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The present work concerns the thermochemical and vapor pressure behavior of the pyrene + 9,10-dibromoanthracene system. The phase diagram of the system has been studied using the thaw melt method and the results show the formation of non-eutectic multiphase mixtures. The temperatures of crystallization, and enthalpies of fusion and crystallization of the system were determined by differential scanning calorimetry. The system behavior can be divided into 5 regions. The X-ray diffraction results also indicated the existence of multiple phase characteristics. The solid-vapor equilibrium studies showed that for mixtures with high mole fractions of pyrene, two different preferred states exist that determine the vapor pressure. For those mixtures with moderate and low mole fractions of pyrene, only one preferred state exists that determines vapor pressure behavior. It was also demonstrated that the vapor pressure of the mixtures is independent of the mixture preparation technique.
Although today much is known about the thermodynamic properties of polycyclic aromatic compounds (PAC)[1–4] and eutectic organic mixtures[5–14], there remains a paucity of thermodynamic data available on PAC mixtures. There is a need for more such data, considering the environmental importance of these chemicals[15, 16].
This article presents the phase diagram, enthalpy of phase change, crystal structure and vapor pressure of the pyrene + 9,10-dibromoanthracene system. The interaction energies of mixtures with different compositions are also calculated using enthalpies of crystallization obtained via differential scanning calorimetry. Effects of different mixture preparation techniques were also examined.
Pyrene (CAS Reg. No. 129-00-0, 97 % purity) and 9,10-dibromoanthracene (CAS Reg. No. 523-27-3, 98 % purity) were obtained from TCI America. Purity was verified by gas chromatography-mass spectrometry (GC-MS) analysis. The melting points of pyrene and 9,10-dibromoanthracene were found to be 424 ± 1 K(423 K), see also [6, 7] and 498 ± 1 K(499 K), see also, respectively.
Mixtures of pyrene and 9,10-dibromoanthracene were prepared at various compositions by using a melt and quench-cool technique. The desired quantities of pyrene and 9,10-dibromoanthracene were measured to ± 0.05 mg and sealed in a brass vessel. The vessel was heated to 503 ± 5 K in a fluidized bath and agitated for 5 min. Then the vessel was immediately placed into liquid nitrogen, which provided cooling at approximately 70 – 80 K·s−1 for the first 4 sec. The heating and cooling procedure was repeated 3 additional times.
Melting temperatures of pure samples and mixtures were measured using a Thermo Scientific 1001D MEL-TEMP® Capillary Melting Point Apparatus. 1 to 2 mg of each sample was placed in a capillary and heated at 1 ± 0.5 K·min−1. The liquidus and thaw points were determined according to the method proposed by Pound and Masson. The thaw temperature is the temperature at which the first droplet of liquid appears in the capillary. The liquidus temperature is the maximum temperature at which both solid crystals and liquid are observed coexisting.
Temperatures and enthalpies of fusion and crystallization were measured using a DuPont 2910 differential scanning calorimeter (DSC). A 1 to 3 mg sample was placed into a hermetically sealed DSC pan and was scanned in heating and cooling modes. The rates of heating and cooling were 10 K·min−1 and 2.5 K·min−1, respectively. This procedure leads to crystallization enthalpy peaks that are more convenient to integrate than the fusion enthalpy peaks because the baseline of the cooling scan is stable, making it easy to distinguish the start and end points of phase transition peaks. Since the values of enthalpy and transition temperatures were generally insensitive to changes in heating and cooling rate in the range of 2.5 – 10 K· min−1, the enthalpy of crystallization was used to characterize the enthalpy of the phase transition. It was also determined that the cooling scan enthalpy of crystallization agreed to within ± 2 % of the total enthalpy of fusion, for all fusion peaks. Note that the crystallization typically occurs at a subcooled liquid condition in these experiments.
The Knudsen effusion technique was used to measure the vapor pressure of pure pyrene, 9,10-dibromoanthracene and their mixtures. This is an indirect measurement technique based on the molecular effusion of a vapor through an orifice into a high vacuum chamber and is generally used to measure the vapor pressure of low volatility substances. The vapor pressure of a substance can be obtained from effusion data using
where ω is the weight loss during the effusion time interval, A is the orifice area, t is the effusion time, R is the universal gas constant, T is the sample temperature, M is the molecular weight, and W is the Clausing correction factor. W ranges from 0 to 1 and is an essential factor for the calculation of vapor pressure because it quantifies the probability of molecular escape from the effusion cell. W is calculated by the empirical formula,
where l is the orifice effusion length and r is the orifice radius. In these experiments, W was always very close to unity, both by calculation using (2) and by experimental calibration.
Samples were placed inside effusion cells prepared from steel shim stock. The cells were sealed except for a single, circular orifice 0.60 ± 0.01mm in diameter, and then placed on one arm of a continuously recording microbalance in a high vacuum chamber. The pressure inside of the chamber was reduced to 10−4 Pa in the experiments.
Powder X-ray diffraction patterns were obtained using a Bruker AXS D8-Advanced diffractometer with CuKa radiation (λ = 1.5418 Å). The samples were scaned with 2θ angle 20° – 60° and a step-size 0.02°.
The composition of mixtures was determined by GC-MS. The samples were dissolved in dichloromethane to about 100μg· ml−1 and analyzed using a Varian CP3800 gas chromatograph and Saturn 2200 mass spectrometer. The analysis procedure followed the EPA Method 8270C.
The computer simulation studies on pure pyrene and 9,10-dibromoanthracene were carried out using Spartan'08 software. The geometries of pyrene and 9,10-dibromoanthracene were optimized by density functional theory (DFT) using DFT (B3LYP/6-31 G*).
The phase diagram of the pyrene (1) + 9,10-dibromoanthracene (2) system determined by thaw-melt method is given in Fig. 1. The phase diagram shows the non-ideality of the pyrene + 9,10-dibromoanthracene system, which is attributed to the difference in size and shape of the two components, see Table 1. The surface area and volume of 9,10-dibromoanthracene is larger than that of pyrene. The phase diagram 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 x2 = 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.
The full heating, cooling and reheating scan of a pyrene + 9,10-dibromoanthracene mixture at x2 = 0.48 (in region C) is shown in Fig. 2 where Φ is heat flow of 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.
The temperature and enthalpy of crystallization (subcooled), shown in Fig. 3, 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 Fig. 3, 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.
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 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 interaction energy is defined by,
where Einter is interaction energy, (ΔHfus)1 and (ΔHfus)2 are enthalpies of fusion of pyrene and 9,10-dibromoanthracene, respectively, (ΔHfus)cal is the calculated enthalpy of fusion of the mixtures, (ΔHfus)exp is enthalpy of fusion of the mixtures determined from the DSC measurements, x1 and x2 are the mole fractions of pyrene and 9,10-dibromoanthracene, respectively.
where (ΔHcry)1 and (ΔHcry)2 are enthalpies of crystallization of pyrene and 9,10-dibromoanthracene, respectively, (ΔHcry)cal is the calculated enthalpy of crystallization of the mixtures, (ΔHcry)exp is enthalpy of crystallization of the mixtures determined from the DSC measurements.
This suggests that pyrene and 9,10-dibromoanthracene form mixtures with a higher energy state than that of the pure components, and that different kinds of interaction exist in the mixtures than in the pure compounds. There is weaker interaction between molecules in the mixtures with 0.55 and 0.75 mole fraction of 9,10-dibromoanthracene than that in the mixture with 0.65 mole fraction of 9,10-dibromoanthracene, which suggests that the latter mixture is in a lower energy state than others, as already noted above. The energy state of this 2:1 mixture is close to that of pure materials.
The powder X-ray diffraction method was used to study the crystal structures of pure pyrene, 9,10-dibromoanthracene and their mixtures (see Fig.5). 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.
The Knudsen effusion technique was used to measure the vapor pressure of pyrene and 9,10-dibromoanthracene mixtures in two different types of experiments. The experimentally measured vapor pressures are compared to those calculated by Raoult's law, an ideal weighted average of pure component vapor pressures.
Fig. 6 shows the vapor pressure and the change in concentration of pyrene with sample mass loss in an experiment that begins with a mixture at x1 = 0.75 and 333.3 K. The temperature was changed to 358.7 K, after about 20 % of the original mass was lost. Compositions were determined by interruption of the experiment, and removal of some of the sample for analysis. At the beginning, the vapor pressure of mixture with x1 = 0.75 closely follows a Raoult's law prediction corresponding to roughly 0.70 mole fraction of pyrene. After the mixture in the cell loses about 35% mass, the vapor pressure of the mixture falls to another stable state which is at Raoult's law prediction for a 0.35 mole fraction of pyrene. This further suggests that the pyrene and 9,10-dibromoanthracene prefer to form a mixture at x1 = 0.35, even though the initial mole fraction of pyrene before melting is different. These results strongly indicate that the system has a preferred composition.
Fig. 7 shows the vapor pressures of pure pyrene, consistent with , and 9,10-dibromoanthracene, consistent with . The vapor pressure of pyrene is about 2 orders of magnitude higher than that of 9,10-dibromoanthracene. From linear regression of the data, the enthalpy of sublimation can be obtained. The enthalpies of sublimation of pyrene and 9,10-dibromoanthracene are 93.1 kJ·mole−1 in a temperature range of 324 – 359 Kand 110.1kJ·mole−1 in a temperature range of 359 – 392 K, respectively. Roux et al. remark that most sublimation enthalpy of pyrene published over the past 50 years range from 93 to 101 kJ·mole−1 in a temperature range of 322 – 423 K. Our value fits well within this range.
Pmax is the maximum vapor pressure that can be achieved in the Knudsen effusion cell containing pyrene + 9,10-dibromoanthracene mixtures. By assuming that these components are present as two separate phases,
where P10 and P20 are the vapor pressure of pure pyrene and 9,10-dibromoanthracene, respectively.
For the pyrene rich mixtures not allowed to lose significant mass (see Fig.8), the vapor pressure follows the Raoult's law prediction at x1 = 0.70, which suggests that a preferred state for pyrene rich mixture systems exists at x1 = 0.70. This state exists at the boundary between region A and B and is the limit of a single phase system.
After mixtures initially at x1 = 0.70, 0.75 and 0.90 lose about 19 %, 35 % and 69 % mass, respectively, the vapor pressure transitions to the Raoult's law prediction curve for x1= 0.35 (see Fig. 9). The vapor pressure stays at this value for a significant degree of mass loss, which suggests that mixtures of pyrene and 9,10-dibromoanthracene form a stable state at x1 = 0.35 that behaves like an azeotrope. This is consistent with the DSC measurements (Fig. 3) showing that this mixture composition is a stable state. Thereafter the vapor pressure falls to the vapor pressure of 9,10-dibromoanthracene, even though there is 14 ± 1 % pyrene still left in the mixture in region E. Hence, the observed behavior is not that of a true azeotrope.
The vapor pressure behavior of mixtures with moderate initial mole fraction of pyrene is shown in Fig. 10. Mixtures at x1 = 0.50 and 0.60 are in an unstable state. The vapor pressures of these mixtures transition to the Raoult's law prediction value at x1 = 0.35 after they lose about 3 % and 5 % mass, respectively. This again reflects that the mixture with pyrene mole fraction of 0.35 is a stable mixture, whose behavior dominates the solid-vapor equilibrium.
Fig. 11 further demonstrates that a stable state of pyrene and 9,10-dibromoanthraene mixtures exists at x1 = 0.35. Even if the initial pyrene mole faction in the mixture is lower than 0.35 in region D and E, the stable state 2:1 mixture is formed.
Other methods were used to prepare the pyrene + 9,10-dibromoanthracene mixtures, such as slow cooling the melts and ball milling the components together at room temperature. The vapor pressures of pyrene and 9,10-dibromoanthracene mixtures did not depend on the mixture preparation techniques.
The pyrene (1) + 9,10-dibromoanthracene (2) system is complicated and non-ideal. It is not a eutectic system. The melting and crystallization studies show that the phase character depends on the composition of the mixture and that a stable state of 9,10-dibromoanthraene + pyrene mixture is formed at a 2:1 molar ratio. Interaction energies calculated from enthalpies of crystallization and XRD data also indicate that the phase behavior depends on mixture composition. The vapor pressure measurements further indicate the existence of the stable 2:1 mixture state. Mixtures with high mole fraction of pyrene also have another stable state at x1 = 0.70. Additionally, mixtures with x1 < 0.35 can also form the stable 2:1 mixture. The vapor pressure of mixtures with near equimolar composition quickly transitions to that of the Raoult's law prediction curve for a 2:1 mixture.
The project described was supported by Grant Number P42ES013660 from the National Institute of Environmental Health Sciences (NIEHS), NIH and the contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS, NIH.
Jinxia Fu, PhD candidate in the Department of Chemistry at Brown University, is a graduate student in the Fluid, Thermal, and Chemical Processes group, and the Superfund Research Program. She is interested in the area of thermochemistry and the environment. Her recent research is related to the phase behavior and vapor pressure of semi-volatile organic compound mixtures and the prediction of the vapor pressure of these mixtures.
James Rice, PhD candidate in the Division of Engineering at Brown University, is a student in Brown's Fluid, Thermal, and Chemical Processes group and a Superfund Research Program (SRP) trainee supported by the SRP Training Core. He is also a teaching assistant for undergraduate fluid mechanics and thermodynamics courses. He is interested in the areas of petroleum chemistry and the environment. His research is largely focused on the thermodynamics of coal tar contaminants, specifically the phase behavior and vapor pressure of semi-volatile organic compound mixtures.
Eric Suuberg, Professor of Engineering at Brown University, is co-Director of Brown's Superfund Research Program, and leads its Research Translation Core. He is also a principal editor of the journal Fuel. Dr. Suuberg's research interests are in the areas of energy and the environment, including fuels chemistry, coal tars, chars, ash, and on the heterogeneous reduction of NO on carbons. Recently, he has been interested in the thermodynamics of semi-volatile organics (particularly vapor pressures), and leads a new effort at understanding vapor intrusion, in which contaminant vapors move through soil and enter structures built atop or near contaminated sites.
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