Near-interfacial halogen atom exchange in collisions of Cl2 with 2.7 M NaBr-glycerol

Gas-liquid scattering experiments are used to investigate reactions of Cl(2) with a 2.7 M NaBr-glycerol solution at 291 K. Only the single and double halogen exchange products, BrCl and Br(2), are observed to desorb from solution. When Cl(2) molecules strike the surface at thermal collision energies, 76% desorb as Cl(2) before reacting, 1% react to form BrCl, and 23% react to form Br(2). Residence time measurements, modeled by mass-transfer equations for absorption, diffusion, reaction, and evaporation, were used to determine the time and depth scales for Cl(2) escape and BrCl and Br(2) production. This modeling indicates that Cl(2) molecules desorb from the interfacial region in less than 1 μs or are attacked within this time by Br(-) ions and irreversibly captured as Cl(2)Br(-). The products BrCl and Br(2) are created primarily within the top few monolayers of the solution and then evaporate on average 12 and 28 μs after Cl(2) initially reacts with Br(-). Notably, Br(2) is not generated from BrCl via Cl(2)Br(-) → BrCl + Cl(-) and BrCl + Br(-) → Br(2)Cl(-) but from the parallel reaction Cl(2)Br(-) + Br(-) → Br(2)Cl(-)+ Cl(-) that bypasses the BrCl intermediate. Br(2) is then likely released through two pathways, Br(2)Cl(-) → Br(2) + Cl(-) and Br(2)Cl(-) + Br(-) → Br(3)(-) + Cl(-), followed by Br(3)(-) ↔ Br(2) + Br(-). The experiments demonstrate that single and double halogen exchange reactions can occur rapidly and close to the surface even when the products are created by multiple sequential reactions.