Thermodynamic Properties of Polycyclic Aromatic Hydrocarbons in the Subcooled Liquid State

Abstract

A knowledge of the solution behavior of polycyclic aromatic hydrocarbons (PAHs) at ambient temperatures is required for predictions of their fate in the environment. Unsubstituted PAHs are crystalline at ambient temperatures, but their behavior in solution depends on the properties of the hypothetical subcooled liquid state, and these properties must be estimated. Enthalpies, entropies, and Gibbs free energies of fifteen PAHs in the subcooled liquid state are calculated using available sublimation, melting, and heat capacity data. Because PAH melting points are as much as 250 K above ambient temperatures, heat capacity corrections can be important for the accurate extrapolation of thermodynamic data to the subcooled liquid state. For example, subcooled liquid vapor pressures, p(l), at 298 K calculated without heat capacity corrections are up to 6 times higher than those estimated with heat capacity corrections. Calculated values for p(l) are expressed as log p (l) = -A/T+B and used to develop a group contribution method for the calculation of p(l) for other unsubstituted PAHs.

Key Words:

• PAH
• enthalpy
• entropy
• Gibbs free energy
• sublimation pressure
• vapor pressure