Sodium Drives Interfacial Equilibria for Semi-Soluble Phosphoric and Phosphonic Acids of Model Sea Spray Aerosol Surfaces

Abstract

Organic phosphates and phosphonates represent important yet understudied constituents in our molecular understanding of the ocean. Herein, we determined the critical concentration of sodium relating to the onset of surface activity of alkyl phosphates and phosphonates at the air–water interface to further understand the interfacial environment of sea spray aerosols emitted from the ocean’s surface. A low pH range (1–5.6) was chosen to represent a model system for aged, acidic marine aerosols. The protonation state and sodium binding properties of C16–C18 alkyl phosphoric and phosphonic acids were explored using surface pressure–area isotherms and infrared reflection–absorption spectroscopy. We found that increasing pH and headgroup charge led to significant desorption of these semi-soluble phosphorus-containing acids into bulk solution, while the neutral, fully protonated, and sodium complexed species were favored at the interface. For the phosphonate species, the competition between sodium complexation and protonation reveals a critical sodium chloride concentration of ≥2 M at pH 2 necessary to outcompete the acid–base equilibrium. The onset of this equilibrium shift begins at concentrations as low as 0.1 M NaCl at pH 2, which demonstrates that ion pairing-mediated surface activity is highly relevant in sea spray aerosol systems. We also show that competitive interfacial equilibria between speciation and binding cannot be modeled by known bulk processes for the fully soluble methylphosphonic acid or through theoretical predictions from the Gouy–Chapman model.