Reactions of Interfacial Electrons Initiated by Collisions of Sodium Atoms with Water Microjets in Vacuum

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Many aspects of our natural world and industrial society rely on reactions that occur in water. Probably the most reactive species in water is the electron itself, surrounded by water molecules. It is called the "solvated electron" and it has a fleeting existence, lasting only 1/1000 of a second in water. During this short time, a solvated electron buried deep inside water can react with molecules of all types, reaching 1600 recorded reactions so far. Much less is known about solvated electrons at the surface of water, where it might be even more reactive. With support from the Chemical Structure, Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Gilbert Nathanson of the University of Wisconsin-Madison is exploring reactions of solvated electrons to learn how its bulk and surface reactivity differ. Working with his students, Professor Nathanson scatters a beam of sodium atoms off a narrow jet of water to create the solvated electron, which then reacts with organic molecules at the water surface. The group's discoveries could provide unusual routes to destroy toxic waste that floats water surfaces, resulting in important societal benefits. The project is also training graduate and undergraduate students to become creative, critical, and independent scientists. Outreach efforts at local science festivals, elementary schools, and senior centers stimulate excitement and curiosity through hands-on surface chemistry experiments for all ages.As a powerful nucleophile and reductant, the solvated electron often breaks bonds and creates reactive free radicals at near diffusion-limited rates. This research is directed at investigating how solvated electrons react at the water-vacuum interface using gas-liquid scattering experiments coupled with water microjets and mass spectroscopy. Among the uniquely interfacial routes that are being explored are the escape of reaction intermediates into the gas phase and reactions of solvated electrons with functional groups on oriented surfactant molecules. Interfacial solvated electrons are generated by collisions of sodium atoms with a water microjet. These atoms ionize within the first few monolayers, where they react with surface-active molecules. As one example, organic ammonium cations composed of a benzyl group and three alkyl chains form monolayers at the surface of water. Solvated electrons can break the benzyl-nitrogen bond, releasing the benzyl radical and neutral amine into the interfacial region. This reaction is monitored by evaporation of the radical and amine as a function of surfactant packing, alkyl chain length, and counterion identity to learn how solvated electrons react in the abruptly changing surfactant-water environment. These studies reveal the competition for surface and bulk reaction sites as solvated electrons are released sodium atom ionization within or below the surfactant layer. Parallel studies are exploring a negatively charged monolayer of benzoate anions and a monolayer of the alkylated amino acid cysteine, a zwitterion that releases hydrogen sulfide following solvated electron attack.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.