T8 PM Measurement/ Estimation of Environmentally Relevant Physico-Chemical Properties
Tuesday, 15 November 2005: 1:50 PM - 5:30 PM in 337-338

(MAC-1117-797533) Estimating the Temperature Dependence of Liquid Vapor Pressure of Nonpolar Substances Using Trouton's Rule.

MacLeod, Matt¹, Scheringer, Martin, Hungerbühler, Konrad¹, ¹ Swiss Federal Institute of Technology, Zurich, Switzerland

ABSTRACT- The fate of persistent chemicals in the environment is determined to a large extent by their physico-chemical properties. Variability in environmental temperatures can strongly influence these properties, and thus partitioning between air, water and soils, overall persistence and potential for long-range transport. Therefore, the temperature dependence of vapor pressure (quantified by the enthalpy of vaporization, H_VAP) is an important factor in the overall fate of chemicals in the environment. We demonstrate that H_VAP of liquids and sub-cooled liquids at an arbitrary system temperature can be reliably estimated for nonpolar substances from vapor pressure (P_L) using Trouton's rule and assuming that H_VAP is linearly dependent on temperature between the boiling temperature and the temperature of interest. Our relationship successfully describes H_VAP and P_L data at 298 K for nonpolar chemicals with subcooled liquid vapor pressures ranging over 15 orders of magnitude. The uncertainty associated with estimating H_VAP from P_L is low, and is likely acceptable for most practical applications. We apply the relationship based on Trouton's rule to assess the accuracy of a popular method for determining H_VAP and P_L from relative retention times on a gas chromatographic column compared to a reference substance. Recently reported H_VAP and P_L data for polybrominated diphenyl ethers and polychlorinated napthalenes appear to be biased by errors associated with extrapolating temperature dependent vapor pressure data for the reference compound from the solid state to the subcooled liquid state, and ignoring temperature dependence of the associated enthalpy and entropy changes. To reduce uncertainties and avoid bias, substances with a complete set of temperature dependent thermodynamic properties should be selected as reference compounds for gas chromatographic estimation of H_VAP and P_L, and determination of thermodynamic properties for commonly used reference substances such as p,p'-DDT should be a high research priority.

Key words: Trouton's Rule, Enthalpy of vaporization, polybrominated diphenyl ethers, polychlorinated napthalenes