Linking surface coverage with surfactant activity to refine the role of surfactants for air-sea gas exchange

Abstract. The sea surface microlayer (SML) is enriched with organic matter. Surface-active molecules (surfactants) assemble right at the air-water interface to form a molecular layer of nanometer thickness that lowers the surface tension. At elevated surface concentrations of surfactants and substantial surface coverage, the nanolayer suppresses the formation of capillary waves and effectively decreases turbulent air-sea gas exchange mechanisms. This study provides a physico-chemical framework to evaluate the SML surfactant surface coverage. A non-linear surface sensitive laser spectroscopic method, vibrational sum-frequency generation Spectroscopy (VSFG), has been employed in combination with tensiometric measurements and AC voltammetry to investigate the formation of organic molecular films. Integral VSFG signal intensity in the spectral range of C-H stretch vibrations has been used to operationally define a surfactant surface coverage parameter, sc, where two model surfactants have been investigated as laboratory reference compounds, namely the phospholipid DPPC (1,2-dipalmitoylphosphatidylcholine) as a monolayer-forming insoluble (dry) surfactant and Triton-X 100 (TX-100) as a soluble (wet) surfactant in bulk-interface adsorption equilibrium. A comparison with natural SML samples from (i) the SURF mesocosm campaign (Wilhelmshaven, Germany, May/June 2023), which investigated the formation of the SML during an pytoplankton bloom in North Sea water, and (ii) a cruise near Helgoland (Germany, North Sea, July/August 2024) targeting the differences of SML characteristics in slick- and non-slick covered areas yielded a consistent picture with regard to the quantification of surfactant surface coverage. A 100 % surface coverage resulting from the complex mixture of biosurfactants in natural SML samples is comparable to the proxy compound monolayers in their liquid-condensed 2D phase state. Finally, by combining available literature data, a first Langmuir-type adsorption correlation between the more common AC voltammetry based surfactant activity measure, c^Teq (in TX-100 equivalents, Teq), and VSFG based surface coverage has been established: sc / sc_max = c^Teq K_L / (1 + c^Teq K_L), with K_L = (4.0 ± 1.0) × 10^-3 L / (µg Teq) and sc_max = (118 ±14) %. First-level estimated global surfactant activity and surfactant surface coverage maps were generated based on satellite-derived primary productivity data, revealing that the surfactant pool in mesotrophic and eutrophic marine environments holds the potential for high surfactant surface coverages. This supports the frequently suggested significant role of biosurfactants in modulating air-sea gas exchange.