Surface–bulk photochemical coupling of nonanoic acid and 4-benzoylbenzoic acid: the dual role of the photosensitizer and environmental influences

Organic compounds concentrated at air-water interfaces, such as the sea-surface microlayer (SML), or aerosol and cloud droplets, are key contributors to complex photochemical reactions. Nonanoic acid (NA) is surface active, and 4-benzoylbenzoic acid (4-BBA) is a photosensitizer. This study combines investigations at the air-water interface by sum-frequency generation (SFG) spectroscopy and in the liquid and gas phases by mass spectrometry (MS) to study the photochemical interactions of both substances at the air-water interface. We identify a novel interfacial aromatic signal via SFG, revealing surface-localized photoproducts previously undetected (C9H10O5). The study demonstrates that 4-BBA not only acts as photosensitizer but also undergoes photodegradation. Our experiments show the critical influence of the UV portion of the solar spectrum on photoproduct formation, as significant amounts of benzene (C6H6) and benzaldehyde (C7H6O) were detected as degradation products of 4-BBA in the gas phase particularly for the lowest wavelengths (280-310 nm). In the liquid phase, we identified the following photoreaction products: C8H12O4, C8H12O5, C9H10O5, C9H14O3, C9H16O3, C9H16O4, and C10H14O4. ... mehrAmong these, C8H12O4, C9H10O5, and C9H16O3 were most enhanced (by a factor of similar to 11) in the presence of 4-BBA, with C9H10O5 strongly dependent on the presence of oxygen. The formation of C9H10O5 increased at shorter wavelengths due to photodegradation of 4-BBA, while C8H12O4 and C9H16O3 were dominant at longer wavelengths, consistent with photooxidation of NA. While pH and salinity were not varied systematically in this study, we found that decreasing the pH from 8 to 5.4 affected the water restructuring at the interface and increased the rate of photoproduct formation. Increasing salinity from zero to similar to 38 ppt enhanced the photoreaction rates by a factor of 2. These findings highlight the importance of environmental factors in modeling interfacial photochemistry and demonstrate how SFG reveals surface-bulk coupling relevant to product formation in diverse aquatic systems.