Hydrophobic Model Systems for Oil Film Photooxidation: Part I: Sensitizer Effects on Hydrocarbon Photodegradation

Photochemical degradation is a significant pathway controlling the fate of surface oil after marine spills; however, the mechanisms acting within oil films remain poorly understood due to the complexity of crude oil mixtures. To address this, we optimized a hydrophobic model system with a representative polycyclic aromatic hydrocarbon (PAH) and sensitizer, specifically benzo[a]pyrene (BaP) and tetracene (TCN), and then validated it with a broader set of hydrocarbons, including both saturated compounds and PAHs. Scavenger experiments confirmed that TCN generates singlet oxygen (¹O₂) as the dominant oxidant, while TCN transformation products, particularly quinones, sustained photoreactivity over time. Application of the system to mixtures of hydrocarbons and crude oil showed that only larger PAHs (≥3 rings) underwent significant, sensitizer‑enhanced loss, indicating that indirect photooxidation via ¹O₂ dominated over direct photolysis. In contrast, monoaromatics, naphthalenes, and saturated compounds remained stable under the studied conditions; after a spill, these compounds would instead be expected to undergo other weathering processes such as biodegradation, evaporation, or dissolution. In a complementary set of experiments, replacing TCN with crude oil as a sensitizer mirrored the degradation pattern observed in the TCN-based system, validating the role of indirect pathways in petroleum. These results indicate that indirect, sensitizer-driven pathways selectively control the transformation of PAHs in oil films and that evolving sensitizer pools prolong photoreactivity. Overall, the model provides a tractable framework for dissecting the oil photooxidation mechanisms of various hydrocarbon classes in marine oil slicks.