Surfactant-Promoted Reactions of Cl2 and Br2 with Br- in Glycerol

Gas-liquid scattering experiments are used to explore reactions of gaseous Cl-2 and Br-2 with a 0.03 M solution of the surfactant tetrahexylammonium bromide (THABr) dissolved in glycerol. At thermal collision energies, 79 +/- 2% of incident Cl-2 molecules react with Br- to form Cl2Br- in the interfacial region. This reaction probability is three times greater than the reactivity of Cl-2 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 Br-2 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 Cl-2 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 Br-2 molecules dissolve in 0.03 M THABr for 1(0)4 times longer than in 3 M NaBr. We attribute this change to formation of stabilizing interfacial and bulk-phase THA(+)Br(3)(-) ion pairs, in analogy with the capture of Cl-2 and formation of THA(+)Cl(2)Br(-) pairs. The THA(+) ion appears to be a powerful interfacial catalyst for promoting reaction of Cl-2 and Br-2 with Br- and for ferrying the resultant ions into solution.