Kinetics of heterogeneous reaction of ozone with linoleic acid and its dependence on temperature, physical state, RH, and ozone concentration

Heterogeneous reaction between ozone and linoleic acid (LA) thin film was investigated by a flow reactor coupled to attenuated total reflection infrared spectroscopy (FR-ATR-IR) over wide ranges of temperature, relative humidity (RH), and ozone concentration under atmospheric pressure condition. Pseudo-first-order rate constants k_app and overall reactive uptake coefficients γ were acquired on the basis of changes in absorbance from peaks located near 1743, 1710, 1172, and 1110 cm^-1, which can be assigned to C=O in ester, C=O in acid, and C - C and C - O stretching modes, respectively. Results showed that the k_app and γ increased nearly by a factor of 6 with increasing temperatures from 258 to 314 K. It was noted the temperature effect on the reaction kinetics was much more pronounced at lower temperatures. Such behavior can be explained by a change in the physical state of LA at lower temperatures. In addition, k_app and γ were enhanced by 2-fold as the RH increased from 0 to 80%. Moreover, the effect of ozone concentration on the reaction kinetics was reported for the first time. k_app was found to display a Langmuir-Hinshelwood dependence on ozone concentration with K_O3 = (1.146 ± 0.017) × 10^-15 molecules cm^-3 and k[S] = 0.0522 ± 0.0004 s^-1, where K_O3 is a parameter that describes the partitioning of ozone to the thin film surface, and k[S] is the maximum pseudo-first-order coefficient at high ozone concentration. Furthermore, yields and hygroscopic properties of reaction products were also investigated by FTIR spectroscopy. The intensity ratio of two C=O stretching bands, A ₁₇₄₃/A₁₇₁₀, which was utilized as an indicator of the product yields, increased sharply with increasing temperatures in the lower temperature region (258-284 K), and then remained nearly constant in the higher temperature region (284-314 K). The product yields showed no significant variation with RH, for the intensity ratio of A₁₇₄₃/A₁₇₁₀ barely changed in the wide RH range 0-80%. Water uptake studies showed that the LA thin film absorbed water with an increasing RH, and the hygroscopicity of the thin film was enhanced after ozone exposure.