Autoionization at the surface of neat water: is the top layer pH neutral, basic, or acidic?

Vácha, Robert; Buch, V.; Milet, Anne; Devlin, J. Paul; Jungwirth, Pavel

doi:10.1039/b704491g

Cited by 158 publications

(206 citation statements)

References 80 publications

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“…In the bulk, both ionic defects can diffuse rather rapidly, on a timescale of 10-100 femtoseconds, consistent with previous reports in the literature (29). However, on segregating to the surface, we find that the mobility of both species is reduced by orders of magnitude.…”

Section: Resultssupporting

confidence: 81%

Point defects at the ice (0001) surface

Watkins

VandeVondele²,

Slater³

2010

Proc. Natl. Acad. Sci. U.S.A.

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Using density functional theory we investigate whether intrinsic defects in ice surface segregate. We predict that hydronium, hydroxide, and the Bjerrum L-and D-defects are all more stable at the surface. However, the energetic cost to create a D-defect at the surface and migrate it into the bulk crystal is smaller than its bulk formation energy. Absolute and relative segregation energies are sensitive to the surface structure of ice, especially the spatial distribution of protons associated with dangling hydrogen bonds. It is found that the basal plane surface of hexagonal ice increases the bulk concentration of Bjerrum defects, strongly favoring D-defects over L-defects. Dangling protons associated with undercoordinated water molecules are preferentially injected into the crystal bulk as Bjerrum D-defects, leading to a surface dipole that attracts hydronium ions. Aside from the disparity in segregation energies for the Bjerrum defects, we find the interactions between defect species to be very finely balanced; surface segregation energies for hydronium and hydroxide species and trapping energies of these ionic species with Bjerrum defects are equal within the accuracy of our calculations. The mobility of the ionic hydronium and hydroxide species is greatly reduced at the surface in comparison to the bulk due to surface sites with high trapping affinities. We suggest that, in pure ice samples, the surface of ice will have an acidic character due to the presence of hydronium ions. This may be important in understanding the reactivity of ice particulates in the upper atmosphere and at the boundary layer.DFT | first-principles | water | interface T he surface of ice acts as a catalyst in the upper atmosphere yielding halogen containing species that facilitate the destruction of ozone (1). More generally, probing the structure/activity relationship of ice with trace gases is important, topical, and yet difficult in the laboratory. Here, we use computational approaches to address the fundamental question of whether charged defects in ice show a tendency to locate at the surface in preference to the crystal interior.

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Section: Resultssupporting

confidence: 81%

Point defects at the ice (0001) surface

Watkins

VandeVondele²,

Slater³

2010

Proc. Natl. Acad. Sci. U.S.A.

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“…Some experimental (9) and theoretical (7,25,26) results were interpreted to signify that the air-water interface is more acidic than bulk water, whereas others reached the opposite conclusion (8,11,21,23,27). The impasse stems in part from the failure to recognize that acidity is a relative concept describing the extent of proton sharing between two conjugate acid/base pairs under specified conditions.…”

mentioning

confidence: 99%

Brønsted basicity of the air–water interface

Mishra

Enami

Nielsen

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

172

203

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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...

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“…At neutral pH, there is the experimental zeta potential evidence for a lower concentration of hydrogen ions than hydroxide ions at the surface. Vácha et al (2008b) has suggested that techniques scrutinizing the water-gas interface from the vapor side can see it as acidic, while those examining it from the aqueous bulk side it will appear as basic. However, there seems little evidence in support as no experimental method has unambiguously shown that the surface is positive but several show it unambiguously as being negative (Hänni-Ciunel et al 2009;Beattie et al 2009).…”

Section: Discussionmentioning

confidence: 99%

“…Although early Monte Carlo studies on isolated clusters indicated that the hydroxonium ion prefers the interior of the clusters (Svanberg and Pettersson, 1998), later studies, including density functional analysis, suggest that they prefer the exterior of the cluster Vácha et al 2007). Note however that such clusters contain mainly surface molecules, are charged particles and not neutral and are acidic with a pH equivalent to less than zero; with hydrogen ion concentration at least three orders of magnitude higher than those present at the lowest isoelectric points proposed.…”

Section: Simulationsmentioning

confidence: 99%

“…In pure water, their calculations show surface enhancement of hydronium but not hydroxide, by one or two orders of magnitude. Vácha et al (2008b) later state that the mechanism for the adsorption of hydroxide ions next to hard attractive walls may not be applicable to the water-air interface. The reasoning behind this conclusion is not clear however.…”

Section: Simulationsmentioning

confidence: 99%

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Untitled

2009

WATER

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SummaryThere is considerable disagreement over whether the gas/liquid surface of water is positive due to the presence of surface-active hydrogen ions or negative due to the presence of surface-active hydroxyl ions. Much has been written and many experimental and simulation studies have been undertaken. We critically analyze these studies to establish what is known unambiguously and what assumptions underlie these opposite views. The conclusion reached after this examination is that there is much misunderstanding over the strength of the evidence for hydrogen ions being surface active and less support for the positive surface than generally regarded. The surface of neutral water is negatively charged.

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Autoionization at the surface of neat water: is the top layer pH neutral, basic, or acidic?

Cited by 158 publications

References 80 publications

Point defects at the ice (0001) surface

Point defects at the ice (0001) surface

Brønsted basicity of the air–water interface

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