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

Abstract

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 Br₂ 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 Br₂ is produced in the presence of THA⁺, implying that the conversion of Br^- to Br₂ is initiated at the 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 Br₂ production rate by 23%, 63%, and 67% of the THABr value, respectively. When CTA⁺ is substituted for THA⁺, Br₂ production drops to 12% of the THABr value. The generation of Br₂ 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.