Air−water interfaces are ubiquitous and central to many essential chemical, biochemical, and environmental processes, ranging from atmospheric reactions on aerosols to ocean acidification and chemical exchanges in the lungs ^{1, 2}. There is now increasing evidence that chemical reactivity at air−water interfaces can be radically different from that in the bulk ^{3, 4}. Some reactions that are slow or unfavorable in the bulk have been observed to proceed much more readily in aqueous microdroplets. This opens up promising green chemistry avenues for catalysis of chemical synthesis. After a series of pioneering experiments, active developments in this rapidly growing field show that the range of reactions accelerated in microdroplets is extremely broad, from the production of hydrogen peroxide ^{5, 6} to the reduction of carbon dioxide and key reactions for the origin of life, such as the prebiotic assembly of peptides and nucleic acids ^{4, 7}. Understanding the unique features of interfacial reactivity is therefore essential for its application to catalysis, but also for improving current climate models whose main source of uncertainty is aerosol reactivity.

However, while some reactions are facilitated in microdroplets compared to the bulk, others are impeded, and a molecular explanation for these contrasting effects remains elusive. The molecular properties of aqueous interfaces are markedly different from those of the bulk liquid due to the abrupt transition from the liquid to the vapor phase. Several explanations have been proposed to explain the observed catalytic effect. The formation of dynamic dangling water OH groups which are not involved in hydrogen bonding was first proposed to explain the catalytic role of the interface, but many other causes can be invoked, including confinement of reagents, partial solvation ⁸, low permittivity of the interfacial water, preferential orientations of solutes and water molecules, curvature in nanodroplets, microdroplet pH ⁹, strong interfacial electric fields ^{10, 11}, and optical confinement ¹². An additional challenge is to bridge the gap between the description of reactivity in model microdroplets and in the complex conditions found in atmospheric aerosols ¹³.

This workshop will bring together theorists and experimentalists to address these issues. Recent developments in molecular simulations now offer a diverse set of techniques to address the complexity of these systems, including ab initio molecular dynamics, mixed quantum-classical simulations, enhanced sampling, reaction transition-path sampling and neural network potentials ^{9, 14}. In parallel, experimental developments in, e.g., X-ray photoelectron spectroscopy ¹⁵, nonlinear sum-frequency generation vibrational spectroscopy³ and electrospray – mass spectroscopy ¹⁶ are now providing an improved characterization of the molecular processes, but the bridge to molecular simulations would need to be strengthened.

We anticipate a fruitful dialogue between physical chemists, atmospheric chemists and theoretical chemists that will allow the identification of critical questions that can be addressed by a combination of approaches to gain a much-needed molecular understanding of interfacial reactivity.