Absorption or Adsorption? Insights from Molecular Probesn-Alkanes and Cycloalkanes into Modes of Sorption by Environmental Solid Matrices

Environmental solid matrices such as soils and aerosols contain a variety of absorbents (e.g., organic matter) and adsorbents (rigid carbonaceous geosorbents, minerals) but the contribution of both modes of sorption to the overall sorption behavior is often uncertain. Absorption of a cycloalkane from air to bulk phases is generally stronger than that of the n-alkane of the same number of carbon atoms, while adsorption onto surfaces does not differ between these two compounds, or rather favors the n-alkane. The presentstudy explores this characteristic sorption behavior of alkanes and eventually claims that determination of n-alkane-to-cycloalkane sorption coefficient ratios (Kn/Kc) helps elucidate the mode of sorption by complex mixtures in the environment. Differences in sorption coefficients from air (K) between n- and cycloalkanes were explained based on the linear free energy relationship (LFER) models in the form log K = - a V+ b MR + constant, where V and MR are the molar volume and the molar refraction, respectively. The LFER models predict Kn/Kc 1), clarifying the contrast between the two sorption modes even more. The application to environmental solid matrices is demonstrated using sorption data for diesel soot, aerosols and snow. The results are in excellent agreement with previous discussions on the modes of sorption in these materials.