Gas/solid and gas/liquid partitioning of organic compounds: Critical evaluation of the interpretation of equilibrium constants

Gas/solid and gas/liquid partition processes are pivotal for the transport and residence time of organic pollutants in the atmosphere. Commonly, experimentally determined partition constants (K) between air and condensed phases (i.e., aerosols, rain, fog, snow, soils, plants) of a series of compounds are evaluated as a function of their (subcooled) liquid saturation vapor pressure Frequently, a linear free energy relationship of the form ln K = m ln + constant has been found for groups of structurally related compounds. In many cases, in the literature, deviations of the slope m from −1 have been considered to be in contradiction to theory and have thus been taken as indicative of nonequilibrium conditions or experimental artifacts. In this paper, it is shown by theoretical considerations as well as by experimental data from the literature that m may deviate significantly from −1 for equilibrium adsorption and absorption and that such deviations do not necessarily indicate nonequilibrium effects. In fact,...