Reactions of N2O5 with Salty and Surfactant-Coated Glycerol: Interfacial Conversion of Br- to Br2 Mediated by Alkylammonium Cations

Gas liquid scattering and product-yield experiments are used to investigate reactions of N2O5 with glycerol containing Br- and surfactant ions. N2O5 oxidizes Br- to Br-2 for every solution tested: 2.7 M NaBr, 0.03 M tetrahexylammonium bromide (THABr), 0.03 M THABr + 0.5 M NaBr, 0.03 M THABr + 0.5 M NaCl, 0.03 M THABr + 0.01 M sodium dodecyl sulfate (SDS), and 0.01 M cetyltrimethylammonium bromide (CTABr). N2O5 also reacts with glycerol itself to produce mono- and dinitroglycerin. Surface tension measurements indicate that 0.03 M THABr and 2.7 M NaBr have similar interfacial Br- concentrations, though their bulk Br- concentrations differ by 90-fold. We find that twice as much Br-2 is produced in the presence of THA(+), implying that the conversion of Br- to Br-2 is initiated interface, perhaps mediated by the charged, hydrophobic pocket within the surface THA(+) cation. The addition of 0.5 M NaBr, 0.5 M NaCl, or 0.01 M SDS to 0.03 M THABr lowers the Br-2 production rate by 23%, 63%, and 67% of the THABr value, respectively. When CTA(+) is substituted for THA(+), Br-2 production drops to 12% of the THABr value. The generation of Br-2 under such different conditions implies that trace amounts of surface-active alkylammonium ions can catalyze interfacial N2O5 reactions, even when salts and other surfactants are present.