanik et al., 1963; Szczepanik

Phenanthrene(1) + Chrysene(2) (Szczepanik et al., 1963; Szczepanik

Anthracene(1) + Carbazole(2) (Szczepanik et al., 1963; Szczepanik

Anthracene(1) + Chrysene(2) (Szczepanik et al., 1963; Szczepanik

Anthracene(1) + 2-methylanthracene(2) (Szczepanik et al., 1963; Szczepanik

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

Fluorene(1) + Acenaphthene(2) (Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

Phenanthrene(1) + Acenaphthene(2) (Szczepanik et al., 1963; Szczepanik

(Szczepanik et al., 1963; Szczepanik

Phenanthrene(1) + Fluorene(2) (Szczepanik et al., 1963; Szczepanik

Phenanthrene(1) + Pyrene(2) (Szczepanik et al., 1963; Szczepanik

Phenanthrene(1) + Biphenyl(2) (Szczepanik et al., 1963; Szczepanik


Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 521

The eutectic point for the anthracene (1) + pyrene (2) system occurs at 404 K at *x*1 = 0.22 (see Figure 2). Only solid state exists below the thaw curve, i.e. eutectic temperature, and only liquid state exists above the liquidus curve. The areas between these two curves exhibit the

Fig. 3. Full DSC scan of an equimolar anthracene (1) + pyrene (2) mixture (Rice et al., 2010).

Figure 2 also displays the correlation between phase behavior and enthalpy of fusion, Δfus*H* for the system. The Δfus*H* observed for a DSC peak near the eutectic temperature of 404 K indicates the heat input for the initial melting of a eutectic solid phase to occur. The total Δfus*H* shown in Figure 2 is a summation of both endothermic phase transition peaks observed in the DSC scan, i.e. the eutectic phase melting and the non-eutectic phase melting (see Figure 3). It is worth noting that the total Δfus*H* is very similar to that of pure pyrene over a wide range of compositions and thus the Δfus*H* for both pure pyrene and the eutectic mixture are very similar. This means that when the mixture contains only a modest amount of anthracene, energetically it behaves quite similarly to pure pyrene, and this persists until the mixture is nearly pure anthracene (see Figure 2). There is a slight increase in fusion enthalpy when the mixtures are enriched in anthracene beyond the eutectic composition,

coexistence of both solid and liquid phases.

Table 1. Melting temperatures of previously reported binary PAH eutectic systems

Fig. 2. Phase diagram and enthalpy of fusion of the anthracene (1) + pyrene (2) system (Rice et al., 2010).

Table 1. Melting temperatures of previously reported binary PAH eutectic systems

Fig. 2. Phase diagram and enthalpy of fusion of the anthracene (1) + pyrene (2) system (Rice

368.5 353.2 0.417 323.2

368.5 372.5 0.492 329

368.5 387.2 0.582 337.7

368.5 489.7 0.914 361.2

Acenaphthene (1) + Naphthalene (2) (Szczepanik et al., 1963; Szczepanik

Acenaphthene (1) + Phenanthrene(2) (Szczepanik et al., 1963; Szczepanik

Acenaphthene (1) + Fluorene(2) (Szczepanik et al., 1963; Szczepanik

Acenaphthene (1) + Anthracene(2) (Szczepanik et al., 1963; Szczepanik

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

and Ryszard, 1963)

et al., 2010).

The eutectic point for the anthracene (1) + pyrene (2) system occurs at 404 K at *x*1 = 0.22 (see Figure 2). Only solid state exists below the thaw curve, i.e. eutectic temperature, and only liquid state exists above the liquidus curve. The areas between these two curves exhibit the coexistence of both solid and liquid phases.

Fig. 3. Full DSC scan of an equimolar anthracene (1) + pyrene (2) mixture (Rice et al., 2010).

Figure 2 also displays the correlation between phase behavior and enthalpy of fusion, Δfus*H* for the system. The Δfus*H* observed for a DSC peak near the eutectic temperature of 404 K indicates the heat input for the initial melting of a eutectic solid phase to occur. The total Δfus*H* shown in Figure 2 is a summation of both endothermic phase transition peaks observed in the DSC scan, i.e. the eutectic phase melting and the non-eutectic phase melting (see Figure 3). It is worth noting that the total Δfus*H* is very similar to that of pure pyrene over a wide range of compositions and thus the Δfus*H* for both pure pyrene and the eutectic mixture are very similar. This means that when the mixture contains only a modest amount of anthracene, energetically it behaves quite similarly to pure pyrene, and this persists until the mixture is nearly pure anthracene (see Figure 2). There is a slight increase in fusion enthalpy when the mixtures are enriched in anthracene beyond the eutectic composition,

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 523

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

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

System *T*fus1/K *T*fus2/K *x*<sup>M</sup> *T*M/K *x*<sup>E</sup> *T*E/K

(Rai and Pandey, 2002) 330.2 423.2 0.025 416.5 0.744 328.5

(Singh et al., 1985) 330.2 373.2 0.225 363.2a 0.975 ~328.2

(Gupta and Singh, 2004) 388.2 423.2 0.324 392.2 0.792 376.2

(Gupta and Singh, 2004) 362.2 423.2 0.301 363.2 0.702 361.2

(Gupta and Singh, 2004) 413.2 423.2 0.902 413.2 0.299 403.2

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

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

378.2 353.2 0.316 357.7 0.838 344.2

**3. Monotectic systems** 

eutectic systems.

indistinguishable.

exists.

2,4-Dinitrophenol(1) + Naphthalene(2)

(Singh et al., 2001; Singh et al., 2007)

Succinonitrile (1) + Phenanthrene (2)

Succinonitrile (1) + Pyrene(2)

p-benzoquinone(1) + Pyrene(2)

m-dinitrobenzene(1)+ Pyrene(2)

m-nitrobenzoic acid(1) +Pyrene(2)

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 amounts of pyrene.

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 that of pure anthracene.

Fig. 4. X‐ray diffraction patters of pure components and mixtures of anthracene (1) + pyrene (2) (Rice et al., 2010).
