Anions dramatically enhance proton transfer through aqueous interfaces

Abstract

Proton transfer (PT) through and across aqueous interfaces is a fundamental process in chemistry and biology. Notwithstanding its importance, it is not generally realized that interfacial PT is quite different from conventional PT in bulk water. Here we show that, in contrast with the behavior of strong nitric acid in aqueous solution, gas-phase HNO_3 does not readily dissociate upon collision with the surface of water unless a few ions (>1 per 10^6 H2O) are present. By applying online electrospray ionization mass spectrometry we monitor in situ the surface of aqueous jets exposed to HNO3_(3(g)) beams and found that NO_3 − production increases dramatically on >30-μM inert electrolyte solutions. We also performed quantum mechanical calculations confirming that HNO3 dissociation on the surface of small water clusters is hindered by a sizable barrier, which is drastically lowered in the presence of an anion. Anions electrostatically assist in drawing the proton away from NO_3 −, which lingers outside the cluster because its incorporation is hampered by the energetic cost of opening a cavity therein. Present results provide both direct experimental evidence and mechanistic insights on the counterintuitive slowness of PT at water- hydrophobe boundaries and its remarkable sensitivity to electrostatic effects.

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