The inhibition of N2O5 hydrolysis in sulfuric acid by 1-butanol and 1-hexanol surfactant coatings.

Gas-liquid scattering experiments are used to measure the fraction of N2O5 molecules that are converted to HNO3 after colliding with 72 wt % H2SO4 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 N2O5 to HNO3 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 N2O5 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 N2O5 and near-interfacial H3O+ or H2O. 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.