Influence of molecular and supramolecular structure on the gas-liquid interfacial reactivity of hydrocarbon liquids with O(3P) atoms

We have investigated the interfacial reactivity of gas-phase 0(3 P) atoms with a representative range of long-chain liquid hydrocarbons. These consisted of two branched molecules, squalane (C(30)H(62), 2,6,10,15,19,23-hexamethyltetracosane) and pristane (C(19)H(40), 2,6,10,14-tetramethylpentadecane), and three linear ones, n-docosane (C(22)H(46)), n-tetracosane (C(24)H(50)) and n-octacosane (C(28)H(58)). This represents the first systematic investigation of reactions of this type for molecules other than squalane. The O((3)P) atoms were generated by 355-nm laser photolysis of a low pressure of NO(2) above the liquid surface. The nascent gas-phase OH radical products were detected by laser-induced fluorescence (LIF). Measurements for the linear hydrocarbons were constrained by their vapor pressures to single temperatures slightly (similar to 1 K) above their respective melting points. Pristane was studied at the lowest temperature practically achievable. Squalane was compared as a reference at the full set of temperatures. Appearance profiles for all of the liquids showed similar characteristic differences between OH v'=O and 1. LIF excitation spectra were obtained for each of the vibrational levels at both the rising edge and peak of the appearance profiles. We conclude that the observed variations in rotational temperatures are consistent with dual contributions to the reaction mechanism for all the liquids, involving both direct escape and trapping-desorption components of the observed OH, as has previously been proposed for squalane. The relative yields of OH showed some surprising dependences on the liquid, including an unexpectedly strong variation with linear hydrocarbon chain length. These cannot all be explained by the relative reactivity of primary, secondary, and tertiary H-C units. We discuss the possibility that the known "surface freezing" phenomenon for linear hydrocarbons may play a role.