Laboratory Measurements of Thermodynamics of Adsorption of Small Aromatic Gases to n-Hexane Soot Surfaces

The adsorption isotherms of a series of aromatic hydrocarbons on n-hexane soot were measured as a function of temperature and partial pressure in a coated-wall flow tube coupled to an electron-impact mass spectrometer. The specific surface area was determined for each of the samples by measuring the BET isotherm of Kr at 77 K. The gas-to-surface uptakes were fully reversible with the extent of adsorption increasing with decreasing temperature and increasing partial pressures. At low partial pressures, the isotherms were well modeled by the Langmuir isotherm for all experimental conditions, and the adsorption was found to saturate at one monolayer of coverage at ≈2 × 10¹⁴ molecule cm^-2. For the less volatile species, evidence for multilayer adsorption was observed and the BET isotherm was used instead. The experimental enthalpies of adsorption were consistently higher than the enthalpies of vaporization for all compounds. A linear free-energy relationship was developed between the Langmuir equilibrium constant for adsorption and the compound's (sub-cooled) liquid vapor pressure, providing validation for the use of such relationships in assessing gas-particle partitioning of aromatic hydrocarbons to soot aerosols in the environment. The experimental results were compared to the Junge−Pankow gas-to-aerosol partitioning model.