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

We examine synthesized standards of structurally related β-caryophyllene ozonolysis products at the air/water and air/ammonium sulfate (aq) interfaces using polarization-resolved standard- and high-resolution vibrational sum frequency generation (SFG) spectroscopy in the C–H and O–H stretching regions. β-Caryophyllene aldehyde, the most surface-active molecule in the suite, appears to undergo a change in its molecular orientation distribution when transitioning from low to high relative surface coverage. We also report that the water molecules present at the outermost layer of the aqueous interface are displaced by or hydrogen-bonded to β-caryophyllene aldehyde on both the ammonium sulfate (aq) and water subphases. Additionally, the more weakly hydrogen-bonded water network observed around ∼3400 cm–1 seems to be perturbed to a greater extent by β-caryophyllene aldehyde than the more strongly hydrogen-bonded population observed at ∼3200 cm–1 on the ammonium sulfate (aq) subphase, while this effect is not observed on the water subphase. We provide surface spectroscopic evidence for the “salting out” effect of ammonium sulfate in aqueous environments and discuss the implications for understanding the organization of surface-localized molecules present in aqueous cloud-forming droplets containing dissolved aerosol cloud condensation nuclei (CCN) in the atmosphere. Despite the importance of elucidating the surface activity and interfacial organization of individual molecules in aqueous media, future work will focus on how the combined effects of several surface-active constituents dictate overall cloud activation potentials of aerosol particles.