Calcium Bridging Drives Polysaccharide Co-Adsorption to a Proxy Sea Surface Microlayer

Abstract

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.

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…Ab initio MD and quantum mechanics/molecular mechanics (QM/MM) simulations, which are based on electronic structure, are computationally expensive and limited to systems of no more than $100 molecules, whereas empirical potential functions can be used to reach larger system sizes into the thousands of molecules. 124 Accordingly, MD simulations have been applied to surfactant systems ranging in complexity from simple surfactant/salt mixtures [125][126][127][128][129][130][131][132][133][134] to multi-component mimics for sea spray aerosol 93,[135][136][137][138][139] and SOA. 140 Quantum chemical methods such as density functional theory (DFT) 141 have also been applied to heterogeneous processes on atmospheric aerosol particles, 142 oen in combination with spectroscopic measurements.…”