Empirical prediction of heats of vaporization and heats of adsorption of organic compounds

Partitioning between the gas phase and ambient condensed phases is an impor tant process in determining the transport and fate of organic chemicals in the atmosphere as well as in other environmental compartments exhibiting a vadose zone (e.g., soils). In general, partition processes including the ga s phase are strongly temperature dependent and the respective enthalpies of transfer need to be known. Unfortunately, such data are often not availabl e. In this paper, we evaluate the possibilities of estimating both the enth alpies of vaporization from the pure liquid and the enthalpies of gas/surfa ce adsorption of organic compounds from either their (subcooled) liquid vap or pressure or their equilibrium adsorption constant at a particular temper ature. Such an approach becomes possible when linear relationships between the enthalpy and entropy, and hence between the enthalpy and the logarithm of the partition constant, exist. Using literature data reported for almost 200 compounds covering a wide range of compound classes we have derived an empirical relationship that can be used to estimate the enthalpy of vapori zation, Delta(vap)H(i), of a given compound i from its saturated liquid vap or pressure, p(iL)*, a t25 degrees C: Delta(vap)H(i) (kJ/mol)= -3.82-(+/-0. 03) In p(iL)* (Pa, 25 degrees C)+ 70.0(+/-0.2); n = 195; r(2) = 0.99. An an alogous equation is given for the estimation of the enthalpy of adsorption of organic vapors to mineral surfaces. The application of this equation to other surfaces including liquid and solid organic phases as well as the liq uid water surface is discussed. The equations presented are useful practica l tools for approximating the temperature dependence of liquid vapor pressu re and of vapor/surface adsorption constants of organic chemicals in the am bient temperature range.