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 dissociate upon collision with the surface of water unless a few ions (>1 per 10 6 H 2 O) are present. By applying online electrospray ionization mass spectrometry to monitor in situ the surface of aqueous jets exposed to HNO 3ðgÞ beams we found that NO 3 − production increases dramatically on >30-μM inert electrolyte solutions. We also performed quantum mechanical calculations confirming that the sizable barrier hindering HNO 3 dissociation on the surface of small water clusters is drastically lowered in the presence of anions. Anions electrostatically assist in drawing the proton away from NO 3 − lingering outside the cluster, whose 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.air-water interface | acid-base | catalysis | nitric acid dissociation P roton transfers (PTs) at water interfaces, such as water boundaries with air (1, 2) or lipid membranes (3), intervene in fundamental phenomena. Arguably the most important PTs are those that take place through and across water boundaries rather than in the bulk liquid. Interfacial PTs participate in the acidification of the ocean (4), the chemistry of atmospheric gases and aerosols (1,5,6), the generation of the electrochemical gradients that drive energy transduction across biomembranes (3,7,8), and in enzymatic function (9, 10) because the activation of neutral species is most generally accomplished via acid-base catalysis (11). Interfacial PT, in contrast with conventional PT in bulk water, depends sensitively on the extent of ion hydration because the density of water in interfacial layers vanishes within 1-nm (12). The acidity of hydronium at the interface, H 3 O þ ðifÞ , is therefore expected to bridge that of H 3 O þ ðgÞ , which protonates most nonalkane species in the gas-phase (13), and H 3 O þ ðaqÞ , which neutralizes only relatively strong bases in solution. Critically controlled by ion hydration in thin yet cohesive interfacial water layers that resist ion penetration, PT "on water" clearly confronts unique constraints. Species that behave as strong acids "in water" may become weak ones on water if dissociation were hindered by kinetic and/or thermodynamic factors in the interfacial region (14, 15).Herein we address these important issues and report the results of experiments in which we monitor the dissociation of gaseous nitric acid HNO 3ðgÞ molecules in collisions with interfacial water, H 2 O ðifÞ , reaction 1 (Eq. 1):[1]The Technique Experiments were conduc...

Brønsted basicity of the air–water interface

“…S3). This contrasts with our observations on the dissociation of the strong HNO 3 at the airwater interface (14). This dissociation of the weak CH 3 COOH on the surface of water is hindered by the intrinsic kinetic barrier limiting this process "in water" [a process previously investigated via Car-Parrinello quantum-mechanical metadynamics (43)], and by the additional cost of creating a cavity to accommodate the resulting CH 3 COO − inside the bulk liquid (14).…”

“…The above observations are conveniently framed in terms of the thermodynamics of proton transfer from the prototypical carboxylic acid CH 3 COOH to X = H 2 O or OH − (14,(39)(40)(41) 3 ] contact ion pairs is sufficient to render R2 exoergic (40). Of course, exoergic proton transfer could nevertheless be hindered by a significant kinetic barrier that would prevent R2 from proceeding fast enough during CH 3 COOH(g) collisions with the surface of water (42,43).…”

“…Despite its importance, the characterization of acid-base chemistry at aqueous interfaces remains fraught with uncertainties (7-11). Basic questions linger about the thickness of interfacial layers (12), how acidity changes through the interfacial region (13), and the mechanistic differences between proton transfer (PT) across interfacial (IF) versus in bulk (B) water (10,14). Because aqueous surfaces are usually charged relative to the bulk liquid (15), the thermodynamic requirement of uniform electrochemical activity throughout (including the interfacial regions) implies that the chemical activity of protons (pH) in IF could be different from that in the B liquid.…”

Long-range specific ion-ion interactions in hydrogen-bonded liquid films

“…11 Detailed descriptions of our experimental setup have been presented before. [20][21][22] Here, we summarize the key events that give rise to our mass spectral signals. Liquid solutions (injected as jets into the spraying chamber of the mass spectrometer) are sheared into primary drops by means of a co-directional highspeed nebulizer gas.…”

Proton Transfer in Nucleobases is Mediated by Water

“…Autoionization in water also drives a variety of processes which are critical to life and biology [12], while PT through nanopores, artificial membranes and structures have major ramifications in energy conversion and storage technologies. PT in nano confined geometries [31] have implications for catalysis and solar energy conversion, while ions have been shown to enhance the transfer of protons through aqueous interfaces [32].…”