201200949 10228..10232 - 10228.full.pdf

Anions dramatically enhance proton transfer

through aqueous interfaces

Himanshu Mishra

a,b,c,1

, Shinichi Enami

a,d,1

, Robert J. Nielsen

, Michael R. Hoffmann

, William A. Goddard III

b,c

, and

Agustín J. Colussi

a,2

Ronald and Maxine Linde Center for Global Environmental Science,

Materials Science, and

Materials and Process Simulation Center, California Institute

of Technology, Pasadena, CA 91125; and

The Hakubi Center, Kyoto University, Kyoto 606-8302, Japan

Edited by Richard J. Saykally, University of California, Berkeley, CA, and approved May 1, 2012 (received for review January 17, 2012)

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

does not dissociate upon collision with

the surface of water unless a few ions (

>

1 per 10

O) are present.

By applying online electrospray ionization mass spectrometry to

monitor in situ the surface of aqueous jets exposed to HNO

beams we found that NO

−

production increases dramatically on

>

μ

M inert electrolyte solutions. We also performed quantum

mechanical calculations confirming that the sizable barrier hinder-

ing HNO

dissociation on the surface of small water clusters is dras-

tically lowered in the presence of anions. Anions electrostatically

assist in drawing the proton away from NO

−

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 counterin-

tuitive slowness of PT at water-hydrophobe boundaries and its

remarkable sensitivity to electrostatic effects.

air

–

water interface

∣

acid-base

∣

catalysis

∣

nitric acid dissociation

roton transfers (PTs) at water interfaces, such as water bound-

aries 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 acidifi-

cation 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

, is there-

fore expected to bridge that of H

3

, which protonates most

nonalkane species in the gas-phase (13), and H

3

, which

neutralizes only relatively strong bases in solution. Critically con-

trolled 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 re-

sults of experiments in which we monitor the dissociation of gas-

eous nitric acid HNO

3

molecules in collisions with interfacial

water, H

2

, reaction 1 (Eq.

HNO

3

2

→

3

−

3

[1]

The Technique

Experiments were conducted by intersecting continuously re-

freshed surfaces of free-flowing aqueous jets with HNO

3

∕

2

beams at ambient temperature and pressure. The formation of

interfacial nitrate, NO

3

−

, was monitored in situ via surface-

specific online electrospray mass spectrometry (ESMS) (16, 17)

(

SI Text

and

Figs. S1

and

). ESMS is routinely used to analyze

the composition of bulk liquids. However, we have demonstrated

that by changing the instrumental configuration and operating

parameters it is possible to sample the interfacial layers of

the liquid jet. We have previously taken advantage of the high

sensitivity, surface selectivity, and unequivocal identification cap-

abilities of our modified electrospray mass spectrometer to inves-

tigate fast gas

–

liquid reactions on the surface of aqueous jets (5,

18). The claim that the mass spectra obtained in our instrument

mostly reflect the ion composition of the outermost layers of the

jet has been validated by showing that: (

) the relative anion abun-

dances (i.e., the relative mass spectral signal intensities) mea-

sured on jets consisting of equimolar solutions of mixed salts are

not identical but follow a normal Hofmeister series (as expected

at the air

–

water interface and confirmed by other surface-sensi-

tive techniques), and are specifically affected by surfactants

(cationic or anionic) (19); and (

) mass spectra of jets exposed

to reactive gases reveal the presence of species necessarily pro-

duced at the interface rather than in the bulk liquid (20, 21).

Mass spectrometers report the net charge that arrives at the

detector per unit time. Therefore, the NO

3

−

produced in

reaction 1 on the surface of the electroneutral liquid jet can be

detected after it has been separated from H

3

counterions.

Separation is brought about during the pneumatic breakup of the

liquid jet by a fast annular N

2

nebulizer gas flow, which shears

the outermost liquid layers into droplets carrying net charges

of either sign. These droplets have size and net charge distribu-

tions, and carry more surface and electrostatic energies than the

original jet, at the cost of the kinetic energy lost by the nebulizer

gas. A key feature of our ESMS instrumental configuration is

that the jet is orthogonal to the inlet to the mass spectrometer

(

Figs. S1

and

). This geometry overwhelmingly favors the

detection of ions emanating from the peripheral layers of the jet.

Ions are ultimately ejected to the gas-phase because of severe

charge crowding in the nanodroplets that result from extensive

solvent evaporation (22). We have presented detailed data

analysis (16), based on mass balances and the application of the

kinetic theory of gases to fast gas

–

liquid reactions, which suggests

Author contributions: A.J.C. designed research; H.M., S.E., and R.J.N. performed research;

M.R.H. contributed new reagents/analytic tools; H.M., S.E., R.J.N., M.R.H. and W.A.G.

edited paper; M.R.H. provided financial support; H.M., S.E., R.J.N., W.A.G., and A.J.C.

analyzed data; and A.J.C. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

H.M. and S.E. contributed equally to this work.

To whom correspondence should be addressed. E-mail: ajcoluss@caltech.edu.

This article contains supporting information online at

www.pnas.org/lookup/suppl/

doi:10.1073/pnas.1200949109/-/DCSupplemental

10228

–

10232

∣

PNAS

∣

June 26, 2012

∣

vol. 109

∣

no. 26

www.pnas.org/cgi/doi/10.1073/pnas.1200949109

that the thickness of the interfacial layers sampled in these experi-

ments is certainly within a few nm, and most likely approximately

1 nm (see below and

SI Text

Results

Fig. 1 displays mass spectral NO

3

−

(

∕

62

) signal intensities,

62

, as a function of pH (of the bulk aqueous solution) on liquid

jets exposed to HNO

3

62

remains above detection limits on

the surface of pH 4.5 to 9.5 jets, but sharply increases both on

more basic and more acidic solutions to limiting values, I

62

max

above pH 11 and below pH 3. Notably, we found that I

62

max

values are uniformly reached at all pH values on

1

-mM NaCl

jets. The previously reported uptake coefficient of HNO

3

deionized water (

γ

0

1

) (23), reveals that only a small fraction

of the HNO

3

molecules colliding with the surface of water are

incorporated into the bulk liquid, where they fully dissociate

[pK

HNO

3

Þ¼

−

1

4

]. Therefore, the small NO

3

−

signals de-

tected in our experiments on pure water jets indicate that we do

sample their outermost interfacial layers (24, 25), and confirms

that most of the mass-accommodated HNO

3

diffuses in undisso-

ciated form through such layers. The fact that the production of

3

−

is dramatically enhanced by inert anions on water hints

at the possibility that the barrier preventing HNO

3

dissociation

at the interface might be kinetic rather than thermodynamic (26,

27). In summary, the results of Fig. 1 and

Fig. S3

provide evidence

that HNO

3

behaves as a weak acid on the surface of water, and

extrinsic inert ions can significantly catalyze HNO

3

dissociation

therein (15, 28).

The air

–

water interface of electrolyte solutions is preferen-

tially populated by anions. This is borne out by the negative

surface potential of most electrolyte solutions (29), by surface-

specific spectroscopic studies (30

–

33), and by theoretical predic-

tions. The adsorption of ions to the surface was surmised long ago

from the surface tension minima observed in electrolyte solutions

at approximately 1 mM. They were accounted for by electrostatic

interactions among ions that saturate the surface of water at ap-

proximately 1 mM (34)

—

i.e., in the concentration range in which

we observe an increase of HNO

3

dissociation on water (30, 33).

The saturation dependence of NO

3

−

production on electrolyte

concentration (

Fig. S3

) can be ascribed to catalysis by anions

−

adsorbed to identical, noninteracting sites of the air

–

water

interface

—

i.e., I

62

62

;

max

−

∕

1

∕

2

þ½

−

(30). We derive

1

∕

2

128

M (NaCl) and K

1

∕

2

77

M (MgSO

4

) (i.e., the

concentrations at which the interface would be half-saturated

with catalyzing anions), which are commensurate with the values

(

NaCl

max

400

M and

MgSO

4

max

200

M) deduced from

SHG experiments (30) (

Fig. S3

). Although neither Cl

−

nor

4

2

−

are as surface active as I

−

or ClO

4

−

(32), they should ap-

proach the air

–

water interface far closer than the (

ion

–

ion

) se-

parations prevalent at the onset of catalytic effects (see below).

Hydronium, H

3

, the counterpart of NO

3

−

in reaction

1, was tracked by using hexanoic acid (PCOOH) as a proton

scavenger. PCOOH is both a weak acid and a weak base in water:

PCOOH

4

8

,pK

PCOOH

2

−

3

. However,

we have shown that PCOOH is protonated on the surface of

mildly acidic water, where it behaves as a stronger base:

PCOOH

2

2

5

(17). Fig. 2 displays I

117

(PCOOH

2

118

(PCOOHD

), and I

119

(PCOOD

2

) signal intensities

from 1 mM PCOOH in

1

∶

1

∕

2

∶

2

O jets (initially at pH 7)

as functions of gas-phase HNO

3

or DNO

3

concentrations.

Fig. 2 (

Inset

) shows the corresponding I

62

and I

115

(PCOO

−

) sig-

nal intensities versus HNO

3

concentration. It is apparent that:

(

) PCOO

−

is promptly neutralized upon exposure to the lowest

HNO

3

∕

DNO

3

concentrations, whereas (

) the protonation/

deuteration (hydronation) of the weaker base PCOOH requires

exposure to at least

2

×

10

12

molecules cm

−

3

. The fact that

HNO

3

dissociates on water containing the anions of either a

stronger acid [pK

HCl

−

7

versus pK

HNO

3

−

1

4

] or a weaker one [pK

PCOOH

4

8

] supports the

assertion that anions function as catalysts rather than proton ac-

ceptors. The appearance of hydronated species [(PCOOH

2

(PCOOHD

), and (PCOOD

2

)] reveals that the surface of

the jet has been acidified (from pH 7) to pH <

2

5

. Because this

is achieved under conditions in which the number of hydrons

Fig. 1.

Electrospray mass spectral nitrate signal intensities (I

62

) detected on

water or 1-mM NaCl microjets exposed to

3

×

10

12

molecules cm

−

3

of gas-

eous nitric acid for approximately 10

s as functions of pH. Solid, dashed lines

are linear regression and 95% confidence limits, respectively, to the data on

1-mM NaCl. Error bars estimated from reproducibility tests. All experiments

under 1 atm of N

2

at 293 K.

Fig. 2.

Electrospray mass spectral signal intensities of protonated iso-

topologues of hexanoic acid (PCOOH):

∕

117

PCOOH

2

∕

118

PCOOHD

, and

∕

119

PCOOD

2

, detected on 1-mM PCOOH

solutions in

1

∶

1

∕

2

∶

2

O microjets (initially at pH 7) exposed to variable

concentrations of gaseous HNO

3

(

)orDNO

3

(

). The inflection point

corresponds to pK

PCOOH

2

2

5

(17).

Inset

shows the evolution of

the PCOO

−

(

∕

115

) and NO

3

−

(

∕

62

) signals detected in negative

ion mode. All experiments under 1 atm of N

2

at 293 K.

Mishra et al.

PNAS

∣

June 26, 2012

∣

vol. 109

∣

no. 26

∣

10229

CHEMISTRY

delivered by HNO

3

∕

DNO

3

on interfacial layers is much

(approximately

10

3

times) smaller than those carried by the

50

L min

−

1

1

∶

1

∕

2

∶

2

O aqueous jet, the former must be

confined to thin (

) interfacial layers during the lifetime of

the jet. The relative abundances of the PCOOH

2

, PCOOHD

and PCOOD

2

isotopologues are appreciably different under

HNO

3

or DNO

3

(Fig. 2

and

) and corroborate that the

hydrons delivered by gaseous nitric acid remain (i.e., do not

diffuse into and rapidly scramble their isotopic labels with the

bulk solvent) in the interfacial layers sampled herein (

SI Text

and

Fig. S4

). The assumption that our experiments probe reactive

events taking place in interfacial layers of molecular depth is

therefore based on substantial evidence. From the frequency of

HNO

3

collisions with the jet given by the kinetic theory of

gases, we estimate that

is approximately

1

×

10

−

7

cm (17)

(

SI Text

What is the minimum number of additional water molecules

that renders reaction 1 exoergic? The free energy required

to produce a hydrated contact ion pair at the air

–

water interface,

0

1

, can be estimated as the sum of the gas-phase process

[

0

2

HNO

3

2

→

3

−

3

Þ¼

160

kcal mol

−

1

]

(13); plus the electrostatic energy released as the infinitely distant

gas-phase point charges reach an approximately 3.3-Å separation

in the contact ion pair (E

−

100

kcal mol

−

1

); plus the free

energy of hydrating H

3

[

0

3

3

2

→

2

O·H

3

]

0

1

0

2

0

3

60

kcal mol

−

1

0

3

Extant thermochemical data on (

2

O·H

3

) clusters (35) show

that

0

3

≥

4

−

60

kcal mol

−

1

(i.e., reaction 1 is thermo-

dynamically allowed for

≥

4

,evenifNO

3

−

were not hydrated

at all) (36). The hydration of NO

3

−

will, of course, contribute

to the exoergicity of reaction 1. Because HNO

3

is able to interact

with at least four water molecules upon impact with the surface of

water (28), the nature of the barrier-hindering reaction 1 remains

to be elucidated. It has been proposed that acid-base equilibria at

the air

–

water interface are shifted (relative to bulk water) toward

neutral species by approximately

2

units (37). In the case of

nitric acid, pK

HNO

3

Þ¼

−

1

4

, this proposal makes HNO

3

a strong acid at the interface: pK

HNO

3

of approximately 0,

at variance with our observations. We wish to emphasize that in

our experiments, in contrast with most other studies (38), HNO

3

approaches the air

–

water interface from the vapor instead of the

water side. Hence, gas-phase ion thermochemistry (13, 35) is a

more appropriate framework for analyzing our results.

Against this background, we performed density functional

theory calculations on HNO

3

interacting with water decamers

10

≡

2

in the absence and presence of Cl

−

to ascertain

the molecular basis of our experimental observations. Fig. 3

and

display the calculated Gibbs free energy (

0

) and enthal-

py (

0

) profiles at 300 K. We confirmed that HNO

3

embedded

in W

10

clusters dissociates spontaneously, in accordance with

common knowledge, thermodynamics, and Car

–

Parrinello

molecular dynamics (CPMD) calculations (14, 15). In contrast,

HNO

3

binds as a molecule to the periphery of W

10

via two

hydrogen bonds with

0

−

13

0

kcal mol

−

1

and (because of

translational and rotational HNO

3

entropy losses)

0

−

1

2

kcal mol

−

1

. The free energy barrier for transferring a proton

from adsorbed HNO

3

into the cluster while leaving a NO

3

−

on its

surface is quite large:

‡

14

1

kcal mol

−

1

,or

12

9

kcal mol

−

1

above the reactants. Weakly bound undissociated HNO

3

therefore rather stable toward dissociation and highly mobile on

the surface of water. Remarkably, HNO

3

not only binds more

strongly (

0

−

18

6

kcal mol

−

1

0

−

6

9

kcal mol

−

1

)to

clusters containing a Cl

−

, but the free energy barrier for transfer-

ring a proton to W

10

·Cl

−

is dramatically reduced:

‡

1

2

kcal mol

−

1

Discussion

Calculations provide significant clues about the origin of the

barrier to HNO

3

dissociation on water. HNO

3

binds to W

10

both

as H-bond donor and acceptor. However, the NO

3

−

–

proton,

an intrinsic water ion, cannot readily slip into cluster leaving

3

−

behind (Fig. 3

). The barrier to PT on the surface of

water is therefore associated with the fact that (

) overcoming

the electrostatic attraction in a disjoint (NO

3

−

–

) ion pair,

or (

) opening a cavity for NO

3

−

to follow after the proton into

the cluster, entails significant energy costs. Calculations involving

larger water clusters do not eliminate such barrier (28). Clearly, a

chloride lets H

3

advance further into the cluster primarily by

countering the electrostatic bias imposed on H

3

by laggardly

3

−

, rather than binding to it [recall that pK

HCl

−

7

We also noticed that the atomic rearrangements involved in bind-

ing HNO

3

to W

10

clusters are uncorrelated to those required for

subsequent PT. In contrast, the stronger interaction between

HNO

3

and

Cl·W

10

−

clusters primes

Cl·W

10

⋯

HNO

3

−

for

PT. The reaction coordinate for PT on pure water is a combina-

tion of six internal modes involving displacements of heavy O

atoms, whereas PT in the presence of chloride proceeds adiaba-

tically along a three-link proton wire between quasi-degenerate

solvent states (

Fig. S5

and

After establishing the role of electrostatics in the catalysis

of HNO

3

dissociation on small water clusters, we need to under-

stand why catalytic effects are observed on

30

M electrolytes

—

Fig. 3.

Calculated Gibbs free energies (

0

) and enthalpies (

0

) of reac-

tants, adducts, transition states, and products of optimized water clusters in

contact with nitric acid in the absence (

) and presence (

) of interfacial

chloride. Proton wires highlighted. Energies in kcal mol

−

1

10230

∣

www.pnas.org/cgi/doi/10.1073/pnas.1200949109

Mishra et al.

i.e., at (

ion

–

ion

120

-nm interfacial separations (

SI Text

) that

vastly exceed the size of such clusters. On the basis of our calcu-

lations we envision that HNO

3

, after alighting on water, roams

rather freely over its surface as HNO

3

until it approaches an

interfacial Cl

−

, whereupon it falls into a deeper potential well

and undergoes prompt dissociation. In

SI Text

we estimate that

average number of hops required by HNO

3

to reach a Cl

−

on the surface of

30

M solutions would take a few nanoseconds

(i.e., competitively with back desorption into the gas-phase) (16,

39). Recapitulating, present experimental results substantiate a

key role for electrostatics in the mechanism of HNO

3

dissociation

at water-hydrophobe interfaces, and suggest that even sparse an-

ions can effectively catalyze this process.

Implications

Our finding that PT across water-hydrophobic media interfaces

is catalyzed by anions has important implications in many fields.

Whether HNO

3

dissociates on aqueous surfaces, for example,

bears on various environmental issues. Whereas NO

3

−

is a sink

for active nitrogen in the atmosphere because it can be removed

by dry and wet deposition, undissociated HNO

3

may react via:

2

HNO

3

→

3

2

2

, thereby sustaining the

atmospheric impact of nitrogen oxides (40). Adsorption of

HNO

3

on ice also depends critically on whether HNO

3

dissoci-

ates therein

—

i.e., whether coverage is a function of

1

∕

2

(

≡

HNO

3

partial pressure) (41). Our results suggest that

HNO

3

will dissociate upon impact on most environmental

aqueous surfaces, including premelted films on ice that contain

electrolytes impurities at least at millimolar levels. Our demon-

stration that PT across internal water-hydrophobe interfaces is

facilitated by electrostatics related to the concept of anion-

mediated water bridges for PT in proteins (42) is at least consis-

tent with the assumption that charge transfer events at water

–

protein interfaces are driven by electrostatic preorganization

(43, 44). It also accounts for the fact that even weakly basic,

mobile anions, such as chloride, may enhance proton motion

along membrane surfaces without providing localized proton-

binding sites (45, 46).

Experimental Methods

In our experiments, continuously refreshed, uncontaminated sur-

faces of free-flowing aqueous microjets exposed to <

8

×

10

12

HNO

3

molecules cm

−

3

for approximately 10

s are monitored

by online negative or positive ion ESMS. Fifty

L min

−

1

of deio-

nized water or aqueous electrolyte solutions (pH-adjusted using

concentrated NaOH or HCl) are injected as a microjet into

the spraying chamber of an ES mass spectrometer held at 1 atm,

293 K via an electrically grounded, stainless steel pneumatic noz-

zle (100

m) internal diameter (

SI Text

and

Figs. S1

and

) (47).

A high-speed (approximately

300

−

1

) annular nebulizer N

2

flow tears up the much slower (

11

cms

−

1

) microjet into droplets

charged with ion excesses of either sign. Ions are eventually

ejected to the gas-phase, charge selected by a polarized inlet

port orthogonal to the nozzle, and detected by mass spectrome-

try. We note that the velocity at which the liquid jet emerges from

the nozzle is approximately 500 times slower than that required

for observing electrokinetic effects in our experiments (48)

(

SI Text

Computational Methods

Gibbs free energies (

G) at 298 K were computed from calcu-

lated enthalpies (

H) and entropies (

) according to

elec

ZPE

vib

−

vib

. Geometries of energy minima and

transition states were optimized using the X3LYP functional (ex-

tended hybrid functional combined with Lee

–

Yang

–

Parr correla-

tion functional) (49), the 6-31G** basis for light atoms (50),

and 6-311G**++ for Cl

−

(51). Hessians at these geometries pro-

vided harmonic zero-point energies, vibrational enthalpies, and

entropies. Neglect of anharmonicity effects (<

1

kcal mol

−

1

) may

not affect the main conclusions. After geometry optimization, the

electronic energy E

elec

was evaluated with the 6-311G**++ basis

on all atoms. The free energies of nitric acid and nitrate at 1 atm

were calculated using statistical mechanics for ideal gases

(

SI Text

ACKNOWLEDGMENTS.

S.E. thanks the Japan Society for the Promotion of

Sciences Postdoctoral Fellowship for Research Abroad. Research supported

by National Science Foundation Grant AGS-964842 (to M.R.H.).

1. Ravishankara AR (1997) Heterogeneous and multiphase chemistry in the troposphere.

Science

276:1058

–

1065.

2. Enami S, Mishra H, Hoffmann MR, Colussi AJ (2012) Protonation and oligomerization

of gaseous isoprene on mildly acidic surfaces: Implications for atmospheric chemistry.

J Phys Chem A

, doi: 10.1021/jp2110133.

3. Mitchell P (1961) Coupling of phosphorylation to electron and hydrogen transfer by a

chemi-osmotic type of mechanism.

Nature

191:144

–

150.

4. Hoegh-Guldberg O, et al. (2007) Coral reefs under rapid climate change and ocean

acidification.

Science

318:1737

–

1742.

5. Enami S, Hoffmann MR, Colussi AJ (2008) Acidity enhances the formation of a persis-

tent ozonide at aqueous ascorbate/ozone gas interfaces.

Proc Natl Acad Sci USA

105:7365

–

7369.

6. Kinugawa T, et al. (2011) Conversion of gaseous nitrogen dioxide to nitrate and nitrite

on aqueous surfactants.

Phys Chem Chem Phys

13:5144

–

5149.

7. Sanden T, Salomonsson L, Brzezinski P, Widengren J (2010) Surface-coupled proton

exchange of a membrane-bound proton acceptor.

Proc Natl Acad Sci USA

107:

4129

–

4134.

8. Lane N, Allen JF, Martin W (2010) How did LUCA make a living? Chemiosmosis in the

origin of life.

Bioessays

32:271

–

280.

9. Villa J, Warshel A (2001) Energetics and dynamics of enzymatic reactions.

J Phys Chem

105:7887

–

7907.

10. Benkovic SJ, Hammes-Schiffer S (2003) A perspective on enzyme catalysis.

Science

301:1196

–

1202.

11. Eigen M (1964) Proton transfer acid-base catalysis and enzymatic hydrolysis. I. Elemen-

tary processes.

Angew Chem Int Ed

3:1

–

15.

12. Arnett EM (1973) Gas-phase proton transfer: A breakthrough for solution chemistry.

Acc Chem Res

6:404

–

409.

13. Hunter EPL, Lias SG (1998) Evaluated gas phase basicities and proton affinities of

molecules: An update.

J Phys Chem Ref Data

27:413

–

656.

14. Wang SZ, Bianco R, Hynes JT (2009) Depth-dependent dissociation of nitric acid at an

aqueous surface: Car

–

Parrinello molecular dynamics.

J Phys Chem A

113:1295

–

1307.

15. Shamay ES, Buch V, Parrinello M, Richmond GL (2007) At the water

’

s edge: Nitric acid as

a weak acid.

J Am Chem Soc

129:12910

–

12913.

16. Enami S, Hoffmann MR, Colussi AJ (2010) Proton availability at the air/water interface.

J Phys Chem Lett

1:1599

–

1604.

17. Enami S, Hoffmann MR, Colussi AJ (2010) Superacid chemistry on mildly acidic water.

J Phys Chem Lett

1:3488

–

3493.

18. Enami S, Hoffmann MR, Colussi AJ (2010) Prompt Formation of organic acids in pulse

ozonation of terpenes on aqueous surfaces.

J Phys Chem Lett

1:2374

–

2379.

19. Cheng J, Vecitis C, Hoffmann MR, Colussi AJ (2006) Experimental anions affinities for

the air/water interface.

J Phys Chem B

110:25598

–

25602.

20. Enami S, Hoffmann MR, Colussi AJ (2009) Simultaneous detection of cysteine sulfe-

nate, sulfinate, and sulfonate during cysteine interfacial ozonolysis.

J Phys Chem B

113:9356

–

9358.

21. Yabushita A, et al. (2009) Anion-catalyzed dissolution of NO

2

on aqueous micro-

droplets.

J Phys Chem A

113:4844

–

4848.

22. Nguyen S, Fenn JB (2007) Gas-phase ions of solute species from charged droplets of

solutions.

Proc Natl Acad Sci USA

104:1111

–

1117.

23. Vandoren JM, et al. (1990) Temperature dependence of the uptake coefficients of

HNO

3

, HCl and N

2

5

by water droplets.

J Phys Chem

94:3265

–

3269.

24. Ardura D, Donaldson DJ (2009) Where does acid hydrolysis take place?

Phys Chem

Chem Phys

11:857

–

863.

25. Dempsey LP, Brastad SM, Nathanson GM (2011) Interfacial acid dissociation and proton

exchange following collisions of DCl with salty glycerol and salty water.

J Phys Chem

Lett

2:622

–

627.

26. Cox MJ, Siwick BJ, Bakker HJ (2009) Influence of ions on aqueous acid-base reactions.

ChemPhysChem

10:236

–

244.

27. Otten DE, Shaffer PR, Geissler PL, Saykally RJ (2012) Elucidating the mechanism of se-

lective ion adsorption to the liquid water surface.

Proc Natl Acad Sci USA

109:701

–

705.

28. Bianco R, Wang SZ, Hynes JT (2007) Theoretical study of the dissociation of nitric acid

at a model aqueous surface.

J Phys Chem A

111:11033

–

11042.

29. Jarvis NL, Scheiman MA (1968) Surface potentials of aqueous electrolyte solutions.

J Phys Chem

72:74

–

78.

30. Petersen PB, Saykally RJ (2005) Adsorption of ions to the surface of dilute electrolyte

solutions: The Jones

–

Ray effect revisited.

J Am Chem Soc

127:15446

–

15452.

31. Lu SY, Jiang YJ, Zhou P, Zou JW, Wu TX (2010) Geometric characteristics and energy

landscapes of halogen

–

water

–

hydrogen bridges at protein-ligand interfaces.

Chem

Phys Lett

485:348

–

353.

32. Tian CS, Byrnes SJ, Han HL, Shen YR (2011) Surface propensities of atmospherically

relevant ions in salt solutions revealed by phase-sensitive sum frequency vibrational

spectroscopy.

J Phys Chem Lett

2:1946

–

1949.

Mishra et al.

PNAS

∣

June 26, 2012

∣

vol. 109

∣

no. 26

∣

10231

CHEMISTRY

33. Petersen PB, Saykally RJ (2006) Probing the interfacial structure of aqueous electro-

lytes with femtosecond second harmonic generation spectroscopy.

J Phys Chem B

110:14060

–

14073.

34. Dole M (1938) A theory of surface tension of aqueous solutions.

J Am Chem Soc

60:904

–

911.

35. Meot-Ner M (2005) The ionic hydrogen bond.

Chem Rev

105:213

–

284.

36. Leopold KR (2011) Hydrated acid clusters.

Annu Rev Phys Chem

62:327

–

349.

37. Zhao XL, Subrahmanyan S, Eisenthal KB (1990) Determination of pK

at the air/water

interface by second harmonic generation.

Chem Phys Lett

171:558

–

562.

38. Lewis T, et al. (2011) Does nitric acid dissociate at the aqueous solution surface?

J Phys

Chem C

115:21183

–

21190.

39. Harper K, et al. (2009) Surface residence and uptake of methyl chloride and methyl

alcohol at the air/water interface studied by vibrational sum frequency spectroscopy

and molecular dynamics.

J Phys Chem A

113:2015

–

2024.

40. Mochida M, Finlayson-Pitts BJ (2000) FTIR studies of the reaction of gaseous NO with

HNO

3

on porous glass: Implications for conversion of HNO

3

to photochemically active

in the atmosphere.

J Phys Chem A

104:9705

–

9711.

41. Molina MJ, et al. (1993) Physical-chemistry of the H

2

4

∕

HNO

3

∕

2

O system: Implica-

tions for polar stratospheric clouds.

Science

261:1418

–

1423.

42. De Vivo M, et al. (2007) Proton shuttles and phosphatase activity in soluble epoxide

hydrolase.

J Am Chem Soc

129:387

–

394.

43. Kamerlin SCL, Warshel A (2010) At the dawn of the 21st century: Is dynamics the

missing link for understanding enzyme catalysis?

Proteins

78:1339

–

1375.

44. Warshel A, et al. (2006) Electrostatic basis for enzyme catalysis.

Chem Rev

106:3210

–

3235.

45. Springer A, Hagen V, Cherepanov DA, Antonenko YN, Pohl P (2011) Protons migrate

along interfacial water without significant contributions from jumps between ioniz-

able groups on the membrane surface.

Proc Natl Acad Sci USA

108:14461

–

14466.

46. Ben-Amotz D (2011) Unveiling electron promiscuity.

J Phys Chem Lett

2:1216

–

1222.

47. Zilch LW, Maze JT, Smith JW, Ewing GE, Jarrold MF (2008) Charge separation in

the aerodynamic breakup of micrometer-sized water droplets.

J Phys Chem A

112:13352

–

13363.

48. Duffin AM, Saykally RJ (2007) Electrokinetic hydrogen generation from liquid water

microjets.

J Phys Chem C

111:12031

–

12037.

49. Xu X, Goddard WA (2004) The X3LYP extended density functional for accurate descrip-

tions of nonbond interactions, spin states, and thermochemical properties.

Proc Natl

Acad Sci USA

101:2673

–

2677.

50. Clark T, Chandrasekhar J, Spitznagel GW, Schleyer PV (1983) Efficient diffuse function-

augmented basis sets for anion calculations. 3. The 3-21+G basis set for 1st-row

elements, LI-F.

J Comput Chem

4:294

–

301.

51. Francl MM, et al. (1982) Self-consistent molecular orbital methods. 23. A polarization

type basis set for 2nd-row elements.

J Chem Phys

77:3654

–

3665.

10232

∣

www.pnas.org/cgi/doi/10.1073/pnas.1200949109

Mishra et al.