Impurity Effects at Hydrophobic Surfaces
Yuki Uematsu
Department of Chemistry, Kyushu University, Fukuoka 819-0395, Japan

Douwe Jan Bonthuis and Roland R. Netz
Fachbereich Physik, Freie Universität Berlin, 14195 Berlin, Germany(Dated: September 8, 2018)
The effective charge of hydrophobic surfaces and in particular of the air-water interface is a crucial
parameter for electrochemistry, colloidal chemistry and interfacial science, but different experiments
give conflicting estimates. Zeta-potential and disjoining-pressure measurements point to a strongly
negative surface charge, often interpreted as being due to adsorbing hydroxide ions. In contrast,
surface tension measurements of acids and bases suggest the hydronium ion to be surface active, inagreement with some surface-specific non-linear spectroscopy results. The air-electrolyte interfacial
tension exhibits a characteristic minimum at millimolar electrolyte concentration for all salts, theso-called Jones-Ray effect, which points to competitive adsorption mechanisms present in diluteelectrolyte solutions. We show that all these puzzling experimental findings can be explained by thepresence of trace amounts of surface-active charged impurities, most likely anionic surfactants.

INTRODUCTION

The interface between electrolytes and air, oil or otherhydrophobic materials is a very popular experimental
model system since it lacks complications due to dissociable surface groups. Consequently, it has been intensely
investigated in studies of the surface tension [1–6] and its
dynamics [7, 8], zeta potential [9–12], bubble coalescence[13, 14], stability of nano-bubbles [15–17], and disjoiningpressure [18–21]. Although the air-electrolyte interface
is the simplest possible electrochemical system, its effective surface charge is still debated [22–33]. Since a longtime it has been known that the electrophoretic mobility of gas bubbles in pure water is negative, which led
to the conclusion that OH− ions adsorb at the air-waterinterface [9, 10]. The negative sign of the air-water interface charge is also supported by the stability of thinwater films on negatively charged silica surfaces [18–21],
whereas surface-sensitive nonlinear spectroscopy suggests
H3 O+ ions to be present at the air-water interface [22, 23]instead. From the molecular modeling side, the situationis also not clear; some simulations support hydroxide adsorption [25, 28] and others not [24, 29–32]. So while
the interface between electrolytes and hydrophobic surfaces is conceptually simple, its basics properties seem
surprisingly complex. This review summarizes puzzling
experimental measurements of the surface tension, thezeta potential, and the disjoining pressure of electrolytehydrophobe interfaces and provides an interpretation interms of the presence of trace amounts of charged surfaceactive impurities.

ELECTROLYTE SURFACE TENSION

Basic insight is provided by surface tension studiesof the air-electrolyte interface. The Gibbs adsorption

isotherm,dγ = −

X

Γi dµi ,(1)

irelates the change of the surface tension γ to the sum of
the surface excesses of molecules of type i multiplied bychanges of the respective chemical potentials µi . Fig. 1ashows the experimental surface tension increment ∆γ [1,3] as a function of added salt concentration cbsalt together
with linear fits according to ∆γ = Acbsalt [3, 4, 14]. Most
salts, except the acids, increase the surface tension in an
ion-specific fashion. For ideal solutions Eq. (1) predicts
the total ionic surface excess to be proportional to −A;
Fig. 1a thus implies that most ions are repelled fromthe interface while acids exhibit a net attraction. The
ionic repulsion from the air-electrolyte interface is partly
due to dielectric image charge repulsion [2], while the ion
specificity is caused by interfacial hydration effects [36].
Fig. 1b shows the experimental coefficients A of different electrolytes [4, 14] sorted in columns with respect
to the anionic surface affinity, which increases according
to the Hofmeister series (from left to right) and basicallyreflects the anion size (with OH− constituting a noteworthy and debated anomaly). The surface affinities of
the cations are rather similar, except for H3 O+ , which
stands out and gives rise to a net positive interface excess
for all anions (except in combination with OH− where
the surface affinity is zero by virtue of chemical equilibrium). Neglecting correlated adsorption of H3 O+ withanions and incomplete acid dissociation at surfaces [37],this suggests that H3 O+ has a strong affinity to the airwater interface, while OH− is repelled. This contrasts
with many other interpretations [24–28, 30–33, 37], so
microscopic insight is direly needed.
Figs. 2a/c show snapshots of force-field based molecular dynamics simulations of HCl and NaOH solution-air
interfaces and Figs. 2b/d present the extracted poten-

2

3

NaOH

HCl

LiBr

HNO3

2

Li

2

A [ (mN/m)/M ]∆γ [ mN/m ]LiI
NaClO4

1
0

1.8

+

Na

NaCl

+

KCl

K+

1

+

Cs

+
NH4

0

+

H 3O

∆γ [ mN/m ]

4

CsCl

1.2

NaF

0.6

-1

αNa = αK = αCs = 1.2-1αCl = 1.0

-2

-2
0

(a)

0.5

1
1.5
cbsalt [ M ]2OH- F-

Cl- Br- NO3 I- ClO4(b)

αF = 1.3

0
0

(c)

0.4
0.8
cbsalt [ M ]1.2

FIG. 1. (a) Surface tension increment of different electrolyte solutions with respect to pure water at relatively large salt
concentrations, the lines are linear fit functions according to ∆γ = Acbsalt . The experimental data of NaOH, LiBr, LiI, HCl,and HNO3 are taken from Ref. 1, whereas the data of NaClO4 is taken from Ref. 3. (b) Experimental linear coefficients A are
sorted with respect to the anions [4, 14]. Anions exhibits pronounced ion specificity according to the anionic Hofmeister series;
except for the acids, the cations are rather similar. (c) Surface tension of NaCl, KCl, CsCl, and NaF solutions. The points are
the experimental data [5], the lines follow from the mean-field model Eq. (2) using ε = 78, T = 298 K, and z ∗ = 0.5 nm. The
ionic surface affinities αNa = 1.2 and αCl = 1.0 are extracted from previous simulations of the potentials of mean force of Na+
and Cl – at the air-water interface [34, 35]. αK = 1.2, αCs = 1.2, and αF = 1.3 are obtained by fits to the experimental data.
The black solid line shows the model result for α+ = α− = 0.8 for comparison.

tials of mean force for the H3 O+ , Cl− , Na+ , and OH−
ions [32]. Of all the ions, only H3 O+ has a small but significant affinity for the interface, which is caused by its
favorable orientation at the interface, in agreement withab initio simulation results [32]. As shown in Fig. 2e,the simulations reproduce the experimental surface tension of NaCl, NaOH, and HCl solutions quantitatively[32]. For this the ion force fields must be very accurate, which is achieved by thermodynamic force-field optimization [34, 38]. It can be concluded that the negative
surface tension of acids is indeed caused by a significant
H3 O+ affinity to the electrolyte-air interface.
JONES-RAY EFFECT

A closer look at experimental NaCl surface tension
data from different labs in H2 O and D2 O (data points[39, 40]) for ultra-low electrolyte concentration in Fig. 3a
is revealing. The surface tension exhibits a pronounced
minimum in the concentration range of cbsalt = 0.01 −0.001 M, with similar results being obtained for all electrolytes [39–44]. Various explanations for this so-called
Jones-Ray effect have been offered, involving instrumental wetting artifacts [45], anion adsorption [46, 47] andOH− adsorption [48, 49], but its mechanism is still debated [35, 47, 50]. Clearly, OH− ions cannot cause thesurface tension minimum since they are repelled from theinterface, as demonstrated in Fig. 1b and Fig. 2d; also therather weak surface affinity of H3 O+ ions is not sufficient
to explain the experimental data in Fig. 3a [35]. However, the assumption of trace amounts of surface-active
charged impurities allows reproduction of the Jones-Ray

effect: the solid lines in Fig. 3a are modeling results for
impurity concentrations in the nM range with a surface
affinity of typical surfactants such as sodium dodecylsulfate (SDS) [35]. This is not unreasonable, as it isdifficult to experimentally detect charged impurities atlower concentration than the inevitable water ions, which
is 100 nM at neutral pH, by conductivity measurements.
Thus even when using ultrapure water, purified salts, andcleaned glassware, impurities are unavoidable and havebeen previously invoked to explain dynamic features of
air-water interfaces [51–55].
In the theoretical model used for Fig. 3, the Gibbs dividing surface is located at z = 0 and all ionic potentials
of mean force are approximated by box profiles. The ion
distribution in the water phase (z > 0) is assumed tofollow the mean-field Poisson-Boltzmann equation
εε0X∗
d2 ψ(z)= −eqi cbi e−eqi ψ(z)/kB T −αi θ(z −z) ,
2
dz
i

(2)

where ε is the solution dielectric constant, ε0 the vacuum dielectric permittivity, ψ(z) the local electrostatic
potential, e the elementary charge, qi the valency of thei-th ion type, cbi the bulk concentration of the i-th ion
type, kB T the thermal energy, αi the surface affinity of
the i-th ion type, θ(z) the Heaviside function, and z ∗the thickness of the surface adsorption potential. The
boundary conditions for an intrinsically neutral interface
are dψ(z)/dz|z=0 = 0 and ψ(z)|Rz→∞ = 0. Theionic sur∞face excess Γi is defined by Γi = 0 ci (z) − cbi dz where∗ci (z) = cbi e−eqi ψ(z)/kB T −αi θ(z −z) is the local concentration of ion type i and ψ(z) follows from the solution of eq.
(2). Finally, the electrolyte surface tension is obtained

3
(a)

(c)

HCl
air

(e)

NaOH

Surface tension

water
3

NaOH
NaCl
HCl

-1

0

z [nm]

1

2

0z [nm]

1

2

(d)
H3O+

5

Cl

VPMF / kBTVPMF / kBT

(b)

-1

-

0
-1

0

z [nm]

1

2

Na+
OH-

5

1

0

0
-1

Δγ [mN/m]2-10z [nm]

1

2

0

0.5
1.0
cbsalt (M)1.5

FIG. 2. (a,c) Snapshots of force-field based molecular dynamics simulations of 0.5 M HCl and NaOH solutions [32]. Oxygenand hydrogen atoms of H3 O+ and OH− are colored in red and white, Cl− and Na+ in orange and blue, respectively, watermolecules are transparent. The water Gibbs dividing surface at z = 0 is indicated by dashed lines. H3 O+ is weakly attracted
to the interface and its favorable orientation is illustrated, whereas Cl− , Na+ , and OH− are depleted from the interface. (b,d)
Potentials of mean force for H3 O+ , Cl− , Na+ , and OH− [32]. (e) Comparison of linear fits to experimental surface tension data(lines and color shaded areas which indicate the 90% confidence intervals) [4, 14] and molecular dynamics simulation results(points with error bars) [32].

by integrating Eq. (1) using the ideal-gas approximationdµi = kB T dcbi /cbi . For experiments in ambient air the solution pH is around 5.6 due to CO2 dissolution [21, 56].
In the model, H3 O+ , OH− , and HCO−
3 are included
with bulk concentrations given as cbH3 O = 10−pH M,
cbOH = 10pH−14 M, and cbHCO3 = cbH3 O − cbOH . In addition, we consider impurities which completely dissociate into surface-active anions and their counterions. The
surface affinities αNa = 1.2 and αCl = 1.0 are extracted
from previous simulations [34] and reproduce the experimental surface tension of NaCl in Fig. 1c. The values
αH3 O = −0.9, αOH = 1.6, αHCO3 = −0.4 are taken fromfits to experimental surface tension data of HCl, NaOH,
and NaHCO3 solutions [35], and the impurity surface
affinity αimp = −15.6 is extracted from surface tension
data of SDS [35].
For the modeling results in Fig. 3a the fitted impurity concentrations are cbimp = 1.6 nM, 18 nM, and 34 nM(black, red and blue lines) and are all well below thewater ion concentration. The convincing agreement between the model results and the experimental data overthe entire salt concentration range, together with the reasonable fit parameters, suggest contamination of waterwith surface-active charged impurities to be the cause of
the Jones-Ray effect. This conclusion is corroborated by
the fact that this hypothesis also explains zeta potential

and disjoining pressure data, as will be explained below.
Figs. 3b and c show the impurity surface excess Γimp
and the surface potential ψ0 = ψ(z)|z=0 as a function of
the added salt concentration for the same parameters as
in Fig. 3a. It is the increase of Γimp with cbsalt , caused byscreening of the electrostatic repulsion between adsorbed
impurity molecules, which produces the minimum in thesurface tension [35]. The typical surface area per impurity is of the order of 100 nm2 , in agreement with recent
experimental estimates [52–54]. The adsorption of dilute impurities takes a few minutes [57], which elegantly
explains why the Jones-Ray effect shows slow dynamics[43]. Interestingly, even though the impurity concentration is nano-molar, the resultant surface potential ψ0 is
substantial, in particular at low salt concentrations.

DISJOINING PRESSURE

The disjoining pressure data of a thin water film ona negatively charged silica surface (shown in Fig. 4a fordifferent fluoride electrolytes) is positive, decays roughly
exponential as a function of film thickness D and exhibits
significant ion-specificity [21]. This immediately suggeststhat the air-electrolyte interface possesses a significant
negative charge [59]. To model these data, the surface

4

-1

10

Γimp [ /nm2 ]distilled water
ultrapure water
99.9% D2O

0

10

-3

10-2
10-3

(b)

10

-3

-10

-10-2
-10-1

NaCl

0

-10

ψ0 [ mV ]∆γ [ mN/m ]

-2

10

-1

10

0

-50

-100
10-4

10-3

10-2

10-1

100

101

cbsalt [ M ]

(a)

10-7

10-5

cbimp = 0 M

18 nM

1.6 nM

34 nM

10-3

10-1

101

cbsalt [ M ](c)FIG. 3. (a) Surface tension increment ∆γ of NaCl solutions at ultra-low concentration. The black circles are the original
Jones-Ray experimental data obtained by the capillary rise method [39], colored symbols are obtained by the Wilhelmy plate
method in H2 O and in D2 O [40]. The lines are model predictions based on Eq. (2) in the presence of surface active H3 O+ ,OH− , HCO−
3 , charged impurities and their neutralizing counterions. Parameters used are ε = 78, T = 298 K, pH = 5.6, surface
potential width z ∗ = 0.5 nm and ionic surface affinities in units of kB T of αNa = 1.2, αCl = 1.0, αH3 O = −0.9, αOH = 1.6,
αHCO3 = −0.4. The surface affinity of impurities αimp = −15.6 is extracted from fits to SDS experimental surface tension
data [35]. The fitted impurity concentrations are cbimp = 1.6 nM, 18 nM, and 34 nM for the black, red, and blue solid lines, theblack broken line for cbimp = 0 nM is added for comparison. (b) Surface impurity excess Γimp and (c) surface potential ψ0 as afunction of the added salt concentration for the same parameters and the same impurity concentrations as in (a).

1000

b

KCl, cbimp = 500 nM

cimp = 553 nM
219 nM0
pKa = 7

NaF
500

KF
70

100

CsF

103

ζ, ψ0 [ mV ]

Π [ Pa ]76 nM
pKa = 5-50-100

ζ (exp) ψ0 (theory)
cbsalt = 0.1 mM

2

10

101

250

70

(a)

80

1 mM
10 mM

-150

b
cimp = 0 M

90

D [ nm ]

100

1

(b)

3

5

7

9

11

pH

FIG. 4. (a) Disjoining pressure Π of a wetting film of 0.1 mM fluoride salt solutions on a silica surface as a function of film
thickness D. Points are experimental data [21], lines are mean-field model results from Eq. (2) for fixed silica surface potential
ψSi = −135 mV [58]. The fitted impurity concentrations are cbimp = 553 nM, 219 nM, and 76 nM, for NaF, KF, and CsF
solutions, respectively. Otherwise same parameters as in Fig. 3 are used. The inset shows in addition the disjoining pressure in
the absence of surface-active impurities (black solid line) (b) The data points are experimental zeta potentials of hydrophobic
planar polymer films for three different added KCl concentrations from Ref. 11. The lines denote mean-field model results forthe surface potential from Eq. 2 in the presence of acidic charged impurities of fixed bulk concentration cbimp = 500 nM with adissociation constant of pKa = 5 (broken lines) and pKa = 7 (solid lines). Calculations are done in presence of surface-inactive
K+ , Cl− and OH− ions as well as surface-active ions H3 O+ , using the same parameters as in Fig. 3.

5affinities of K+ , Cs+ , and F− ions are extracted from fits
to the experimental surface tension data of NaCl, KCl,and CsCl solutions in Fig. 1c [5], resulting in the same
surface affinity αNa = αK = αCs = 1.2. This suggeststhat the cation specificity of the disjoining pressure in
Fig. 4a does not originate from cation specific surface
affinities. For F− we obtain αF = 1.3 from the data
in Fig. 1c. The model results in Fig. 4a are obtainedfor 0.1 mM added salt assuming pH=5.6 in the presenceof H3 O+ , OH− and HCO−
3 ions and for a fixed silica
surface potential of ψSi = −135 mV, which is the zeta
potential of silica in 0.1 mM KCl solution [58]. In the absence of charged impurities, a very small repulsive pressure is obtained (shown as a black solid line in the inset
of Fig. 4a) which is due to the compression of the counterion layer at the silica surface. The fits to the experimental data yield impurity concentrations of cbimp = 553 nM,219 nM, and 76 nM for NaF, KF, CsF solutions, respectively. These concentration are somewhat larger than theones obtained for the Jones-Ray effect, but are still very
dilute and not unreasonable.

ZETA POTENTIAL

Air bubbles and oil droplets in an electric field movetowards the anode, which indicates a negative effective charge [10–12]. A relation between zeta potentials
and the presence of impurities was fist suggested for oil
droplets in water [12]; alternative mechanisms involving
the surface potential of the pure air-water interface wereshown to be inconsistent [60, 61]. Fig. 4b shows the experimental zeta potential of hydrophobic planar polymer
films [11] as a function of pH for different concentrationsof added KCl. This experimental geometry has the advantage that complications due to surface curvature areabsent [62, 63]. In the experiments, the pH was adjusted
by adding KOH and HCl [11]. Since the zeta potentialof gas bubbles and hydrophobic surfaces is strongly pH
sensitive, impurity charge regulation is included in the−−modeling according to imp− + H3 O+ )−*− impH + H2 O,
described by an acidic dissociation constant Ka . Near theinterface, the impurity dissociation equilibrium is perturbed in the presence of a finite surface potential, which
is fully accounted for in the model. Because the experiment was performed in nitrogen atmosphere [11], HCO−
3
is not included in the model.
Fig. 4b compares the experimental zeta potential datawith the calculated surface potential ψ0 as a functionof added KCl concentration and for different pH values.
The same parameters as in Fig. 3, extracted from theexperimental data in Fig. 1d, are used for the interface
affinities of all ions. The bulk impurity concentration forthe modeling results in Fig. 4b is set to cbimp = 500 nM,
similar to the one used in the fits to the disjoining pressure data in Fig. 4a. For an impurity dissociation con-stant of pKa = 7 (solid lines) the model describes the experimental data better than for pKa = 5 (broken lines),
which points to a very weak acidic character of the impurities, even weaker than carboxyl groups, for which theexperimental value of pKa is around 5 [12, 64]. In thiscontext it is interesting to note that pKa values are expected to be shifted to higher values at low-dielectric surfaces [64, 65]. The experimental data in Fig. 4b exhibits
an isoelectric point around pH = 4, the value of which
sensitively depends on the surface type [66]. The model
predicts slightly positive surface potentials for pH < 2
due to weak adsorption of H3 O+ ; the larger positive experimental zeta potentials at low pH might point to theadditional presence of cationic impurities.
For a hydrophobic surface as used in the experiment in
Fig. 4b, the hydrodynamic boundary condition is complex and the experimentally measured zeta potential notnecessarily coincides with the electrostatic surface potential [67]. In fact, the hydrodynamic boundary condition at an air-water interface depends on the presence ofabsorbed molecules and changes smoothly from perfectslip to no slip with rising adsorbent concentration [9, 52–
54, 68–72], leading to a number of side effects [55, 68, 71].
The comparison of the experimentally measured zeta potential and the calculated electrostatic surface potential
in Fig. 4b is, therefore, only tentative.

SUMMARY AND OUTLOOKAssuming the presence of negatively charged surfaceactive impurities, a simple mean-field description can beused to explain a number of hitherto puzzling experimental observations, namely the presence of stable water films on silica surfaces, the observation of negative
zeta potentials at hydrophobic surfaces, and finally the
Jones-Ray effect. The impurity concentrations that are
obtained from fits to experimental data are in the nM
range, they differ for different measurement techniquesand even for the same measurement done in different labs.
This is not surprising, considering the different cleaning
procedures and varying purity of used chemicals and lab
water in different labs.
It is not clear at present what the chemical characterof the impurities is, and it seems likely that experimental
impurities consist of a broad mixture of different species
that most likely varies from experiment to experiment.
Water might become contaminated during preparationor during experiments, impurities might be present inadded salts or might be ionic surfactants from detergentsused for washing glassware and will most likely consist ofa mixture of cationic and anionic surfactants with different surface affinities. We could explain experimental databy assuming negative impurities with a surface affinity
similar to SDS; future experiments should strive to identify impurities in lab water and in chemical compounds

6
by modern techniques such as high performance liquid
chromatography and mass spectrometry [73]. Careful experiments with added trace amounts of surfactants havealready given important clues about the chemical identity of impurities [12, 54, 69–72].
Trace amounts of impurities presumably play essential
roles also in other anomalies, such as the boundary dynamics of air-water interfaces [51–55], the inverse JonesRay effect [44, 74, 75], and the anomalous nanobubble stability [15–17]. In conclusion, the interface between electrolytes and hydrophobic surfaces is conceptually simple but in reality rather complex, since the effects of trace amounts of surfactants are amplified due tostrong adsorption at the hydrophobic interface.

ACKNOWLEDGEMENTS

YU acknowledge financial supports from Grant-in-Aid
for JSPS Fellows 16J00042.

REFERENCES AND RECOMMENDEDREADING

Papers of particular interest, published within the period of review, have been highlighted as:• Paper of special interest.
•• Paper of outstanding interest.

[1] E. W. Washburn, ed., International Critical Tables of Numerical Data, Physics, Chemistry and Technology, electronic 1st ed., Vol. IV (Knovel, Norwich, NY, 1930) pp.
463–466.
[2] L. Onsager and N. N. T. Samaras, “The surface tensionof Debye-Hückel electrolytes,” J. Chem. Phys. 2, 528–536(1934).
[3] P. K. Weissenborn and R. J. Pugh, “Surface tension of
aqueous solutions of electrolytes: Relationship with ion
hydration, oxygen solubility and bubble coalescence,” J.
Colloid Int. Sci. 184, 550–563 (1996).
[4] L. M. Pegram and M. T. Record, “Hofmeister salt effects
on surface tension arise from partitioning of anions andcations between bulk water and the air-water interface,”
J. Phys. Chem. B 111, 5411–5417 (2007).
[5] N. Matubayasi, Surface tension and related thermodynamic quantities of aqueous electrolyte solutions (CRC
Press, 2014).
[6] Yuki Nagata, Tatsuhiko Ohto, Mischa Bonn,andThomas D. Kühne, “Surface tension of ab initio liquidwater at the water-air interface,” J. Chem. Phys. 144,
204705 (2016), • Technique to obtain interfacial tension
data from ab initio simulation trajectories is developed.
[7] M. Liu, J. K. Beattie, and A. Gray-Weale, “The surface
relaxation of water,” J. Phys. Chem. B 116, 8981–8988(2012).

[8] Ines M. Hauner, Antoine Deblais, James K. Beattie,
Hamid Kellay, and Daniel Bonn, “The dynamic surface
tension of water,” J. Phys. Chem. Lett. 8, 1599–1603(2017), • High precision measurements of the dynamic
surface tension of water drops show slow dynamics on
the millisecond timescale.
[9] N. P. Brandon, G. H. Keksall, S. Levine, and A. L.
Smith, “Interfacial electrical properties of electrogenerated bubbles,” J. Appl. Electrochem. 15, 485–493 (1985).
[10] A. Graciaa, G. Morel, P. Saulner, J. Lachaise, and R. S.
Schechter, “The ζ-potential of gas bubbles,” J. Colloid
Int. Sci. 172, 131–136 (1995).
[11] Ralf Zimmermann, Stanislav Dukhin, and Carsten
Werner, “Electrokinetic measurements reveal interfacial
charge at polymer films caused by simple electrolyte
ions,” J. Phys. Chem. B 105, 8544–8549 (2001), •• Negative zeta potentials of planar hydrophobic surface aremeasured as a function of pH and salinity.
[12] K. Roger and B. Cabane, “Why are hydrophobic/waterinterfaces negatively charged?” Angew. Chem. Int. Ed.
51, 5625–5628 (2012), •• Negative zeta potential of oil
droplets in water is suggested to originate from contamination of oil with fatty acids.
[13] Vincent S. J. Craig, Barry W. Ninham, and Richard M.
Pashley, “The effect of electrolytes on bubble coalescencein water,” J. Phys. Chem. 97, 10192–10197 (1993).
[14] Christine L. Henry, Casuarina N. Dalton, Lehoa Scruton, and Vincent S. J. Craig, “Ion-specific coalescenceof bubbles in mixed electrolyte solutions,” J. Phys. Chem.
C 111, 1015–1023 (2007).
[15] William A. Ducker, “Contact angle and stability of interfacial nanobubbles,” Langmuir 25, 8907–8910 (2009).
[16] Siddhartha Das, Jacco H. Snoeijer, and Detlef Lohse,“Effect of impurities in description of surface nanobubbles,” Phys. Rev. E 82, 056310 (2010).
[17] James R. T. Seddon, Detlef Lohse, William A. Ducker,
and Vincent S. J. Craig, “A deliberation on nanobubbles
at surfaces and in bulk,” Chem Phys Chem 13, 2179–
2187 (2012).
[18] B. V. Derjaguin and M. Kussakov, “Anomalous properties of thin polymolecular films V,” Acta Physicochim.
USSR 10, 153 (1939).
[19] D. Exerowa, “Effect of adsorption, ionic strength and phon the potential of the diffuse electric layer,” KolloidZeitschrift und Zeitschrift für Polymere 232, 703–710(1969).
[20] N.V. Churaev, “Surface forces in wetting films,” Advances in Colloid and Interface Science 103, 197–218(2003).
[21] N. Schelero and R. von Klitzing, “Correlation between
specific ion adsorption at the air/water interface andlong-range interactions in colloidal systems,” Soft Matter 7, 2936 (2011), •• The repulsive disjoining pressure
between the air-electrolyte interface and a silica surface
is measured for different salt solutions and with high precision.
[22] Poul B. Petersen and Richard J. Saykally, “Evidence foran enhanced hydronium concentration at the liquid water surface,” J. Phys. Chem. B 109, 7976–7980 (2005),•• Non-linear surface specific spectroscopic methods areused to detect hydronium adsorption at liquid-water surfaces.
[23] Martin Mucha, Tomaso Frigato, Lori M. Levering,
Heather C. Allen, Douglas J. Tobias, Liem X. Dang, and7Pavel Jungwirth, “Unified molecular picture of the surfaces of aqueous acid, base, and salt solutions,” J. Phys.
Chem. B 2005 109, 7617–7623 (2005).
[24] R. Vácha, D. Horinek, M. L. Berkowitzc, and P. Jungwirth, “Hydronium and hydroxide at the interface between water and hydrophobic media,” Phys. Chem.
Chem. Phys. 10, 4975–4980 (2008).
[25] C. J. Mundy, I-F. W. Kuo, M. E. Tuckerman, H.-S. Lee,and D. J. Tobias, “Hydroxide anion at the air/water interface,” Chem. Phys. Lett. 481, 2–8 (2009).
[26] Shinichi Enami, Michael R. Hoffmann, and A. J. Colussi,“Proton availability at the air/water interface,” J. Phys.
Chem. Lett. 1, 1599–1604 (2010).
[27] Himanshu Mishra, Shinichi Enami, Robert J. Nielsen,
Logan A. Stewart, Michael R. Hoffmann, William
A. Goddard III, and Agustn J. Colussi, “Bronsted basicity of the air-water interface,” PNAS 109, 18679–18683(2012).
[28] M. D. Baer, I-F. W. Kuo, D. J. Tobias, and C. J. Mundy,“Toward a unified picture of the water self-ions at the
air-water interface: A density functional theory perspective,” J. Phys. Chem. B 118, 8364–8372 (2014).
[29] Jochen S. Hub, Maarten G. Wolf, Carl Caleman, Paul J.
van Maaren, Gerrit Groenhoff, and David van der Spoel,“Thermodynamics of hydronium and hydroxide surface
solvation,” Chem. Sci. 5, 1745 (2014).
[30] T. T. Duignan, D. F. Parsons, and B. W. Ninham, “Hydronium and hydroxide at the air-water interface with acontinuum solvent model,” Chem. Phys. Lett. 635, 1–12
(2015).
[31] Y-L. S. Tse, C. Chen, G. E. Lindberg, R. Kumar, and
G. A. Voth, “Propensity of hydrated excess protons andhydroxide anions for the air-water interface,” J. Am.
Chem. Soc. 137, 12610–12616 (2015).
[32] S. Mamatkulov, C. Allolio, R. R. Netz, and D. J. Bonthuis, “Orientation induces adsorption of the hydrated
proton at the air-water interface,” Angew. Chem. Int.
Ed. 56, 15846–15851 (2017), • Hydronium adsorption at
air-electrolyte interfaces is explained by favorable orientation of its dipole by using force-field molecular dynamics simulation with optimized force fields.
[33] X. Yan, M. Delgado, J. Aubry, O. Gribelin, A. Stocco,
F. Boisson-Da Cruz, J. Bernard, and F. Ganachaud,“Central role of bicarbonate anions in charging water/hydrophobic interfaces,” J. Phys. Chem. Lett. 9, 96–
103 (2018).
[34] D. Horinek, A. Herz, L. Vrbka, F. Sedlmeier, S. I. Mamatkulov, and R. R. Netz, “Specific ion adsorption at
the air/water interface: The role of hydrophobic solvation,” Chem. Phys. Lett. 479, 173–183 (2009).
[35] Y. Uematsu, D. J. Bonthuis, and R. R. Netz, “Charged
surface-active impurities at nanomolar concentration induce Jones-Ray effect,” J. Phys. Chem. Lett. 9, 189–193(2018), • The Jones-Ray effect is quantitatively explainedby assuming surface-active charged impurity at nanomolar concentration.
[36] Nadine Schwierz, Dominik Horinek, Uri Sivan, andRoland R. Netz, “Reversed Hofmeister series - the rulerather than the exception,” Current Opinion in Colloid
and Interface Science 23, 10–18 (2016).
[37] Pavel Jungwirth, “Ions at aqueous interfaces,” Faraday
Discuss. 141, 9–30 (2009).
[38] Douwe Jan Bonthuis, Shavkat I. Mamatkulov, andRoland R. Netz, “Optimization of classical nonpolariz-

able force fields for OH− and H3 O+ ,” J. Chem. Phys
144, 104503 (2016).
[39] G. Jones and W. A. Ray, “The surface tension of solutions of electrolytes as a function of the concentrationiii. sodium chloride,” J. Am. Chem. Soc. 63, 3262–3263(1941).
[40] Yixing Chen, Halil I. Okur, Nikolaos Gomopoulos, Carlos Macias-Romero, Paul S. Cremer, Poul B. Petersen,
Gabriele Tocci, David M. Wilkins, Chungwen Liang,
Michele Ceriotti, and Sylvie Roke, “Electrolytes induce
long-range orientational order and free energy changes inthe H-bond network of bulk water,” Sci. Adv. 2, e1501891(2016).
[41] G. Jones and W. A. Ray, “The surface tension of solutions of electrolytes as a function of the concentration. I.
a differential method for measuring relative surface tension,” J. Am. Chem. Soc. 59, 187–198 (1937), •• First
experimental report of a surface tension minimum of electrolyte solutions as a function of salt concentration.
[42] M. Dole and J. A. Swartout, “A twin-ring surface tensiometer. I. the apparent surface tension of potassium
chloride solutions,” J. Am. Chem. Soc. 62, 3039–3045(1940).
[43] G. Passoth, “Uber den Jones-Ray-Effekt und die
Oberflächenspannung verdünnter Elektrolytlosungen,”Z. Physik. Chem. 211O, 129–147 (1959).
[44] J. E. B. Randles and D. J. Schiffrin, “Surface tension ofdilute acid solutions,” Trans. Faraday Soc. 62, 2403–2408(1966).
[45] I. Langmuir, “Repulsive forces between charged surfaces
in water and the cause of the jones-ray effect,” Science88, 430–432 (1938).
[46] M. Dole, “A theory of surface tension of aqueous solutions,” J. Am. Chem. Soc. 60, 904–911 (1938).
[47] P. B. Petersen and R. J. Saykally, “Adsorption of ions tothe surface of dilute electrolyte solutions: The Jones-Ray
effect revisited,” J. Am. Chem. Soc. 127, 15446–15452(2005).
[48] K. A. Karraker and C. J. Radke, “Disjoining pressures,zeta potentials and surface tensions of aqueous non-ionicsurfactant electrolyte solutions: theory and comparison to experiment,” Adv. Colloid Int. Sci. 96, 231–264(2002).
[49] M. Manciu and E. Ruckenstein, “Specific ion effects via
ion hydration: I. surface tension,” Adv. Colloid Int. Sci.
105, 63–101 (2003).
[50] H. I. Okur, Y. Chen, D. M. Wilkins, and S. Roke, “Thejones-ray effect reinterpreted: Surface tension minima of
low ionic strength electrolyte solutions are caused by electric field induced water-water correlations,” Chem. Phys.
Lett. 684, 433–442 (2017).
[51] José A. Nicolás and José M. Vega, “A note on the effect of
surface contamination in water wave damping,” J. Fluid
Mech. 410, 367–373 (2000).
[52] Ofer Manor, Ivan U. Vakarelski, Xiaosong Tang, Sean J.
OShea, Geoffrey W. Stevens, Franz Grieser, Raymond R.
Dagastine, and Derek Y. C. Chan, “Hydrodynamic
boundary conditions and dynamic forces between bubbles and surfaces,” Phys. Rev, Lett 101, 024501 (2008).
[53] Ofer Manor, Ivan U. Vakarelski, Geoffrey W. Stevens,
Franz Grieser, Raymond R. Dagastine, and Derek Y. C.
Chan, “Dynamic forces between bubbles and surfaces
and hydrodynamic boundary conditions,” Langmuir 24,
11533–11543 (2008).

8[54] A. Maali, R. Boisgard, H. Chraibi, Z. Zhang, H. Kellay,
and A. Würger, “Viscoelastic drag forces and crossoverfrom no-slip to slip boundary conditions for flow near airwater interfaces,” Phys. Rev. Lett. 118, 084501 (2017),•• The presence of impurities at the air-water interface isconcluded from dynamic AFM measurements at bubbles.
[55] M. Arangalage, X. Li, F. Lequeux, and L. Talini, “DualMarangoni effects and detection of traces of surfactants,”
Soft Matter 14, 3378 (2018).
[56] A. Persat, R. D. Chambers, and J. G. Santiago, “Basicprinciples of electrolyte chemistry for microfluidic electrokinetics. part i: Acid/base equilibria and ph buffers,”
Lab on a Chip 9, 2437–2453 (2009).
[57] Chaodong Yang and Yongan Gu, “Modeling of the adsorption kinetics of surfactants at the liquid-fluid interface of a pendant drop,” Langmuir 20, 2503–2511 (2004).
[58] Grinnell Jones and Lloyd A. Wood, “The measurement ofpotentials at the interface between vitreous silica and solutions of potassium chloride by the streaming potential
method,” J. Chem. Phys. 13, 106 (1945).
[59] Sven Holger Behrens and Michal Borkovec, “Exact
poisson-boltzmann solution for the interaction of dissimilar charge-regulating surfaces,” Phys Rev E 60, 7040–
7048 (1999).
[60] Douwe Jan Bonthuis, Dominik Horinek, Lyderic Bocquet, and Roland R. Netz, “Electrohydraulic power conversion in planar nanochannels,” Phys. Rev. Lett. 103,
144503 (2009).
[61] D. J. Bonthuis, K. Falk, C. N. Kaplan, D. Horinek,
A. N. Berker, L. Bocquet, and R. R. Netz, “Commenton ”pumping of confined water in carbonnanotubes byrotation-translation coupling”,” Phys. Rev. Lett. 105,
209401 (2010).
[62] D. C. Henry, “The cataphoresis of suspended particles.
part i. the equation of cataphoresis,” Proc. R. Soc. London, Ser. A 133, 106 (1931).
[63] R. W. O’Brien and L. R. White, “Electrophoretic mobility of a spherical colloidal particle,” J. Chem. Soc.,
Faraday Trans. 2 74, 1607 (1978).
[64] Bethany A. Wellen, Evan A. Lach, and Heather C. Allen,“Surface pka of octanoic, nonanoic, and decanoic fatty
acids at the air-water interface: applications to atmospheric aerosol chemistry,” Phys. Chem. Chem. Phys. 19,

26551–26558 (2017).
[65] R. R. Netz, “Charge regulation of weak polyelectrolytes
at low- and high-dielectric-constant substrates,” J. Phys.:
Condens. Matter 15, S239–S244 (2003).
[66] Thomas W. Healy and Douglas W. Fuerstenau, “The isoelectric point/point-of zero-charge of interfaces formedby aqueous solutions and nonpolar solids, liquids, andgases,” Journal of Colloid and Interface Science 309, 183–
188 (2007).
[67] Douwe Jan Bonthuis and Roland R. Netz, “Unravelingthe combined effects of dielectric and viscosity profiles on
surface capacitance, electro-osmotic mobility, and electric surface conductivity,” Langmuir 28, 16049–16059(2012).
[68] Derek Y. C. Chan, Evert Klaseboer, and Rogerio Manica, “Film drainage and coalescence between deformable
drops and bubbles,” Soft Matter 7, 2235–2264 (2011).
[69] Laurent Joly, François Detcheverry, and Anne-Laure
Biance, “Anomalous ζ potential in foam films,” Phys.
Rev. Lett. 113, 088301 (2014).
[70] Alexia Barbosa De Lima and Laurent Joly, “Electroosmosis at surfactant-laden liquid-gas interfaces: beyond
standard models,” Soft Matter 13, 3341–3351 (2017).
[71] Oriane Bonhomme, Baptiste Blanc, Laurent Joly,
Christophe Ybert, and Anne-Laure Biance, “Electrokinetic transport in liquid foams,” Adv. Colloid Int. Sci.
247, 477–490 (2017).
[72] Baptiste Blanc, Oriane Bonhomme, Pierre-Francois
Brevet, Emmanuel Benichou, Christophe Ybert, andAnne-Laure Biance, “Electroosmosis near surfactant
laden liquid-air interfaces,” Soft Matter 14, 2604–2609(2018).
[73] Tarun Anumol, Sylvain Merel, Bradley O. Clarke, andShane A. Snyder, “Ultra high performance liquid chromatography tandem mass spectrometry for rapid analysis of trace organic contaminants in water,” Chemistry
Central Journal 7, 104 (2013).
[74] Philip B. Lorenz, “The specific adsorption isotherms of
thiocyanate and hydrogen ions at the free surface of aqueous solutions,” J. Phys. Chem. 54, 685–690 (1950).
[75] J. K. Beattie, A. M. Djerdjev, A. Gray-Weale, N. Kallay,
J. Lützenkirchen, T. Preočanin, and A. Selmani, “phand the surface tension of water,” J. Colloid and Int. Sci.
422, 54–57 (2014).