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.