Surface pH and Interfacial Properties of Aqueous Aerosols

Aqueous interfaces are ubiquitous in nature. The characteristic interfacial properties at/near the aqueous interface are crucial in many fundamental processes in the aerosol science, atmospheric chemistry, marine science, biological chemistry and electrochemistry.1-3 For instance, due to the vast sea/air interface, the interfacial properties of the sea surface microlayer inherently determine the chemical compositions of sea spray aerosols that constitute a major source of atmospheric aerosols. These surface-specific properties may become even more outstanding and crucial for nanoscaled aqueous aerosols, due to their relatively large surface-to-volume ratio.

The research team led by Prof. Chia C. Wang reported for the first time the valence electronic properties of phenol, catechol, resorcinol and hydroquinone aqueous nanoaerosols via the recently built aerosol VUV photoelectron spectroscopy. The chemical composition and surface pH at the aqueous aerosol interface deviate considerably from the bulk. The surface pH of phenol aqueous nanoaerosols is estimated ~2.2 ± 0.1 pH units more acidic than the bulk interior. Considering that the valence electronic structures of most organic solute species are strongly pH-dependent, the modified acidity on the aqueous nanoaerosol interface is crucial as it means that the chemical activities of aqueous nanoaerosols, particularly their tendency to undergo oxidation could be drastically different from the bulk. It reveals that, with increasing degrees of hydration, increasing extents of deprotonation accompanying with increasing pH, and increasing numbers of –OH group may substantially lower the first ionization energy for phenolic species, thereby allowing them to lose the outermost electrons more readily upon phenolic oxidation.

Reference:

Lin, P.-C., Wu, Z.-H., Chen, M.-S., Li, Y.-L., Chen, W.-R., Huang, T.-P., Lee, Y.-Y. and Wang, C. C.* (2017): Interfacial solvation and surface pH of phenol and dihydroxybenzene aqueous nanoaerosols unveiled by aerosol VUV photoelectron spectroscopy. J. Phys. Chem. B 121, 1054-1067. (link)