Surfactant-promoted reactions of Cl2 and Br2 with Br- in glycerol

Gas-liquid scattering experiments are used to explore reactions of gaseous Cl2 and Br2 with a 0.03 M solution of the surfactant tetrahexylammonium bromide (THABr) dissolved in glycerol. At thermal collision energies, 79 ± 2% of incident Cl2 molecules react with Br(-) to form Cl2Br(-) in the interfacial region. This reaction probability is three times greater than the reactivity of Cl2 with 3 M NaBr-glycerol, even though the interfacial Br(-) concentrations are similar in each solution. We attribute the high 79% uptake to the presence of surface THA(+) ions that stabilize the Cl2Br(-) intermediate as it is formed in the charged, hydrophobic pocket created by the hexyl chains. Cl2Br(-) generates the single exchange product BrCl in a 1% yield close to the surface, while the remaining 99% desorbs as the double exchange product Br2 over >0.1 s after diffusing deeply into the bulk. When NaCl is added to the surfactant solution in a 20:1 Cl(-)/Br(-) ratio, the Cl2 reaction probability drops from 79% to 46 ± 1%, indicating that Cl(-) in the interfacial region only partially blocks reaction with Br(-). In parallel, we observe that gaseous Br2 molecules dissolve in 0.03 M THABr for 10(4) times longer than in 3 M NaBr. We attribute this change to formation of stabilizing interfacial and bulk-phase THA(+)Br3(-) ion pairs, in analogy with the capture of Cl2 and formation of THA(+)Cl2Br(-) pairs. The THA(+) ion appears to be a powerful interfacial catalyst for promoting reaction of Cl2 and Br2 with Br(-) and for ferrying the resultant ions into solution.