Structure-Reactivity Trends of Fatty Acid Monolayer Ozonolysis Studied by Time-Resolved & Polarization-Dependent VSFG Spectroscopy
Monomolecular films of surface-active substances are model systems for organic-covered environmental interfaces. Oxidative processing of monolayers at the air-water interface modulates air-sea gas exchange and aerosol ageing and is discussed as a secondary source of volatile organic compounds. Vibrational sum-frequency generation spectroscopy (VSFG) has been used as a quantitative tool to determine the surface rate constants and uptake coefficients of a series of mono-unsaturated long-chain fatty acid monolayers with systematically shifted double bond position. Careful calibration procedures have been worked out to convert time-resolved SFG signal trends to concentration-time profiles. Petroselinic acid (PA), oleic acid (OA), and vaccenic acid (VA) were used as test systems with in-chain double bond and are compared with previous data for docosenoic acid (DA) as an example for a double bond terminated monolayer. Observed VSFG product spectra are consistent with a mechanistic model where the oxygenated products, simply depending on their volatility and solubility, stay at or escape from the interface. Moreover, a distinct reactivity trend in the order PA < OA < VA < DA has been found. Compared to gas phase reactivity, these rate constants imply a factor of 5.5 (PA), 8.4 (OA), 12.0 (VA), and 111 (DA) rate enhancement of the surface reaction. In consideration of previous experimental and molecular dynamics simulation results, both the rate enhancement effect and reactivity trend are discussed in view of closely interlinked thermodynamic and dynamic effects such as ozone adsorption, solubility, diffusion, and collision number. Whereas the noticeably higher relative rate enhancement of DA is most likely due to high local ozone concentrations resulting from an ozone absorption layer, the interpretation of the observed reactivity trend for PA, OA, and VA is more challenging, but most probably is a consequence of structural order effects in the monolayer. In order to gain insight to what extent a loose structural alignment and associated structural disorder of the alkyl chains within the monolayer affect the effective reactivity, monolayer ozonolysis has been performed at variable surface concentrations using linoleic acid (LA) as a test system. LA gives rise to strong VSFG signatures such that experiments could be performed at surface concentrations corresponding to the liquid-condensed (LC), liquid-expanded (LE), and gaseous (G) 2D phase state of LA. It was indeed possible to show that the reactivity increases with decreasing surface concentration. Finally, polarization null angle (PNA) measurements have been performed for PA, OA, and VA monolayers to reach conclusions about the structural order of the alkyl chains. Although the absolute values of the extracted average tilt angles of the terminal methyl group was found to be strongly dependent on the assumed local field factor, a robust trend of increasing tilt angles with lower surface concentration and in the order PA < OA < VA was found. As a higher tilt angle reflects a lower degree of overall chain alignment, this trend is consistent with both the reactivity and surface concentration trends outlined above.
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