Bubbles bursting at the ocean surface are one of the largest sources of atmospheric aerosols. These marine aerosols are comprised of common ocean ions of Na, K, Ca, Mg and Cl. Additionally, organic matter from the sea-surface microlayer (SSML) forms a surfactant coating on the surface of the aerosols, which impacts aerosol reactivity, volatility, and gas exchange. Divalent cations Ca2+ and Mg2+ are known to aggregate at the air-water interface by binding to the organic surfactants. This binding then affects the surface coating in different ion-specific ways. Ca2+ and Mg2+ are known to have different binding affinities to various moieties such as phosphate and carbonyl groups, but the specificity of preference to different groups is still under debate. Here binding affinities of Ca2+ and Mg2+ to the phosphate headgroup of di-palmitoylphosphatidic acid (DPPA) are investigated via surface tensiometry and infrared reflection-absorption spectroscopy (IRRAS). Surface pressure isotherms reveal that in solutions containing both cations, Mg2+ preferentially adsorbs to the phosphate headgroup compared to Ca2+. However, when the relative concentrations of Ca2+ to Mg2+ equals or exceeds a 2:1 ratio, Ca2+ becomes the predominant adsorbed cation. IRRAS spectra probe changes to the asymmetric and symmetric PO2- stretches. These vibrational modes are sensitive to changes in conformation and hydration state and can reveal the mechanism of ion binding.

Influence of Ca2+ vs Mg2+ binding to negatively charged phosphatidic acid monolayers studied by infrared reflection-absorption spectroscopy and surface tensiometry

251st National Meeting (2016)

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