Comparison of subcooled liquid vapor pressures of polychlorinated dibenzo-p-dioxins and dibenzofurans predicted by QSPR and GC-RI methods

Subcooled liquid vapor pressures (P(L)) are of great importance for assessing the persistent behavior of organic pollutants. As P(L) cannot be determined by direct experiments, it is of interest to develop and evaluate various predictive methods. In the current study, gas chromatography retention index (GC-RI) and quantitative structure-property relationship (QSPR) methods were used to develop predictive models for P(L) of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). The model development was based on P(L) values converted from consistent experimental solid vapor pressures (P(S)). The P(L) values predicted by the two methods are highly consistent with each other, and in-between sets of values predicted by others. Since the QSPR method can be regarded as independent of experiments, and can be used to interpret intermolecular interactions that govern the magnitude of P(L), it may be superior to the GC-RI method.     

Solid vapor pressure for five heavy PAHs via the Knudsen effusion method

Polycyclic aromatic hydrocarbons (PAHs) are compounds resulting from incomplete combustion and many fuel processing operations, and they are commonly found as subsurface environmental contaminants at sites of former manufactured gas plants. Knowledge of their vapor pressures is the key to predict their fate and transport in the environment. The present study involves five heavy PAHs, i.e. benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, and dibenz[a,h]anthracene, which are all as priority pollutants classified by the US EPA. The vapor pressures of these heavy PAHs were measured by using Knudsen effusion method over the temperature range of 364 K to 454 K. The corresponding values of the enthalpy of sublimation were calculated from the Clausius-Clapeyron equation. The enthalpy of fusion for the 5 PAHs was also measured by using differential scanning calorimetry and used to convert earlier published sub-cooled liquid vapor pressure data to solid vapor pressure in order to compare with the present results. These adjusted values do not agree with the present measured actual solid vapor pressure values for these PAHs, but there is good agreement between present results and other earlier published sublimation data.     

Henry’s law constant for environmentally significant compounds

Many nonvolatile organic compounds, e.g., polyaromatic hydrocarbons (PAHs), are readily stripped during aerobic biodegradation. This is because of the high infinite dilution activity coefficient resulting from forces generated by the water-organic interactions at the molecular level. Several models have been proposed for air-stripping based on the Henry’s law constant. By definition, the Henry’s law constant is the infinite dilution activity coefficient multiplied by the pure component vapor pressure. In this article, a gas saturation technique was used to measure the very low vapor pressures exhibited by these nonvolatile compounds. Literature values of other PAHs have been tabulated and are presented. For determining infinite dilution activity coefficients, a differential ebulliometery apparatus has been constructed. In this technique, the boiling point difference between pure water and a water-organic solution is measured very precisely. Thermodynamics is then used to calculate the infinite dilution activity coefficient. The method’s accuracy has been tested using the phenol-water system.     

Supercooled liquid vapour pressures and related thermodynamic properties of polycyclic aromatic hydrocarbons determined by gas chromatography

A gas chromatographic method using Kováts retention indices has been applied to determine the liquid vapour pressure (P(i)), enthalpy of vaporization (DeltaH(i)) and difference in heat capacity between gas and liquid phase (DeltaC(i)) for a group of polycyclic aromatic hydrocarbons (PAHs). This group consists of 19 unsubstituted, methylated and sulphur containing PAHs. Differences in log P(i) of -0.04 to +0.99 log units at 298.15K were observed between experimental values and data from effusion and gas saturation studies. These differences in log P(i) have been fitted with multilinear regression resulting in a compound and temperature dependent correction. Over a temperature range from 273.15 to 423.15K, differences in corrected log P(i) of a training set (-0.07 to +0.03 log units) and a validation set (-0.17 to 0.19 log units) were within calculated error ranges. The corrected vapour pressures also showed a good agreement with other GC determined vapour pressures (average -0.09 log units).