Control of Interfacial Cl₂ and N₂O₅ Reactivity by a Zwitterionic Phospholipid in Comparison with Ionic and Uncharged Surfactants

Abstract

Gas-liquid scattering experiments reveal that charge-separated but neutral (zwitterionic) surfactants catalyze the oxidation of dissolved Br^- to Br₂ by gaseous Cl₂ at the surface of a 0.3 M NaBr/glycerol solution. Solutions of NaBr dissolved in glycerol with no surfactant were compared with solutions coated with zwitterionic, cationic, and anionic surfactants at dilute surface concentrations of 1.1 to 1.5 × 10¹⁴ cm^-2 (less than 65% of maximum chain packing). The zwitterionic phospholipid enhances Cl₂ conversion of Br^- to Br₂ by a factor of 1.61 ± 0.15, in comparison with a 14-fold enhancement by a cationic surfactant (tetrahexylammonium) and a five-fold suppression by an anionic surfactant (dodecyl sulfate). Further studies indicate that even an uncharged surfactant, monododecanoylglycerol, enhances Cl₂ → Br₂ production. Similar behavior is observed for the oxidation of Br^- to Br₂ by N₂O₅; it is just slightly suppressed by the phospholipid and strongly enhanced by the cationic surfactant. Collectively, these results suggest that attractions and repulsions between the negative Br^- ions and the positive and negative charges of the surfactant headgroups draw Br^- ions to the surface or repel them away. At low coverages, ion-induced dipole and dispersion interactions between the CH₂ groups and Br^- or Cl₂ may also enhance reactivity. These results demonstrate that the hydrocarbon chains of loosely packed surfactants do not necessarily block gas-liquid reactions but that positively charged, and even uncharged, groups can instead facilitate reactions by bringing gas-phase and solution-phase reagents together in the interfacial region.