Inception of Acetic Acid/Water Cluster Growth in Molecular Beams

The influence of carboxylic acids on water nucleation in the gas phase has been explored in the supersonic expansion of water vapour mixed with acetic acid (AcA) at various concentrations. The sodium-doping method has been used to detect clusters produced in supersonic expansions by using UV photoionisation. The mass spectra obtained at lower acid concentrations show well-detected Na⁺-AcA(H2O)n and Na⁺-AcA2(H2O)n clusters up to 200 Da and, in the best cooling expansions, emerging Na⁺-AcAm(H2O)n signals at higher masses and unresolved signals that extend beyond m/e values >1000 Da. These signals, which increase with increasing acid content in water vapour, are an indication that the cluster growth taking place arises from mixed water-acid clusters. Theoretical calculations show that small acid-water clusters are stable and their formation is even thermodynamically favoured with respect to pure water clusters, especially at lower temperatures. These findings suggest that acetic acid may play a significant role as a pre-nucleation embryo in the formation of aerosols in wet environments.