Case studies on the role of acidity in droplet chemistry

The influence of acidity on bulk phase chemistry is well established. For chemical reactions that take place in droplets, its influence is less clear and less well studied. For example, the long-debated question of air-water interfacial acidity becomes criticial in droplets due to their extremely high surface-to-volume ratio, while it is negligible in bulk phase reactions. In my presentation, I will discuss specific examples of unexpected chemical reactions in droplets where the acidity is a key factor. This ranges from our recent observation of acid-catalyzed urea formation from carbon dioxide and ammonia in aqueous droplets [1] to challenges related to the distribution of acids and bases between the gas and droplet phase. Our experiments utilize the advantages of single-droplet studies. We combine immobilization of single aerosol droplets in optical traps with sensitive in-trap detection methods, such as Raman and photoacoustic spectroscopy [2].



[1] M. Azizbaig Mohajer, P. Basuri, A. Evdokimov, G. David, D. Zindel, E. Miliordos, R. Signorell, Science, 388, 1426-1430 (2025)

[2] J.W. Cremer, K.M. Thaler, C. Haisch, R. Signorell, Nat. Commun., 7, 10941 (2016)