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

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 diffraction results.

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

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 527

Benzo[*a*]pyrene rich mixture with *x*

Eutectic-like mixture with *x*

1

1

Benzo[*a*]pyrene

= 0.10

= 0.26

Pyrene

10 15 20 25 30 35

**2/º**

The influence of halogen substitution on the interaction energy between PAH molecules has also been investigated. Unlike the anthracene + pyrene mixture system, bromine substitution on anthracene induces a different kind of interaction in the pyrene (1) + 9,10 dibromoanthracene (2) mixture system, which also results in non-idealities in solid-liquid equilibrium (see Figure 8). The surface area and volume of the 9,10-dibromoanthracene

Fig. 7. X‐ray diffraction patters of pure components and mixtures of anthracene (1) +

benzo[*a*]pyrene (2) system (Rice and Suuberg, 2010).

**5.2 Pyrene + 9,10-dibromoanthracene system** 

molecule is much larger than that of pyrene.

**Intensity/arb unit**

similarity between the eutectic mixture scan and those of the pure components. This result is consistent with the melting point studies that imply that the mixtures form a single, amorphous solid phase at the eutectic composition.

Fig. 6. Phase diagram of anthracene (1) + benzo[a]pyrene (2) system (Rice and Suuberg, 2010).

similarity between the eutectic mixture scan and those of the pure components. This result is consistent with the melting point studies that imply that the mixtures form a single,

Fig. 6. Phase diagram of anthracene (1) + benzo[a]pyrene (2) system (Rice and Suuberg, 2010).

amorphous solid phase at the eutectic composition.

Fig. 7. X‐ray diffraction patters of pure components and mixtures of anthracene (1) + benzo[*a*]pyrene (2) system (Rice and Suuberg, 2010).

#### **5.2 Pyrene + 9,10-dibromoanthracene system**

The influence of halogen substitution on the interaction energy between PAH molecules has also been investigated. Unlike the anthracene + pyrene mixture system, bromine substitution on anthracene induces a different kind of interaction in the pyrene (1) + 9,10 dibromoanthracene (2) mixture system, which also results in non-idealities in solid-liquid equilibrium (see Figure 8). The surface area and volume of the 9,10-dibromoanthracene molecule is much larger than that of pyrene.

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 529

The full heating, cooling and reheating scan of a pyrene + 9,10-dibromoanthracene mixture at x2 = 0.48 (in region C) is shown in Figure 9, where Φ is heat flow in the DSC. During the heating scan, two peaks appear at 428 K and 440 K, which indicates the two-phase character of the mixture. Two peaks are also observed in the cooling scan, in which the 9,10 dibromoanthracene like phase crystallizes first at 418 K, and then the pyrene like phase crystallizes at 410 K. The cooling scan also suggested two-phase behavior of the mixture just as did the melting behavior. When reheated, the phase transition enthalpies and associated

Fig. 9. Full DSC scan of a pyrene (1) + 9,10-dibromoanthracene (2) mixture at *x*2 = 0.48

(Fu et al., 2010).

temperatures matched those of the initial heating scan.

Fig. 8. Phase diagram of pyrene (1) + 9,10-dibromoanthracene (2) mixture system (Fu et al., 2010).

The phase diagram of this system can be crudely divided into 5 regions. The mixtures with relatively low mole fraction of 9,10-dibromoanthracene (< 0.30), in region A, form a pyrene like phase. When the mole fraction of 9,10-dibromoanthracene is between 0.30-0.41, in region B, the mixtures transition from a pyrene-like phase to two phases that both have low melting temperatures. The divergence of the liquidus and thaw curve is 2-9 K. In region C, mixtures containing about *x*2 = 0.41-0.50 also show two-phase character and start to transition to 9,10-dibromoanthracene behavior. Mixtures with x2 = 0.50-0.75, in region D, also have two phases with 9,10-dibromoanthracene like behavior and high melting temperature. Only one of the phases evolves while the other gives a constant low melting temperature (corresponding to the thaw point). In region E, a 9,10-dibromoanthracene like phase is defined based upon the thermal behavior, shown below.

Fig. 8. Phase diagram of pyrene (1) + 9,10-dibromoanthracene (2) mixture system

phase is defined based upon the thermal behavior, shown below.

The phase diagram of this system can be crudely divided into 5 regions. The mixtures with relatively low mole fraction of 9,10-dibromoanthracene (< 0.30), in region A, form a pyrene like phase. When the mole fraction of 9,10-dibromoanthracene is between 0.30-0.41, in region B, the mixtures transition from a pyrene-like phase to two phases that both have low melting temperatures. The divergence of the liquidus and thaw curve is 2-9 K. In region C, mixtures containing about *x*2 = 0.41-0.50 also show two-phase character and start to transition to 9,10-dibromoanthracene behavior. Mixtures with x2 = 0.50-0.75, in region D, also have two phases with 9,10-dibromoanthracene like behavior and high melting temperature. Only one of the phases evolves while the other gives a constant low melting temperature (corresponding to the thaw point). In region E, a 9,10-dibromoanthracene like

(Fu et al., 2010).

The full heating, cooling and reheating scan of a pyrene + 9,10-dibromoanthracene mixture at x2 = 0.48 (in region C) is shown in Figure 9, where Φ is heat flow in the DSC. During the heating scan, two peaks appear at 428 K and 440 K, which indicates the two-phase character of the mixture. Two peaks are also observed in the cooling scan, in which the 9,10 dibromoanthracene like phase crystallizes first at 418 K, and then the pyrene like phase crystallizes at 410 K. The cooling scan also suggested two-phase behavior of the mixture just as did the melting behavior. When reheated, the phase transition enthalpies and associated temperatures matched those of the initial heating scan.

Fig. 9. Full DSC scan of a pyrene (1) + 9,10-dibromoanthracene (2) mixture at *x*2 = 0.48 (Fu et al., 2010).

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 531

Since the enthalpies of crystallization of the mixtures with 9,10-dibromoanthracene mole fractions of 0.55 and 0.75 are significantly lower than that of other mixtures, these are at a higher energy state and are less stable than other mixtures with nearby compositions. Moreover, the mixture with 0.65 mole fraction of 9,10-dibromoanthracene is in a more stable state than those mixtures with 0.55 and 0.75 mole fraction of 9,10-dibromoanthracene meaning that around a 2:1 molar ratio of 9,10-dibromoanthracene to pyrene, there exists a

The powder X-ray diffraction method was used to study the crystal structures of the pyrene and 9,10-dibromoanthracene mixtures (see Figure 11). The results are qualitative. For the 9,10-dibromoanthracene rich mixture at the region D-E boundary, at x2 = 0.75 (curve E), the XRD data show a 9,10-dibromoanthracene like microstructure though there are distinct differences from 9,10-dibromoanthracene. The pyrene rich mixture in region A at x2 = 0.25 (curve A) has the pyrene like microstructure. However, the mixture at x2 = 0.65 (curve D) reflects neither pyrene nor 9,10-dibromoanthracene like microstructure, and in fact is

Fig. 11. X‐ray diffraction patters of pure components and mixtures of pyrene (1) + 9,10-

particular lower energy configuration.

dibromoanthracene(2) (Fu et al., 2010).

amorphous.

The temperature and enthalpy of crystallization (subcooled), shown in Figure 10, correspond to the results obtained from the phase diagram. Mixtures with a mole fraction of 9,10-dibromoanthraene 0.30-0.75, in regions B, C and D, have two-phase character, which is observed as two distinct phase-transition peaks during the cooling procedure. Region E showed two-phase melting behavior, but in the DSC experiments of Figure 10, the low temperature crystallization peak was absent. Likewise, region B showed two distinct melting peaks, whereas in the DSC experiment only a single peak was observed.

Fig. 10. Crystallization temperature and total enthalpy of crystallization of pyrene (1) + 9,10 dibromoanthracene (2) mixtures. 1st crystallization temperature is the higher temperature peak in the DSC cooling scan, and 2nd crystallization temperature is the lower temperature peak in the DSC cooling scan (Fu et al., 2010).

The temperature and enthalpy of crystallization (subcooled), shown in Figure 10, correspond to the results obtained from the phase diagram. Mixtures with a mole fraction of 9,10-dibromoanthraene 0.30-0.75, in regions B, C and D, have two-phase character, which is observed as two distinct phase-transition peaks during the cooling procedure. Region E showed two-phase melting behavior, but in the DSC experiments of Figure 10, the low temperature crystallization peak was absent. Likewise, region B showed two distinct

Fig. 10. Crystallization temperature and total enthalpy of crystallization of pyrene (1) + 9,10 dibromoanthracene (2) mixtures. 1st crystallization temperature is the higher temperature peak in the DSC cooling scan, and 2nd crystallization temperature is the lower temperature

peak in the DSC cooling scan (Fu et al., 2010).

melting peaks, whereas in the DSC experiment only a single peak was observed.

Since the enthalpies of crystallization of the mixtures with 9,10-dibromoanthracene mole fractions of 0.55 and 0.75 are significantly lower than that of other mixtures, these are at a higher energy state and are less stable than other mixtures with nearby compositions. Moreover, the mixture with 0.65 mole fraction of 9,10-dibromoanthracene is in a more stable state than those mixtures with 0.55 and 0.75 mole fraction of 9,10-dibromoanthracene meaning that around a 2:1 molar ratio of 9,10-dibromoanthracene to pyrene, there exists a particular lower energy configuration.

The powder X-ray diffraction method was used to study the crystal structures of the pyrene and 9,10-dibromoanthracene mixtures (see Figure 11). The results are qualitative. For the 9,10-dibromoanthracene rich mixture at the region D-E boundary, at x2 = 0.75 (curve E), the XRD data show a 9,10-dibromoanthracene like microstructure though there are distinct differences from 9,10-dibromoanthracene. The pyrene rich mixture in region A at x2 = 0.25 (curve A) has the pyrene like microstructure. However, the mixture at x2 = 0.65 (curve D) reflects neither pyrene nor 9,10-dibromoanthracene like microstructure, and in fact is amorphous.

Fig. 11. X‐ray diffraction patters of pure components and mixtures of pyrene (1) + 9,10 dibromoanthracene(2) (Fu et al., 2010).

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 533

The powder X-ray diffraction method was also used to study the crystal structures of pure anthracene, 2-bromoanthracene and their mixtures (see Figure 13). The lattice structure of anthracene crystals is monoclinic with *a* = 8.44 Å, *b* = 5.99 Å, *c* = 11.11 Å, *β* = 125.4° (Jo et al., 2006). The strong diffraction peak at 19.58° in pure anthracene corresponds to the (002) plane, and the spacing between the 002 planes is 4.53 Å. With the increase of the mole fraction of 2-bromoanthracene, *x*2, in the mixture, the (002) plane spacing starts to shift to lower values. Moreover, a new diffraction peak occurs near 2*θ* = 17° with increasing *x*2 in the mixture. This indicates that new mixture crystals are formed. The new peak appears at 2*θ* = 16.38° when *x*1 = 0.70 roughly corresponding to the lowest solid-liquid equilibrium melting point. With increase of *x*1, the peak position increases from 16.38° to 17.06° and disappears in pure anthracene. The diffraction data for mixtures with *x*1 = 0.50 and 0.10 indicate

10 20 30 40 50 60

*x* 1 = 1.00

0.90

0.80

0.72

0.70

0.50 0.18 0.10

*x* 2 = 1.00

**2/û**

Fig. 13. X‐ray diffraction patters of pure components and mixtures of anthracene

relatively amorphous structures.

*Intensity***/arb unit**

(1) + 2-bromoanthracene(2).

#### **5.3 Anthracene + 2-bromoanthracene system**

The influence of bromine substitution on thermochemical properties of PAH mixture systems was further investigated by studying the anthracene (1) + 2-bromoanthracene (2) system. The crystal structure is changed by addition one bromine atom on the aromatic ring. Moreover, the surface area and volume of 2-bromoanthracene is about 10% bigger than those of anthracene.

The solid-liquid equilibrium diagram of anthracene (1) + 2-bromoanthracene (2) system is shown in Figure 12. The diagram suggests the non-ideality of the anthracene + 2 bromoanthracene system. The melting temperature range (thaw to completion) of these mixtures at any given composition is observed to be 1.1 - 2.6 K. The reported solid-liquid equilibrium melting temperature is here taken as the thaw temperature, in Figure 12. The lowest solid-liquid equilibrium temperature for the system is 477.65 K at *x*1 = 0.74, and the melting temperature range of this mixture is 1.8 K.

Fig. 12. Phase diagram and distance between (002) planes of anthracene (1) + 2 bromoanthracene (2) system.

The influence of bromine substitution on thermochemical properties of PAH mixture systems was further investigated by studying the anthracene (1) + 2-bromoanthracene (2) system. The crystal structure is changed by addition one bromine atom on the aromatic ring. Moreover, the surface area and volume of 2-bromoanthracene is about 10% bigger than

The solid-liquid equilibrium diagram of anthracene (1) + 2-bromoanthracene (2) system is shown in Figure 12. The diagram suggests the non-ideality of the anthracene + 2 bromoanthracene system. The melting temperature range (thaw to completion) of these mixtures at any given composition is observed to be 1.1 - 2.6 K. The reported solid-liquid equilibrium melting temperature is here taken as the thaw temperature, in Figure 12. The lowest solid-liquid equilibrium temperature for the system is 477.65 K at *x*1 = 0.74, and the

Fig. 12. Phase diagram and distance between (002) planes of anthracene (1) + 2-

bromoanthracene (2) system.

**5.3 Anthracene + 2-bromoanthracene system** 

melting temperature range of this mixture is 1.8 K.

those of anthracene.

The powder X-ray diffraction method was also used to study the crystal structures of pure anthracene, 2-bromoanthracene and their mixtures (see Figure 13). The lattice structure of anthracene crystals is monoclinic with *a* = 8.44 Å, *b* = 5.99 Å, *c* = 11.11 Å, *β* = 125.4° (Jo et al., 2006). The strong diffraction peak at 19.58° in pure anthracene corresponds to the (002) plane, and the spacing between the 002 planes is 4.53 Å. With the increase of the mole fraction of 2-bromoanthracene, *x*2, in the mixture, the (002) plane spacing starts to shift to lower values. Moreover, a new diffraction peak occurs near 2*θ* = 17° with increasing *x*2 in the mixture. This indicates that new mixture crystals are formed. The new peak appears at 2*θ* = 16.38° when *x*1 = 0.70 roughly corresponding to the lowest solid-liquid equilibrium melting point. With increase of *x*1, the peak position increases from 16.38° to 17.06° and disappears in pure anthracene. The diffraction data for mixtures with *x*1 = 0.50 and 0.10 indicate relatively amorphous structures.

Fig. 13. X‐ray diffraction patters of pure components and mixtures of anthracene (1) + 2-bromoanthracene(2).

Phase Behavior and Crystal Structure of Binary Polycyclic Aromatic Compound Mixtures 535

Chickos, J.S., Acree, W.E., 1999. Estimating solid-liquid phase change enthalpies and

De Kruif, C., 1980. Enthalpies of sublimation and vapor-pressure of 11 polycyclic-

Fu, J., Rice, J.W., Suuberg, E.M., 2010. Phase behavior and vapor pressures of the pyrene +

Goldfarb, J.L., Suuberg, E.M., 2008a. The effect of halogen hetero-atoms on the vapor

Goldfarb, J.L., Suuberg, E.M., 2008b. Vapor pressures and enthalpies of sublimation of ten

Goldfarb, J.L., Suuberg, E.M., 2008c. Vapor pressures and thermodynamics of oxygen-

Gupta, R.K., Singh, R.A., 2004. Thermochemical and microstructural studies on binary organic eutectics and complexes. Journal of Crystal Growth 267, 340-347. Haglund, P., Alsberg, T., Bergman, A., Jansson, B., 1987. Analysis of Halogenated Polycyclic

Hansen, P.C., Eckert, C.A., 1986. An Improved Transpiration Method for the Measurement

Hinckley, D.A., Bidleman, T.F., Foreman, W.T., Tuschall, J.R., 1990. Determination of Vapor-

Horii, Y., Khim, J.S., Higley, E.B., Giesy, J.P., Ohura, T., Kannan, K., 2009. Relative Potencies

Horii, Y., Ok, G., Ohura, T., Kannan, K., 2008. Occurrence and profiles of chlorinated and

Ishaq, R., Naf, C., Zebuhr, Y., Broman, D., Jarnberg, U., 2003. PCBs, PCNs, PCDD/Fs, PAHs

Jo, S., Yoshikawa, H., Fujii, A., Takenaga, M., 2006. Surface morphologies of anthracene single crystals grown from vapor phase. Appl Surf Sci 252, 3514-3519. Kitazawa, A., Amagai, T., Ohura, T., 2006. Temporal trends and relationships of particulate

Koistinen, J., Paasivirta, J., Nevalainen, T., Lahtipera, M., 1994a. Chlorinated Fluorenes and

of Very Low Vapor-Pressures. J Chem Eng Data 31, 1-3.

Chromatographic Retention Data. J Chem Eng Data 35, 232-237.

pressures and thermodynamics of polycyclic aromatic compounds measured via

polycyclic aromatic hydrocarbons determined via the Knudsen effusion method. J

containing polycyclic aromatic hydrocarbons measured using Knudsen effusion.

Aromatic-Hydrocarbons in Urban Air, Snow and Automobile Exhaust.

Pressures for Nonpolar and Semipolar Organic-Compounds from Gas-

of Individual Chlorinated and Brominated Polycyclic Aromatic Hydrocarbons for Induction of Aryl Hydrocarbon Receptor-Mediated Responses. Environ Sci Technol

brominated polycyclic aromatic hydrocarbons in waste incinerators. Environ Sci

and Cl-PAHs in air and water particulate samples - patterns and variations.

chlorinated polycyclic aromatic hydrocarbons and their parent compounds in

Alkylfluorenes in Bleached Kraft Pulp and Pulp-Mill Discharges. Chemosphere 28,

9,10-dibromoanthracene system. Fluid Phase Equilibr 298, 219-224.

the Knudsen effusion technique. J Chem Thermodyn 40, 460-466.

entropies. J Phys Chem Ref Data 28, 1535-1673.

hydrocarbons. J Chem Thermodyn 12, 243-248.

Chem Eng Data 53, 670-676.

Chemosphere 16, 2441-2450.

43, 2159-2165.

2139-2150.

Technol 42, 1904-1909.

Chemosphere 50, 1131-1150.

urban air. Environ Sci Technol 40, 4592-4598.

Environ Toxicol Chem 27, 1244-1249.

The distance between 002 planes in the pure anthracene, pure 2-bromoanthracene and mixtures can be calculated by Bragg's law

$$m\mathcal{X} = \mathcal{Z}d\sin\theta\tag{2}$$

where *n* is an integer, *λ* is the wavelength of the incident wave, *d* is the spacing between the planes in the atomic lattice, and *θ* is the angle between the incident ray and the scattering planes.

Figure 12 also shows changes of the distance between 002 planes in this system, which demonstrates that the spacings between 002 planes are stretched by adding 2 bromoanthracene into anthracene. The distance between 002 planes reaches a maximum when the mixture is near the lowest melting solid-liquid equilibrium point, which is in good agreement with the thermodynamic data in Figure 12, indicating the formation of the least stable solid state near the lowest solid-liquid equilibrium point. Interestingly, the mixture at *x*1 = 0.18 gives a local minimum in the (002) plane spacing.
