Calcium bridging drives polysaccharide co-adsorption to a proxy sea surface microlayer

Area of Science:

• Marine organic chemistry
• Atmospheric science
• Surface chemistry

Background:

• Saccharides are a major component of sea spray aerosol (SSA) organic carbon.
• Mechanisms of saccharide transfer from seawater to SSA are not fully understood.
• Hypothesized cooperative adsorption of saccharides with other organic matter at the sea surface microlayer (SSML).

Purpose of the Study:

• To investigate the co-adsorption mechanism of marine polysaccharides at the air-sea interface.
• To elucidate the role of cations in polysaccharide adsorption to insoluble organic monolayers.
• To characterize the molecular interactions governing saccharide enrichment in the SSML.

Main Methods:

• Surface-sensitive infrared reflection-absorption spectroscopy (IRRAS).
• All-atom molecular dynamics (MD) simulations.
• Investigation of alginate co-adsorption to a palmitic acid monolayer.

Main Results:

• Alginate co-adsorbs to palmitic acid monolayers via divalent cation bridging.
• Calcium ions (Ca2+) significantly enhance alginate co-adsorption (~30% increase in surface coverage).
• Magnesium ions (Mg2+) show limited co-adsorption due to strong hydration; sodium ions (Na+) show none.
• Palmitic acid protonation inhibits cation bridging.

Conclusions:

• Divalent cations, particularly Ca2+, are crucial for polysaccharide enrichment in the SSML.
• This cation-bridging mechanism explains saccharide transfer to the air-sea interface.
• Findings offer insights into SSA composition and formation processes.