Cooperative and competitive effects in pH-dependent surface composition of atmospherically relevant organic ions in...

The molecular surface compositions of aerosols can differ considerably from their bulk counterparts, an aspect often overlooked in climate models. This oversight can potentially affect our understanding of the complex interplay between composition, optical characteristics, and their influence on cloud formation and properties. A substantial portion of aerosol mass often includes organic compounds, such as carboxylic acids and alkyl amines. These organic compounds are surface-active and influence surface tension, an important aspect of cloud droplet activation. To better understand the impact of complex organic mixtures on aerosol surfaces, we report a liquid-jet X-ray photoelectron spectroscopy (XPS) study exploring the pH-dependent surface composition of aqueous solutions of butyric acid and butylamine, both isolated and mixed. Our findings reveal that the surface composition is highly influenced by the ratio between these solutes and their total surface concentration. Around pH 7, where both solutes are charged, the mixed solution demonstrates cooperative surface behavior, leading to an increased presence of organics at the surface. At extreme pH values, where one solute is charged and the other remains neutral, the solutes act independently, with the neutral species dominating the surface enrichment. We also discuss the molecular mechanisms driving these interactions and their broader implications for aerosol behavior in climate models.