N₂O₅ at water surfaces: Binding forces, charge separation, energy accommodation and atmospheric implications

Interactions of N₂O₅ with water media are of great importance in atmospheric chemistry and have been the topic of extensive research for over two decades. Nevertheless, many physical and chemical properties of N₂O₅ at the surface or in bulk water are unknown or not microscopically understood. This study presents extensive new results on the physical properties of N₂O₅ in water and at the surface of water, with a focus on their microscopic basis. The main results are obtained using ab initio molecular dynamics and calculations of a potential of mean force. These include: (1) collisions of N₂O₅ with water at 300 K lead to trapping at the surface for at least 20 ps and with 95% probability. (2) During that time, there is no N₂O₅ hydrolysis, evaporation, or entry into the bulk. (3) Charge separation between the NO₂ and NO₃ groups of N₂O₅, fluctuates significantly with time. (4) Energy accommodation of the colliding N₂O₅ at the surface takes place within picoseconds. (5) The binding energy of N₂O₅ to a nanosize amorphous ice particle at 0 K is on the order of 15 kcal mol^-1 for the main surface site. N₂O₅ binding to the cluster is due to one weak hydrogen bond and to interactions between partial charges on the N₂O₅ and on water. (6) The free-energy profile was calculated for transporting N₂O₅ from the gas phase through the interface and into bulk water. The corresponding concentration profile exhibits a propensity for N₂O₅ at the aqueous surface. The free energy barrier for entry from the surface into the bulk was determined to be 1.8 kcal mol^-1. These findings are used to interpret recent experiments. We conclude with implications of this study for atmospheric chemistry.