| Summary: | Adsorption of organic vapors at a gas-water interface has several physical applications with respect to natural processes in the environment. Understanding the adsorption processes is critical in development of risk assessment and modeling of transport and fate of various chemical species that are abundantly present in the environment. Evidences from the works of several researchers point out deviations in gas-liquid partitioning as predicted by Henrys law. In view of several theories that describe these deviations, adsorption process is hypothesized to be significant in gas-liquid partitioning, in addition to temperature and presence of organic matter, particularly on liquid surfaces with high surface to volume ratio. Implications of these adsorption and thermodynamic parameters of chemical species have been applied to its atmospheric fate and transport.
In this work, inverse gas chromatography has been used to determine surface adsorption partition constant for organic compounds such as benzene, naphthalene and phenanthrene. A water-coated diatomaceous earth was used as the packing support material in the chromatographic column. The probe used to study this gas-liquid partitioning was obtained in vapor phase and injected in the chromatograph with dry support material and water coated support. The retention volume on the water-coated support, depends on two processes, the adsorption and dissolution process in the liquid coated surface, and was primarily used to extract the partition constants. The Gibbs-Helmholtz equation was used to determine the enthalpy of adsorption.
The enthalpy of adsorption indicates that the adsorption process is more favorable at the gas-solid interface than at the gas-water interface, supporting the surface morphology theory based on fractal dimensions. The enthalpy of adsorption at the gas-water interface for all three compounds was larger than the enthalpy of condensation and the enthalpy of aqueous solvation. This supports the prevailing critical cluster model for the dynamics of the transfer of compounds from the gas-water interface. The partition constant at the gas-water interface was correlated with the sub-cooled liquid vapor pressure of the PAHs along with data obtained from other works. Partition constants indicate that the most hydrophobic compound will experience enhanced surface adsorption and hence scavenged the most by wet deposition process.
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