Reactions of NO with Salty and Surfactant-Coated Glycerol: Interfacial Conversion of Br to Br Mediated by Alkylammonium Cations.

Gas-liquid scattering and product-yield experiments are used to investigate reactions of NO with glycerol containing Br and surfactant ions. NO oxidizes Br to Br 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). NO 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 is produced in the presence of THA, implying that the conversion of Br to Br 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 Br production rate by 23%, 63%, and 67% of the THABr value, respectively. When CTA is substituted for THA, Br production drops to 12% of the THABr value. The generation of Br under such different conditions implies that trace amounts of surface-active alkylammonium ions can catalyze interfacial NO reactions, even when salts and other surfactants are present.