High molecular weight aromatic compounds, including polycyclic aromatic hydrocarbons (PAH) and their derivatives, polycyclic aromatic compounds (PAC), are common environmental pollutants, resulting from the incomplete combustion of coal, fossil fuels and other anthropogenic sources. For example, fluorene, from which fluorenone is a known oxidation product, is commonly present in excess of 1% in coal tar mixtures. 9-fluorenone, with its carbonyl group on a central carbon between aromatic rings, readily forms oximes, such as 9-fluorenone oxime [1
]. The seven heteroatomic and heterocyclic aromatic compounds investigated herein are only some examples of compounds of potential interest, and for which few thermodynamic property measurements are available.
Vapor pressure data are important inputs into models that predict the fate and transport of such high molecular weight compounds. Yet few data exist on these classes of compounds, owing to the difficulty in obtaining such data. High molecular weight compounds tend to degrade at the high temperatures needed to measure their relatively low vapor pressures directly, such that indirect methods must be used. Here we employ the isothermal Knudsen effusion technique, which is commonly used for such measurements and which is well described in the literature [2
The vapor pressures measured in the present experiments are sublimation vapor pressures, as these compounds all exist in the solid phase at near-ambient temperature and pressure. Under the common assumption of constant enthalpy of sublimation, ΔHsub
, over the modest temperature ranges employed, the vapor pressure data may be represented by the integrated form of the Clausius-Clapeyron equation:
is the saturation vapor pressure of the compound, R the universal gas constant, T the absolute temperature, and ΔSsub
the entropy of sublimation. This representation of the Clausius-Clapeyron equation considers the sublimed PAC to behave as an ideal gas, which under the current conditions of low pressure and moderate temperature is quite appropriate.
The vapor pressures of pure PAC span many orders of magnitude at ambient temperatures. Vapor pressures are sensitive to variables such as molecular weight, carbon to hydrogen ratio (bond saturation), as well as intramolecular bonding and intermolecular bonding interactions.