Knowledge Bank

Sea spray aerosols (SSA) are known to have a thin organic coating that is largely comprised of fatty acids, but also contains many other organic molecules. Subphase sea water contains metal ions that have the ability to interact with this organic coating at the air-water interface, resulting in transport of metals into the atmosphere. Metal surface binding at pure monolayers has been well studied, however metal surface binding at ocean relevant mixed monolayers has not previously been studied. A model interfacial system with four monolayer ratios of stearic acid to octadecanol (100:0, 95:5, 90:10, and 85:15) is analyzed on subphases containing varying ZnCl2 concentrations with a constant NaCl ionic background. Monolayers are held at a surface pressure of 35 mN/m and an approximate mean molecular area of 21.5 Å2/molecule, maintaining a highly ordered film structure. Surface pressure–area (Π-A) isotherms and infrared reflection absorption spectroscopy (IRRAS) were used in conjunction to determine molecular binding interactions and quantify Zn2+– carboxylate surface binding affinities at each monolayer composition. It was found that: (1) Binding affinities for all four monolayer compositions are ~300 times greater in magnitude than zinc-acetate binding in bulk aqueous solution, indicating that zinc-carboxylate binding is enhanced at the surface, and (2) Mixed monolayer composition impacts trace metal surface binding in both strength and stoichiometry.