Dissolved organic phosphorus species, such as alkyl phosphates and phosphonates, provide an important source of nutrition for oceanic phytoplankton, significantly contributing to marine biogeochemistry. Yet, their molecular interactions in the ocean are understudied. The surface activity and sodium binding properties were determined for C16-C18 alkyl phosphoric and phosphonic surfactants at the air-water interface using organized Langmuir monolayer studies to model a proxy aged sea spray aerosol emitted from the ocean’s surface through wave-breaking processes. Ascending aerosols yield increasingly acidic environments due to bulk desorption, therefore studies were conducted in the low pH regime (1 to 5.6). Surface pressure-area isotherms, equilibrium spreading pressure, Brewster angle microscopy, and infrared reflection-absorption spectroscopy were used to elucidate the difference in phosphoric and phosphonic headgroup interactions as a function of acid-base speciation, sodium ion binding, and the resulting competition of the two equilibria. The results from these surface-sensitive microscopy and spectroscopy techniques show that increasing pH and headgroup charge leads to significant desorption into the bulk solution while both the neutral, fully protonated and sodium complexed species are favored at the air-water interface. Additional studies with phosphonic surfactants on the acid-base and sodium binding competition were done showing a critical sodium concentration &gt;2 M NaCl at pH 2 which shows outcompeting the acid-base equilibria, favoring sodium binding at the surface. Considerably low concentrations of sodium ions can therefore lead to charge neutral stabilized monolayers. These competitive interactions need to be considered as we begin to unravel the molecular complexity of sea spray aerosol surfaces, especially in aged environments.

Competitive binding interactions for phosphoric and phosphonic acids in proxy aged aerosols

ACS Spring Meeting 2021

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