Brønsted basicity of the air–water interface

Abstract

Differences in the extent of protonation of functional groups lying on either side of water-hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol-gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H 2 O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air-water interface predict that such event is hindered by a significant kinetic barrier unless OH − ions are present therein. Thus, by detecting RCOO − we demonstrate the presence of OH − on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me) 3 N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH − densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH − gradients in the interfacial region. This fact could account for the weak OH − signals detected by surface-specific spectroscopies.gas-liquid reactions | surface potential | water surface acidity | interfacial proton transfer A cid-base chemistry at aqueous interfaces lies at the heart of major processes in chemistry and biology. Changes in the degree of dissociation of the acidic/basic residues upon translocation between aqueous and hydrophobic microenvironments orchestrate enzyme catalysis (1), drive proton/electron transport across biomembranes (2, 3), and mediate molecular recognition and self-assembly phenomena (4-6). 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. Reduced hydration of ionic species at the interface could force acids and bases toward their undissociated forms (16).These fundamental issues have been extensively investigated via electrostatic (17) and electrokinetic experiments (11), surface tension studies and analysis (1...

Tropospheric aerosol as a reactive intermediate

“…A full description of the instrument and its operation in similar experiments can be found in previous publications from our laboratory. [41][42][43]47 Specic details are provided as Supplementary Information (ESI), † which includes a diagram of the reaction zone (Fig. S1 †).…”

“…The objective of this paper is to present an explicit chemical mechanism for R1, and show that its operation on wet SOA accounts for a suite of apparently unrelated, hitherto unexplained observations. The paper is organized as follows: we (1) report new laboratory results obtained by a novel technique developed in our laboratory [41][42][43][44] on the catalysis by dicarboxylic acid anions of the hydrolytic disproportionation of NO 2 (g) on aqueous surfaces (reaction R1) [45][46][47] as well as supporting quantum mechanical calculations and (2) show that such anions are normally produced in urban SOA from the photochemical oxidation of volatile organic compounds (VOCs). 48 We then review and summarize pertinent information on aerosol optical depth time series data over megacities, 49,50 and solar photolysis frequencies J(NO 2 ), J(O 1 D) and J(HONO) as functions of season and latitude.…”

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.…”

“…Solvent evaporation is minimal prior to film breakup, as evidence by the fact that the titration curves of carboxylic acids and trimethylammonium determined in this setup were identical with the ionization constants reported in the literature (i.e., pK a ∼ 4.8 and 9.8, respectively). 21,22,33 Droplets eventually become Rayleigh-unstable and undergo a cascade of Coulomb explosions whose outcome is the ejection of bare single ions to the gas-phase. 34,35 Note that Coulomb explosions, in contrast with the aerodynamic breakup of primary droplets described above, arise from repulsion among like charges and therefore preserve the overall net charges of the initial ensemble of negatively charged secondary droplets.…”

Ion-Specific Long-Range Correlations on Interfacial Water Driven by Hydrogen Bond Fluctuations

“…6,7 The fact that the titration curves of carboxylic acids and trimethylammonium determined in this setup are identical with the ionization constants reported in the literature (i.e., pK a ∼ 4.8 and ∼ 9.8, respectively) suggests that solvent evaporation is minimal prior to droplet breakup. [8][9][10] Sub-micrometer-sized droplets eventually become Rayleigh-unstable and undergo a cascade of Coulomb explosions whose outcome is the ejection of bare single ions to the gas-phase. 6,7 Note that Coulomb explosions, in contrast with the aerodynamic breakup of primary droplets described above, arise from repulsion among like-charges and therefore preserve the overall charges of the initial ensemble of (positively and) negatively charged secondary droplets.…”