Absorption or adsorption? Insights from molecular probes n-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 present study explores this characteristic sorption behavior of alkanes and eventually claims that determination of n-alkane-to-cycloalkane sorption coefficient ratios (K-n/K-c) helps elucidate the mode of sorption by complex mixtures in the environment. Differences in sorption coefficients from air W 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 K-n/K-c < 1 for absorption and K-n/K-c similar to 1 for adsorption. An extensive number of experimental K values of C-5-C-8 alkanes for known ab- and adsorbents were evaluated. The data matched the model expectations and indeed exhibited a distinct difference in K-n/K-c, between ab-and adsorption. Steric factors due to sorbate and sorbent geometries generally favored adsorption of n-alkanes over that of cycloalkanes (K-n/K-c > 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.
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