Model Behavior: Characterization of Hydroxyacetone at the Air-Water Interface Using Experimental and Computational Vibrational Sum Frequency Spectroscopy.

Small atmospheric aldehydes and ketones are known to play a significant role in the formation of secondary organic aerosols (SOA). However, many of them are difficult to experimentally isolate, as they tend to form hydration and oligomer species. Hydroxyacetone (HA) is unusual in this class as it contributes to SOA while existing predominantly in its unhydrated monomeric form. This allows HA to serve as a valuable model system for similar secondary organic carbonyls. In this paper the surface behavior of HA at the air-water interface has been investigated using vibrational sum frequency (VSF) spectroscopy and Wilhelmy plate surface tensiometry in combination with computational molecular dynamics simulations and density functional theory calculations. The experimental results demonstrate that HA has a high degree of surface activity and is ordered at the interface. Furthermore, oriented water is observed at the interface, even at high HA concentrations. Spectral features also reveal the presence of both cis and trans HA conformers at the interface, in differing orientations. Molecular dynamics results indicate conformer dependent shifts in HA orientation between the subsurface (∼5 Å deep) and surface. Together, these results provide a picture of a highly dynamic, but statistically ordered, interface composed of multiple HA conformers with solvated water. These results have implications for HA's behavior in aqueous particles, which may affect its role in the atmosphere and SOA formation.