Spontaneous formation of urea from carbon dioxide and ammonia in aqueous droplets.

Urea is a key molecule in the search for the origin of life and a basic chemical produced in large quantities by industry. Its formation from ammonia and carbon dioxide requires either high pressures and temperatures or, under milder conditions, catalysts or additional reagents. In this study, we observed the spontaneous formation of urea under ambient conditions from ammonia and carbon dioxide in the surface layer of aqueous droplets. Single, optically trapped droplets were probed by using Raman bands as markers. We found the surface layer to act like a microscopic flow reactor, with chemical gradients providing access to unconventional reaction pathways. This observation revealed a general mechanistic scheme for distinctive droplet chemistry. Interfacial chemistry is a possible nonenergetic route for urea formation under prebiotic conditions. Editor's summary: Urea, one of the simplest organic molecules containing carbon, nitrogen, and oxygen, is a key compound that connects chemistry and life. It is also one of the most important industrial chemicals, mainly used as fertilizer. Urea production usually relies on the direct reaction between ammonia and carbon dioxide under harsh conditions of high temperature and pressure. Using a combination of single-droplet experiments with quantum chemical modelling, Azizbaig Mohajer et al. detected spontaneous urea formation from these chemicals at the gas-liquid interface of aqueous aerosol droplets under ambient conditions (i.e. non-energetic conditions and without additional catalysts) enabled by chemical gradients across the surface of aqueous droplets. These findings have implications for various scientific disciplines. —Yury Suleymanov [ABSTRACT FROM AUTHOR]