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    Infrared Reflection-Absorption Spectroscopy of alpha-Hydroxyacids at the Water-Air Interface

    Journal: J. Phys. Chem. A

    Authors: Deal, Alexandra M.; Vaida, Veronica

    Organizations: U.S. Army Research Office (Grant ARO); National Science Foundation (Grant CHE); National Science Foundation Graduate Research Fellowship (Grant DGE); [W911NF1710115]; [1611107]; [1650115]

    Organic molecules, including alpha-hydroxyacids, are ubiquitous in the natural environment. Often found at water-air interfaces, organic molecules can alter the structure of the interface or participate in interfacial chemistry. Despite their prevalence in the environment, the structure and ordering of alpha-hydroxyacids have not been widely investigated at water-air interfaces, and the impact of the hydrophobic tail length on structure has not been explored. Here, for the first time, we use infrared reflection-absorption spectroscopy to assess the vibrational structure of alpha-hydroxyacids at a water surface as a function of surface partitioning and surface coverage. We study lactic acid, 2-hydroxyoctanoic acid, and 2-hydroxystearic acid, which have 1 carbon, 6 carbon, and 16 carbon tails, respectively. Vibrational features compared across the set of alpha-hydroxyacids studied here are used to determine the interaction of the polar headgroup with the water subphase and the ordering of the hydrophobic tail. We find that the carbonyl and alpha-hydroxyl groups participate in a complex hydrogen-bonding motif at the water-air interface that can be affected by the hydrophobic tail length and places the polar headgroup in or below the water-air interface. Furthermore, molecular ordering increases with the tail length or the surface coverage. The presence of the alpha-hydroxyl group causes the alpha-hydroxyacids to maintain a tilted orientation with respect to the surface normal even at high surface coverages. A combination of polar headgroup and hydrophobic tail effects dictates the overall orientation of alpha-hydroxyacids and can thus affect their ability to participate in chemistry and alter organic coatings on water surfaces.
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