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 following marine oil spills; however, the mechanisms governing transformation in 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. In contrast, monoaromatics, naphthalenes, and saturated compounds remained stable under the studied conditions. Replacing TCN with crude oil as a sensitizer mirrored the degradation pattern observed in the TCN-based system, validating the role of indirect photooxidation 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 system provides a tractable framework for dissecting hydrocarbon-class-specific oil photooxidation mechanisms in a homogeneous hydrophobic phase.