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

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.

Automated summary

The study found that the ratio of fatty alcohols to fatty acids in organic coatings on sea spray aerosol affects the binding affinity of Zn2+-carboxylate on the surface. It was observed that Zn2+-carboxylate binding at the surface is significantly enhanced compared to bulk solution, and the addition of fatty alcohols can reduce the apparent binding affinity.