Production of Br2 from N2O5 and Br- in Salty and Surfactant-Coated Water Microjets.

Gas-liquid scattering experiments are used to investigate the oxidation-reduction reaction N2O5(g) + 2Br-(aq) → Br2(g) + NO3-(aq) + NO2-(aq), a model for the nighttime absorption of N2O5 into aerosol droplets containing halide ions. The detection of evaporating Br2 molecules provides our first observation of a gaseous reaction product generated by a water microjet in vacuum. N2O5 molecules are directed at a 35 μm diameter jet of 6 or 8 m LiBr in water at 263 or 240 K, followed by detection of both unreacted N2O5 and product Br2 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 ∼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 Br2 signal by 85%, and a 0.050 m solution (58% of a monolayer) causes the Br2 signal to disappear entirely. A detailed analysis suggests that TBA+ efficiently suppresses Br2 evaporation because it tightly bonds to the Br3- intermediate formed in the highly concentrated Br- solution and thereby hinders the rapid release and evaporation of Br2.