Ozone initiated heterogeneous oxidation of unsaturated carboxylic acids by ATR-FTIR spectroscopy

Despite considerable effort has been directed to ozone-initiated heterogeneous oxidation of unsaturated organic species in atmospheric environment, current knowledge about how chemical structure of unsaturated carboxylic acids affects their reaction kinetics remains very limited. Here, kinetics of heterogeneous reaction of ozone with six unsaturated fatty acids (oleic acid, vaccenic acid, eladic acid, myristoleic acid, palmitoleic acid and 2-hexadecenoic acid) were studied via a flow system combined with attenuated total reflection Fourier-transformed infrared spectroscopy (ATR-FTIR). Pseudo-first-order rate constants (k(app)) and overall reactive uptake coefficients (gamma) values were derived according to the changes in absorbance from infrared spectra. Results have shown reaction rates are highly dependent on structures of unsaturated fatty acids. Cis-isomer has faster reaction kinetics than trans-isomer, and conjugated system between C=C and C=O bonds can greatly inhibit reactivity. Also, it is found that a longer carbon chain length between C=C bond and COOH group or a shorter chain length between C=C bond and last carbon seemingly enhances reaction kinetics. In addition, changes in redox activity and hydrophilicity of oleic acid samples before and after exposure ozone have been reported for the first time. Results have revealed that ozone-initiated heterogeneous reaction can markedly increase redox activity and hydrophilicity of unsaturated carboxylic acids. (C) 2019 Elsevier B.V. All rights reserved.