Surface Chemistry of Human-Derived Organic Compounds in Indoor Environments

Butman, Jana Lee

doi:https://doi.org/10.21985/n2-23j2-4126

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Surface Chemistry of Human-Derived Organic Compounds in Indoor Environments

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Human skin oils are significant scavengers of atmospheric oxidants in occupied indoor environments. Many techniques used to study gas-phase transformations of surface films indoors have been limited to off-line bulk analysis, although more surface-selective methodologies are emerging. Here, we present a multi-prong analytical approach to characterizing skin oil ozonolysis. Skin oil is a complex heterogeneous system containing compounds like oleic acid and squalene, a major ozone-active constituent. We employ 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 tend to align with surface normal, resulting in different concentrations of the various types of its double bonds at the interface and thus different reactivities. Skin oil and squalene are found to produce different vibrational sum frequency generation spectra in the C–H stretching region, while exposure to ozone results in surface spectra for both materials that is consistent with a loss of C–H oscillators. Measured contact angles show that the hydrophobicity of the films increases following exposure to ozone, consistent with the reduction in C═C···H2O (“πH”) bonding interactions that is expected from C═C double bond loss due to ozonolysis and indicating that the polar functional groups formed point toward the films’ interiors. Further elucidation of skin oil’s surface partitioning behaviors are revealed through mixing ratio studies of a binary squalene/oleic acid mixture. Implications for heterogeneous indoor chemistry are discussed.

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