Molecular Orientation at the Squalene/Air Interface from Sum Frequency Generation Spectroscopy and Atomistic Modeling

Human skin oils are significant scavengers of atmospheric oxidants in occupied indoor environments, and squalene is a major ozone-active constituent. Here, we present a combined spectroscopic and atomistic modeling approach to elucidate the conformational and orientational preferences of squalene at the air/oil interface and their implications for reactions with ozone. We find that squalene chains have a tendency to align with the surface normal, resulting in different concentrations of the various types of its double bonds and thus different reactivities. We also observe the presence of water at the surface of this hydrophobic compound. Both findings have possible implications for the design and outcomes of kinetic models describing this important aspect of indoor air chemistry.