Discerning Poly- and Monosaccharide Enrichment Mechanisms: Alginate and Glucuronate Adsorption to a Stearic Acid Sea Surface Microlayer

Poly- and monosaccharides are ubiquitous in the marine environment and are enriched in sea spray aerosol but the mechanisms for their enrichment are not fully understood. We expand upon previously defined co-adsorption mechanisms by investigating the co-absorption of alginate and its representative monomeric form, glucuronate, to a stearic acid monolayer as a function of saccharide concentration on an ocean proxy solution. Using Langmuir isotherms, surface-sensitive infrared reflection–absorption spectroscopy, and Brewster angle microscopy, we demonstrate that the mechanism of co-adsorption significantly differs between alginate and glucuronate. We find that film thickness increases from ∼3.5 to 4.8 nm and from ∼3.5 to ∼3.7 nm upon co-adsorption of alginate and glucuronate to stearic-d35 acid monolayers, respectively, indicating that alginate forms multilayers underneath the monolayer. Glucuronate shows a different co-adsorption where it likely intercalates and induces significant reorganization within the monolayer. We quantify the Langmuir adsorption coefficients and half-saturation concentrations of both alginate and glucuronate co-adsorption to stearic-d35 acid. We find that alginate co-adsorption produces the Langmuir adsorption constants K alginate = 0.089 ± 0.015 and C 1/2,alginate = 11 ± 1.9 mg/L alginate and glucuronate co-adsorption produces K glucuronate = 0.081 ± 0.015 and C 1/2,glucuronate = 12 ± 2.3 mg/L glucuronate. Thus, we demonstrate that the use of monosaccharides alone as a proxy for saccharide enrichment is insufficient, and we provide important parameters for better representation of saccharides in marine aerosol investigations.