Reactivity of Volatile Organic Compounds at the Surface of a Water Droplet

Knowledge of the role of water droplets and aerosols in atmospheric chemistry is crucial to significantly improve our understanding of global warming and air quality. Chemistry at the air/water interface, in particular, is still poorly understood. There is a great need to understand how clouds and aerosols process chemistry of organics prevalent in the atmosphere. We report in this study the first computer simulation of a volatile organic compound (formaldehyde) at the air/water interface with explicit description of its ground and excited states electronic properties. We use an elaborated technique that combines molecular dynamics simulations together with a quantum/classical description of the formaldehyde water system. We show that in spite of a large affinity for water, formaldehyde exhibits a preference for the air/water interface with respect to the bulk, roughly by 1.5 kcal/mol. Another important finding in our simulations is that frontier orbitals HOMO and LUMO undergo substantial stabilization at the interface due to surface water reorientation, which induces a local positive electrostatic potential. Such a potential is significantly larger than the one estimated in bulk water suggesting that the reactivity of formaldehyde could change with respect to both gas phase and bulk water. The conclusions presented in this work are expected to help/guide future experiments studying the chemical reactivity of volatile organic compounds at the air/water interface.