Direct observation of the complex S(IV) equilibria at the liquid-vapor interface
Résumé
The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pK a values of the various S(IV) tautomers and reaction barriers for SO 2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO 2 uptake kinetics and reactions involving NO x and H 2 O 2 at aqueous aerosol interfaces.
Domaines
Chimie| Origine | Publication financée par une institution |
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