The sea surface microlayer (SML) is the water layer within the first few millimeters of the air/sea interface. The layer is rich in polysaccharides and calcium cations (10 mM). At the interface, long chain amphiphilic (carboxylic) fatty acids derived from the microorganisms and algae of the neuston ecosystem of the microlayer are adsorbed in monolayers, and complex with the calcium cations and polysaccharides to form an interfacial layer which mediates the physico-chemistry of sea surface interfacial processes. To understand the molecular structure of the microlayer, we focus on two fatty acids, the straight, sixteen hydrocarbon chain palmitic acid (the most abundant fatty acid on the sea surface), and phytanic acid, a palmitic acid homologue with four methyl branches along the chain and present in the thylakoid membranes of the chloroplasts of sea algae. As a characteristic sea polysaccharide we study the negatively charged lambda carrageenan. Langmuir trough surface pressure measurements of spread monolayers of palmitic and phytanic acid atop aqueous subphases indicate that the presence of the methyl branches create a more expanded, liquid-like monolayer relative to the condensed monolayers of the straight chain palmitic acid. Surface pressure and atomic force microscopy measurements of the monolayers on solid supports indicate that the liquid phytanic acid monolayer is more stable to collapse than palmitic under a constant surface pressure. The addition of lambda carrageenan to the sublayer causes an expansion of the layer due to the formation of a complex, and the further addition of calcium leads to a larger expansion. X-ray reflectivity (XR) and x-ray fluorescence near total reflection (XFNTR) are undertaken on spread monolayers using the synchrotron radiation source and the liquid surface spectrometer at Argonne. From the reflectivity data, and slab models, the surface concentrations of carrageenan and fatty acids are obtained, and the calcium directly from the fluorescence data. For phytanic acid, which formed stable monolayers for x-ray interrogation over extended times, the concentration of carrageenan is greater with calcium in the subphase indicating the positive calcium cation acts as a bridge to electrostatically complex the negative charged fatty acid to the negative polysaccharide. These studies are used to to understand and rationalize the composition of sea spray aerosols derived from the surface microlayer.

Complexation of fatty acids, calcium cations and polysaccharides at the sea surface microlayer and Its applications to sea spray aerosols

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