The surface partitioning of a medium chain fatty acid and its conjugate base has been investigated through a combined experimental and theoretical approach of the multi-equilibria involved in the bulk phase and at the air/water interface.

Amino acids (AAs), the building blocks of proteins, are enriched by several orders of magnitude in sea spray aerosols compared to ocean waters. This suggests that AAs may reside at the air–water interface and be highly surface active. Using surface tension measurements, infrared reflection–absorption spectroscopy, and molecular dynamics simulations, we show that AAs are surface active and that salts and low-pH environments are drivers of surface activity. At typical sea spray salt concentrations and pH values, we determine that the surface coverage of hydrophobic AAs increases by approximately 1 order of magnitude. Additionally, divalent cations such as Ca2+ and Mg2+ can further increase AA surface propensity, particularly at neutral pH. Overall, these results indicate that AAs are likely to be found at increased concentrations at the surface of sea spray aerosols, where they can impact the cloud activation properties of the aerosol and enhance peptide formation under certain conditions.

The acidity of atmospheric aerosols controls their impacts on heterogeneous and multiphase reactions, cloud formation, and human health. Recently, it has been shown that multiphase buffering can shift aerosol pH...