Thermodynamic Study of Anthracene + Phenanthrene Solid Mixtures

The combustion of carbon-based fossil fuels currently provides the United States with nearly 70% of its useable energy. Production and use of these fuels often involves concerns regarding polycyclic aromatic hydrocarbons (PAHs).  PAHs usually occur in mixtures both naturally (e.g., in petroleum) and as byproducts of combustion or gasification (e.g., in coal tar) that are sometimes released to the environment. The US EPA considers many PAHs to be hazardous to human health due to their potential to cause cancer. Studying behavior that might affect the environmental fate of PAHs is important in understanding the risks associated with exposure to these contaminants. This study focused on the mixture properties of model PAH mixtures. Differential scanning calorimetry and x-ray diffraction were used to assess the fusion and solidification behavior of mixtures of anthracene and phenanthrene at varying mole fractions. In addition, the vapor pressure of these mixtures was studied via the Knudsen effusion technique and compared with the observed melting and solidification behavior to infer mixture phase behavior. Mixtures with greater than 20% anthracene showed distinct two-phase behavior, but neither phase was pure anthracene or pure phenanthrene. Mixtures with less than 20% anthracene showed behavior somewhat like a single mixture phase with constant enthalpy of fusion and melting temperature. The temperature at which solid anthracene and phenanthrene mixtures first began to melt (the thaw temperature) remained constant and very close to the melting temperature of pure phenanthrene for all mixture compositions. These measurements suggest the presence of a constant fusion enthalpy, phenanthrene-rich phase that behaves similarly to pure phenanthrene and can incorporate up to 20% anthracene into its crystal structure. This information was used to compute the enthalpy of fusion for what is thought to be an anthracene-rich phase that co-exists in the presence of the phenanthrene-rich phase for all mixture compositions with greater than 20% anthracene. It was this phase that determined the final melting temperature (the liquidus temperature) of the mixture. The enthalpic contribution of this anthracene-rich phase was always significant and greater than that of the phenanthrene rich phase, even when only a small mole fraction of that phase was present. The vapor pressure and x-ray diffraction analyses further confirmed that neither pure anthracene nor pure phenanthrene phases exist in these binary mixtures. However, the phenanthrene-rich phase showed vapor pressure behavior similar to that of pure phenanthrene, while that of the anthracene-rich compared with pure anthracene. This information is useful in developing a more complete understanding of the thermodynamics of PAH systems, which can lead to improved predictions of how mixtures of PAHs interact with their surrounding environment.