Production of Br-2 from N2O5 and Br- in Salty and Surfactant-Coated Water Microjets

TitleProduction of Br-2 from N2O5 and Br- in Salty and Surfactant-Coated Water Microjets
Publication TypeJournal Article
Year of Publication2019
AuthorsSobyra, TB, Pliszka, H, Bertram, TH, Nathanson, GM
JournalJournal of Physical Chemistry A
Date PublishedOct
ISBN Number1089-5639

Gas liquid scattering experiments are used to investigate the oxidation-reduction reaction N2O5(g) + 2Br(-)(aq) -> Br-2(fg) + NO3-(aq) + NO2-(aq), a model for the nighttime absorption of N2O5 into aerosol droplets containing halide ions. The detection of evaporating Br-2 molecules provides our first observation of a gaseous reaction product generated by a water microjet in vacuum. N2O5 molecules are directed at a 35 mu m diameter jet of 6 or 8 m LiBr in water at 263 or 240 K, followed by detection of both unreacted N2O5 and product Br-2 molecules by velocity-resolved mass spectrometry. The N2O5 reaction probability at near-thermal collision energy is too small to be measured and likely lies below 0.2. However, the evaporating Br2 product can be detected and controlled by the presence of surfactants. The addition of 0.02 m 1-butanol, which creates similar to 40% of a compact monolayer, reduces Br2 production by 35%. Following earlier studies, this reduction may be attributed to surface butanol molecules that block N2O5 entry or alter the near-surface distribution of Br-. Remarkably, addition of the cationic surfactant tetrabutylammonium bromide (TBABr) at 0.005 m (9% of a monolayer) reduces the Br-2 signal by 85%, and a 0.050 m solution (58% of a monolayer) causes the Br-2 signal to disappear entirely. A detailed analysis suggests that TBA(+) efficiently suppresses Br-2 evaporation because it tightly bonds to the Br-3(-) intermediate formed in the highly concentrated Br solution and thereby hinders the rapid release and evaporation of Br-2.