Reaction of gas phase OH with unsaturated self-assembled monolayers and relevance to atmospheric organic oxidations

Abstract

The kinetics and mechanisms of the reaction of gas phase OH radicals with organics on surfaces are of fundamental chemical interest, as well as relevant to understanding the degradation of organics on tropospheric surfaces or when they are components of airborne particles. We report here studies of the oxidation of a terminal alkene self-assembled monolayer (7-octenyltrichlorosilane, C8 SAM) on a germanium attenuated total reflectance crystal by OH radicals at a concentration of 2.1 × 10⁵ cm⁻³ at 1 atm total pressure and 298 K in air. Loss of the reactant SAM and the formation of surface products were followed in real time using infrared spectroscopy. From the rate of loss of the CC bond, a reaction probability within experimental error of unity was derived. The products formed on the surface include organic nitrates and carbonyl compounds, with yields of 10 ± 4% and ≤7 ± 4%, respectively, and there is evidence for the formation of organic products with C–O bonds such as alcohols, ethers and/or alkyl peroxides and possibly peroxynitrates. The yield of organic nitrates relative to carbonyl compounds is higher than expected based on analogous gas phase mechanisms, suggesting that the branching ratio for the RO₂ + NO reaction is shifted to favor the formation of organic nitrates when the reaction occurs on a surface. Water uptake onto the surface was only slightly enhanced upon oxidation, suggesting that oxidation per se cannot be taken as a predictor of increased hydrophilicity of atmospheric organics. These experiments indicate that the mechanisms for the surface reactions are different from gas phase reactions, but the OH oxidation of surface species will still be a significant contributor to determining their lifetimes in air.