Saccharides comprise a significant mass fraction of organic carbon in sea spray aerosol (SSA), but the mechanisms through which saccharides are transferred from seawater to the ocean surface and eventually...
Trace metals have been found to be selectively enriched in the sea surface microlayer (SSML) due to association with organics, but the underlying cause of this selectivity has not been as well studied. Therefore, a systematic cation affinity study on surfactant lipids containing various functional headgroups (‐OH, ‐H2PO4, ‐COOH, ‐N(CH3)2) was conducted at pH 3 to further explain cation enrichment selectivity using Brewster angle microscopy, surface tension salt titration, and infrared‐reflection absorption spectroscopy (IRRAS). At pH 3, the headgroups chosen are neutral, and therefore the variability of surface charge on binding is eliminated. The phosphate ester headgroup was observed to exhibit the strongest trace metal binding, followed by the carboxylic acid headgroup. Hydroxy and dimethylamine headgroups did not exhibit significant trace metal binding. By applying the Langmuir‐Szyszkowski model to the surface tension data, the cation surface binding affinities for phosphatidic acid were determined, with Al3+ > Fe3+ > Zn2+ > Mg2+ > Ni2+ > Mn2+ ∼ Ca2+, an order not predicted from bulk properties such as solid formation constants of metal‐phosphonates. Thus cation binding to surface active organic molecules with phosphate ester headgroups is indicated to be a significant source of trace metal enrichment within the SSML.
Surface-sensitive vibrational spectroscopy is a common tool for measuring molecular organization and intermolecular interactions at interfaces. Peak intensity ratios are typically used to extract molecular information from one-dimensional spectra but...
Saccharides comprise a significant mass fraction of
organic carbon in sea spray aerosol (SSA), but the mechanisms through which
saccharides are transferred from seawater to the ocean surface and eventually
into SSA are unclear. It is hypothesized that saccharides cooperatively adsorb
to other insoluble organic matter at the air/sea interface, known as the sea
surface microlayer (SSML). Using a combination of surface-sensitive infrared
reflection-absorption spectroscopy and all-atom molecular dynamics simulations,
we demonstrate that the marine-relevant, anionic polysaccharide alginate
co-adsorbs to an insoluble palmitic acid monolayer via divalent cationic
bridging interactions. Ca<sup>2+</sup> induces the greatest extent of alginate
co-adsorption to the monolayer, evidenced by the ~30% increase in surface
coverage, whereas Mg<sup>2+</sup> only facilitates one-third the extent of
co-adsorption at seawater-relevant cation concentrations due to its strong hydration
propensity. Na<sup>+</sup> cations alone do not facilitate alginate co-adsorption,
and palmitic acid protonation hinders the formation of divalent cationic
bridges between the palmitate and alginate carboxylate moieties. Alginate
co-adsorption is largely confined to the interfacial region beneath the monolayer
headgroups, so surface pressure, and thus monolayer surface coverage, only
changes the amount of alginate co-adsorption by less than 5%. Our results
provide physical and molecular characterization of a potentially significant
polysaccharide enrichment mechanism within the SSML.
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