Control of Interfacial Cl-2 and N2O5 Reactivity by a Zwitterionic Phospholipid in Comparison with Ionic and Uncharged Surfactants

Gas liquid scattering experiments reveal that charge-separated but neutral (zwitterionic) surfactants catalyze the oxidation of dissolved Br- to Br-2 by gaseous Cl-2 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 X 10(14) cm(-2) (less than 65% of maximum chain packing). The zwitterionic phospholipid enhances Cl-2 conversion of Br- to Br-2 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 Cl-2 -> Br-2 production. Similar behavior is observed for the oxidation of Br- to Br-2 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 Cl-2 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.