The Inhibition of N sub(2)O sub(5) Hydrolysis in Sulfuric Acid by 1-Butanol and 1-Hexanol Surfactant Coatings

Gas-liquid scattering experiments are used to measure the fraction of N sub(2)O sub(5) molecules that are converted to HNO sub(3) after colliding with 72 wt % H sub(2)SO sub(4) containing 1-hexanol or 1-butanol at 216 K. These alcohols segregate to the surface of the acid, with saturation coverages estimated to be 60% of a close-packed monolayer for 1-hexanol and 44% of a close-packed monolayer for 1-butanol. We find that the alkyl films reduce the conversion of N sub(2)O sub(5) to HNO sub(3) from 0.15 on bare acid to 0.06 on the hexyl- coated acid and to 0.10 on the butyl-coated acid. The entry of HCl and HBr, however, is enhanced by the hexanol and butanol films. The hydrolysis of N sub(2)O sub(5) may be inhibited because the alkyl chains restrict the transport of this large molecule and because the alcohol OH groups dilute the surface region, suppressing reaction between N sub(2)O sub(5) and near-interfacial H sub(3)O super(+) or H sub(2)O. In contrast, the interfacial alcohol OH groups provide additional binding sites for HCl and HBr and help initiate ionization. These and previous scattering experiments indicate that short- chain alcohol surfactants impede or enhance sulfuric acid-mediated reactions in ways that depend on the chain length, liquid phase acidity, and nature of the gas molecule.