Agradecimentos: ANB acknowledges support from FAPESP (the Sao Paulo Research Foundation, Process number 2017/11986-5) and Shell and the strategic importance of the support given by ANP (Brazil's National Oil, Natural Gas, and Biofuels Agency) through the R&D levy regulation, and National Council for Scientific and Technological Development CNPq 401581/2016-0. The authors acknowledge the support for the collaboration from STINT, Sweden, and CAPES, Brazil (grant 9805/2014-01 and 88881.465527/2019-01). O. B. also acknowledges the Swedish Research Council (VR) funding for the VR 2023-04346 project. A. Mocellin also acknowledges funding from the Federal District Research Foundation (FAPDF) for the 410/2022 project. This research used facilities of the Brazilian Synchrotron Light Laboratory (LNLS), part of the Brazilian Center for Research in Energy and Materials (CNPEM), a private non-profit organization under the supervision of the Brazilian Ministry for Science, Technology, and Innovations (MCTI). The PGM beamline staff is acknowledged for their assistance during the experiments Proposals PGM-20170820 and PGM-20180082 Ver menos

Abstract: 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 Ver menos