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

Partitioning between the gas phase and ambient condensed phases is an important 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 gas phase are strongly temperature dependent and the respective enthalpies of transfer need to be known. Unfortunately, such data are often not available. In this paper, we evaluate the possibilities of estimating both the enthalpies of vaporization from the pure liquid and the enthalpies of gas/surface adsorption of organic compounds from either their (subcooled) liquid vapor pressure or their equilibrium adsorption constant at a particular temperature. 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 vaporization, Delta(vap)H(i), of a given compound i from its saturated liquid vapor 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 analogous 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 liquid water surface is discussed. The equations presented are useful practical tools for approximating the temperature dependence of liquid vapor pressure and of vapor/surface adsorption constants of organic chemicals in the ambient temperature range.