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Chemical Reactions at the Air-Water Interface of Aqueous Microdroplets

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Monday, October 3, 2011, 4 pm

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Mike R. Hoffman
California Institute of Technology

Richard Honrath Memorial Lecture

Fundamental processes in chemistry and biology are driven by proton transfer (PT) across water interfaces with hydrophobic media. However, what distinguishes PT 'on water' from conventional PT 'in water' remains unclear. Here we show that PT from gaseous nitric acid to liquid water is dramatically accelerated by non-specific anions. We found that HNO3(g) fails to dissociate on pure water surfaces but is fully deprotonated on 1 mM electrolytes. Quantum mechanical (QM) calculations confirm that HNO3(g) dissociation on pure water is unfavorable and show that anions pre-organize interfacial water, thereby setting the stage for adiabatic PT. Our findings provide direct evidence of the critical role electrostatic pre-organization plays in catalyzing proton transfers across air-water interfaces, such as those involved in cloud- and haze-aerosol chemistry.

We have also shown that gas-phase isoprene is readily protonated and subsequently polymerized by collision with liquid microjets surface of mildly acidic water (1 < pH < 4) within ~10 s. Kinetic isotope effects for the products formation were determined to reach up to 7, showing the direct evidence that the observed phenomena are due to a truly interfacial reactions initiated by proton on water. Since such reactions only occur at superacidic condition in homogeneous bulk media (e.g., pH < -1) or in pure gas phase, the surface of ambient atmospheric aerosol particles will behave as an unusual catalyst for reactive uptake of global gaseous unsaturated hydrocarbons.

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