Hofmeister effects in micromolar electrolyte solutions

Enami, Shinichi; Mishra, Himanshu; Hoffmann, Michael R.; Colussi, A. J.

doi:10.1063/1.4704752

Cited by 48 publications

(75 citation statements)

References 77 publications

(118 reference statements)

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“…We have previously suggested that the closer approach of the larger ions to air-liquid interfaces could be the outcome of the actions of a repulsive (toward the interface) force, f rep , a net attractive force, f atr , due the unbalanced dispersive forces exerted by the whole liquid on interfacial layers, i.e., those accounting for macroscopic liquid cohesion (see Appendix S2 in the supplementary material 67 ), 11,[39][40][41] and the negative entropy of mixing associated with the creation of interfacial concentration gradients. The essential point for present purposes is that anion concentration profiles should peak at different depths, z = 1 − δ, depending on their ionic radii.…”

Section: Resultsmentioning

confidence: 99%

“…We investigate χ for different (X − /Y − ) anion pairs in four solvents as functions of gas velocity, ν G , and third anion (Z − ) additions in the 0.1-100 μM range. 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.…”

Section: Methodsmentioning

confidence: 99%

“…3,[5][6][7][8][9][10] Recent experiments in our laboratory revealed that ions interact specifically at the prototype airwater interface over separations that vastly exceed the range of direct electrostatic forces in any dielectric medium. 11 Such long-range specific ion effects (LR-SIE) may be triggered by electrostatic and electrodynamic forces, but it is obvious that they must be powered by other mechanisms, such as the thermal fluctuations intrinsic to fluid interfaces. Recent simulations have shown that anions at the air-water interface specifically bias the height fluctuations of thermal capillary waves over extended domains.…”

Section: Introductionmentioning

confidence: 99%

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

Enami

Colussi

2013

The Journal of Chemical Physics

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102

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Anions populate fluid interfaces specifically. Here, we report experiments showing that on hydrogenbonded interfaces anions interact specifically over unexpectedly long distances. The composition of binary electrolyte (Na + , X − /Y − ) films was investigated as a function of solvent, film thickness, and third ion additions in free-standing films produced by blowing up drops with a high-speed gas. These films soon fragment into charged sub-micrometer droplets carrying excess anions detectable in situ by online electrospray ionization mass spectrometry. We found that (1) the larger anions are enriched in the thinner (nanoscopic air-liquid-air) films produced at higher gas velocities in all (water, methanol, 2-propanol, and acetonitrile) tested solvents, (2) third ions (beginning at sub-μM levels) specifically perturb X − /Y − ratios in water and methanol but have no effect in acetonitrile or 2-propanol. Thus, among these polar organic liquids (of similar viscosities but much smaller surface tensions and dielectric permittivities than water) only on methanol do anions interact specifically over long, viz.: r i − r j /nm = 150 (c/μM) −1/3 , distances. Our findings point to the extended hydrogenbond networks of water and methanol as likely conduits for such interactions. © 2013 AIP Publishing LLC. [http://dx

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Enami

Colussi

2013

The Journal of Chemical Physics

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102

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“…We have recently shown that different anions populate interfacial layers at depths that are inversely correlated with their relative surface affinities (63). The emerging picture is that surface affinities indicate how close anions approach the interface rather than their relative concentrations within a single subsurfacial layer.…”

Section: Quantum-mechanical Calculationsmentioning

confidence: 99%

Brønsted basicity of the air–water interface

Mishra

Enami

Nielsen

et al. 2012

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

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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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“…Electrospray mass spectrometry also provides evidence for an enhancement of hydronium [75] and a depletion of hydroxide [76] at the air-water interface.…”

Section: Spectroscopymentioning

confidence: 99%

Hydronium and hydroxide at the air–water interface with a continuum solvent model

Duignan

Parsons

Ninham

2015

Chemical Physics Letters

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The distribution of hydronium and hydroxide ions at the air-water interface has been a problem of much interest in recent years. Here we explore what insights can be gained from a continuum solvent model. We extend our model of ionic solvation free energies and surface interaction free energies to include hydronium and hydroxide. The hydronium cation is attracted to the air-water interface, whereas the hydroxide anion is repelled. If the cavity size parameters required by the model are adjusted to reproduce solvation energies, quantitative agreement with experimental surface tensions is achieved. To the best of our knowledge, this is the most accurate theoretical prediction of this property so far. The results indicate that even if 'water structure' is important, its effects can be captured with a relatively simple model. They also contradict the inference from electrophoresis that there is strong hydroxide enhancement at the air-water interface.

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Hofmeister effects in micromolar electrolyte solutions

Cited by 48 publications

References 77 publications

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

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

Brønsted basicity of the air–water interface

Hydronium and hydroxide at the air–water interface with a continuum solvent model

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