Quantitative Time-Resolved Vibrational Sum Frequency Generation Spectroscopy as a Tool for Thin Film Kinetic Studies: New Insights into Oleic Acid Monolayer Oxidation

Abstract

Environmental air-water interfaces are often covered by thin films of surface-active organic substances that play an important role for air-sea gas exchange and aerosol aging. Surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy has been widely used to study the static structure of organic monolayers serving as simple model systems of such films. Probably due to the difficulties to correlate the SFG signal intensity with the surface concentration, corresponding time-resolved studies of surface reactions are scarce. In this study, quantitative time-resolved measurements have been performed on the oleic acid monolayer ozonolysis, which is considered a benchmark system for investigating the reactivity and fate of unsaturated natural organics. Surface concentration calibration data have been obtained by combining the pressure-area isotherm and VSFG spectra acquisition such that the 2D phase behavior of the oleic acid film could be properly taken into account. In contrast to literature reports, surface-active oxidation products were found to be negligible and do not interfere with the VSFG measurements. A pseudo-first-order kinetic analysis of the time-resolved data yielded a bimolecular rate constant of k2(oleic acid + O3 → products) = (1.65 ± 0.64) × 10(-16) cm(3) molecules(-1) s(-1), corresponding to an uptake coefficient of γ = (4.7 ± 1.8) × 10(-6). This result is in very good agreement with most recent monolayer measurements based on alternative methods and underlines the reliability of the time-resolved VSFG approach.

The reaction of oleic acid monolayers with gas-phase ozone at the air water interface: the effect of sub-phase viscosity, and inert secondary components

“…The purity and structure of the partially deuterated sample used in this work was confirmed by the supplier with GC-MS and the new fully deuterated sample is a commercial sample. The results from this current study with the new fully deuterated sample are consistent with other work published in the literature [53][54][55][56] where no final product film was observed. The results are consistent with other studies of the reaction between ozone and oleic acid (or a molecule that contains the oleic moiety) at the air-water interface.…”

“…The results are consistent with other studies of the reaction between ozone and oleic acid (or a molecule that contains the oleic moiety) at the air-water interface. [53][54][55][56][57][58][59][60][61][62][63][64][65] It is also noteworthy that Fig. 5 demonstrates that the stationary point in the kinetic decay shown in Fig.…”

Comparative Adsorption of Saturated and Unsaturated Fatty Acids at the Iron Oxide/Oil Interface

“…The second observation of interest is the emergence of a peak around 3000 cm -1 for oleic and linoleic acid, which is attributed to the C-H stretch from the double-bonded carbons 29 . However, as shown in Figure 8, for both oleic and linoleic acid this peak decreases in intensity proportionally to the other peaks upon heating the sample.…”

Surface-Active β-Caryophyllene Oxidation Products at the Air/Aqueous Interface

“…The presence of surface-active organic species within aerosol particles has been reported to play a critical role in cloud activation due to the propensity of these molecules to localize at the air/aqueous interface and reduce the surface tension of forming droplets containing aerosol nuclei. ,,,,− Moreover, such surface-active molecules may play an important role in cooperative adsorption processes, where the presence of an adsorbed insoluble monolayer at the vapor/aqueous interface may further promote the surface enrichment of soluble organic species in the aerosol bulk, thereby influencing aerosol-mediated cloud droplet nucleation. − Several reports have examined the surface tension depression induced by aerosol samples ,− as well as selected model compounds ,− in aqueous media in order to examine their respective cloud activation potentials. These studies have provided estimates for the relative surface activities of several organic species likely present within aerosol mixtures, yet establishing how the interfacial organization of such compounds on an aqueous subphase alters aerosol surface properties, and thus CCN activities, remains challenging. − Therefore, molecular-level insights into the organization of surface-active molecules at atmospherically relevant air/aqueous interfaces are needed to reveal the molecular origins of CCN-driven cloud activation.…”