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    Zinc-Carboxylate Binding in Mixed Octadecanoic Acid and Octadecanol Monolayers on Proxy Seawater Solution Surfaces

    Year: 2021

    Journal: ACS Earth Space Chem., Volume 5, OCT 21, page 2947–2956

    Authors: Auvil, Nicole C.; de Vasquez, Maria G. Vazquez; Allen, Heather C.

    Organizations: National Science Foundation Center for Aerosols Impacts on the Chemistry of the Environment (NSF-CAICE) [CHE1801971]

    Keywords: air-water interface; binding constant; surface spectroscopy; trace metal; metal ion enrichment; mixed monolayer; stearic acid; stearyl alcohol

    Organic coatings on sea spray aerosol are largely comprised of fatty acids in addition to a vast array of other organic molecules including fatty alcohols. The seawater from which sea spray aerosol originates contains metal ions that interact with the organic coating at the air-seawater interface, resulting in transport of these metals into the atmosphere. Metal binding within single-substance monolayers on aqueous solutions has been previously studied. However, such binding events within mixed monolayers, especially those of ocean-relevant ratios, are not well-explored. Here, we examine four monolayer ratios of octadecanoic acid (stearic acid) to octadecanol (stearyl alcohol) on aqueous solutions with varying ZnCl2 concentrations and a seawaterrelevant NaCl concentration of 0.465 M. Surface pressure-area (Pi-A) isotherms and infrared reflection-absorption spectroscopy (IRRAS) are used to quantify the Zn2+-carboxylate surface binding affinities for each monolayer composition. We find that Zn2+-carboxylate binding is enhanced by similar to 300 times at the surface when compared to the bulk solution. Addition of 10% octadecanol reduces the apparent surface binding affinity by over 50% from 3.6 x 10(3) to 1.4 x 10(3) M-1; this is significantly more than predicted from the slight reduction in viable 1:1 binding sites. Furthermore, 1:2 Zn2+-carboxylate binding is shown to only be viable for the 100 and 95% fatty acid film, whereas 1:1 binding is observed for all film ratios investigated.
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