Molecular dynamics investigation of the influence of surfactants on halogens propensity at the air-water interface
Résumé
In the atmospheric environment, both organic and inorganic compounds are present and have a strong influence on the chemical composition of the troposphere. On the other side, the ocean surface water and sea mist spray are enriched with surface active organic compounds, which may have a significant effect on the distribution of halide ions at the interface [1,2]. For instance, the abundance of bromide (Br
-) ions at the interface is implicated in halogen activation processes relevant for the ozone budget in the troposphere [3]. In this context, we evaluate the surface affinity of cationic hexylammonium, neutral hexylamine, and anionic propylsulfate at the air/liquid interface, and the effect of these ionic surfactants on the abundance of bromide at interfaces. Inspired by liquid X-ray Photoelectron Spectroscopy (XPS) experiments, we conducted Molecular Dynamics (MD) simulations and we were able to speculate about the distribution of these species between bulk and surface area in aqueous systems. Here, the results are in good agreement with the experimental ones, which clearly advocate that those organic molecules dislike the bulk and prefer to reside near the free surface; indicating the effect of salting. In contrast, both solutions containing hexylammonium and hexylamine show an enhancement of the surfactant bromide concentration while the propylsulfate solution do not alter the bromide concentration. In parallel, the same results were observed in the presence of NaCl. These results demonstrate that molecular level properties bring valuable insights on the interfacial behavior of organic and inorganic species on water films, that can be transposed to aerosol particles.
-) ions at the interface is implicated in halogen activation processes relevant for the ozone budget in the troposphere [3]. In this context, we evaluate the surface affinity of cationic hexylammonium, neutral hexylamine, and anionic propylsulfate at the air/liquid interface, and the effect of these ionic surfactants on the abundance of bromide at interfaces. Inspired by liquid X-ray Photoelectron Spectroscopy (XPS) experiments, we conducted Molecular Dynamics (MD) simulations and we were able to speculate about the distribution of these species between bulk and surface area in aqueous systems. Here, the results are in good agreement with the experimental ones, which clearly advocate that those organic molecules dislike the bulk and prefer to reside near the free surface; indicating the effect of salting. In contrast, both solutions containing hexylammonium and hexylamine show an enhancement of the surfactant bromide concentration while the propylsulfate solution do not alter the bromide concentration. In parallel, the same results were observed in the presence of NaCl. These results demonstrate that molecular level properties bring valuable insights on the interfacial behavior of organic and inorganic species on water films, that can be transposed to aerosol particles.