Real-Time Monitoring of the Kinetics and Gas-Phase Products of the Reaction of Ozone with an Unsaturated Phospholipid at the Air−Water Interface

• Published in: Langmuir Vol. 16; no. 24; pp. 9321 - 9330
• Main Authors: Wadia, Y, Tobias, D. J, Stafford, R, Finlayson-Pitts, B. J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28.11.2000
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0006622

While the kinetics and mechanisms of the reaction of O3 with alkenes in the gas and condensed phases are reasonably well understood, those with unsaturated organics in the intermediate regime at the air−water interface are not. Studies of the reaction of ozone at room temperature with the unsaturated phospholipid, 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) at the air−water interface, and, for comparison, the fully saturated dipalmitoyl-l-α-phosphatidylcholine (DPPC) were carried out. The phospholipids were exposed at varying surface areas per phosphocholine molecule on a water subphase to a flow of O3 in air (0.25−1 ppm), and atmospheric pressure ionization mass spectrometry (API-MS) was used to monitor the formation of gaseous products in real time. Nonanal was detected as a major gas-phase product of the reaction of ozone with OPPC; no volatile products were observed in the case of DPPC. The yield of nonanal, defined as the nonanal produced per phosphocholine molecule reacted at the air−water interface, was 51 ± 13% (2σ) over this range of ozone concentrations, after correcting for the solubility of nonanal in the subphase. The nonanal yield was also independent of the available area per molecule over the range from 40 to 158 Å2 molecule-1, 56 ± 11% (2σ), at a constant O3 concentration of 1 ppm. Cyclohexane was used as a scavenger for any OH generated in the reaction, but no evidence for gas-phase OH radical production in the OPPC−O3 reaction was found using this technique. The time-dependence of the generation of nonanal shows that these reactions are enhanced kinetically at the air−water interface compared to that expected for analogous gas-phase reactions. Molecular dynamics simulations of OPPC at the air−water interface show that the insensitivity of the time dependence and yield of nonanal production to the extent of film compression, as well as the kinetic enhancement, can be understood in terms of the structure of the fatty acid chains at the interface. These studies illustrate the utility of real-time monitoring of the gas-phase products of reactions at the air−water interface and the insights into kinetics and mechanisms which can be obtained by combining these experimental data with molecular dynamics simulations.