From the journal Environmental Science: Atmospheres Peer review history

01-Feb-2022

Dear Dr Unger:

Manuscript ID: EA-ART-12-2021-000104
TITLE: The Surface Composition of Amino Acid - Halide Salt Solutions is pH-Dependent

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Associate Editor, Environmental Sciences: Atmospheres

************

Reviewer 1

The authors reported some interesting results on the influence of pH on the relative distribution of inorganic ions in salt solutions. However, I have some concerns regarding the reliability of experiments results and their significance or application in aerosol chemistry. I could support its publication if these concerns are fully addressed.
Comments:
It is not clear how the relative humidity (RH) was controlled during the sample droplet measurement. As is well known, the RH condition has a strong influence on the pH, water content and phase state of aerosol droplets. The RH may also induce liquid–liquid phase separation in aerosol droplets (Freedman, 2020). The liquid–liquid phase separation would alter the relative distribution of inorganic ions and organic components in aerosol droplets. How did the authors judge that their results were influenced by pH rather than RH. Also, how did the authors determine the pH of sample droplets under the measurement? I found that they used the bulk pH instead of aerosol pH. It is my feeling that they have used the droplets with supermicrometer size rather than bulk solutions for (XPS) measurements. If so, without RH control, the droplet pH may significantly differ from the bulk data.
As a result, the current findings in this MS may not advance our understanding in aerosol or atmospheric science.

Miriam Arak Freedman. Liquid–Liquid Phase Separation in Supermicrometer and Submicrometer Aerosol Particles. Accounts of Chemical Research, 2020, 53 (6), 1102-1110.

Reviewer 2

This manuscript reports the surface propensity of halides in presence of amino acids at varying pH. Essentially, this is a demonstration of electrostatic effects being important in the concentration of ions at the aqueous solution - air interface of sea water or sea spray aerosol particles. Amino acids are important, mostly surface active species, naturally present as part of the organic material associated with biologically active ocean surface water and thus also with particles emitted therefrom. In turn, amino acids have both basic and acidic functional groups responding to pH changes in opposing ways. X-ray photoelectron spectroscopy as used in this work is well suited to measure surface chemical composition with high chemical selectivity, including the protonation states of acids and bases, and also the halide ions, chloride in this case. It complements macroscopic surface tension measurements, but the latter cannot replace XPS measurements in terms of chemical detail in complex mixtures, as mentioned by the authors in their response to previous review comments on this manuscript.

As this manuscript has been extended from a previous submission, and the authors have adequately responded to the previous comments, there is not much to add. The manuscript is well written, and within the context and audience addressed for the field of atmospheric chemistry, the results are relevant and significant. Also, the response to the reviewer comments related to the surface sensitivity or probing depth of XPS are correct. The additional analysis in terms of the departure of the local chloride and potassium concentration in presence of the amino acids from that in neat KCl solution helps nicely to discuss the somewhat unexpected trends for proline and cysteine.

I have only a few points:

1) The authors use terminology of 'surface pKa' when discussing the degree of protonation at the aqueous solution - air interface. I would like to caution that the higher abundance of the protonated acid must not be related to a locally different value of the pKa, as the local proton concentration is not known. Therefore, I would prefer saying that the degree of protonation is different in comparison to what is expected from the bulk pK and bulk pH. This avoids the debate about interfacial acidity.

2) The authors talk about ion-pair formation as driving force for attracting Cl- or K+ towards the interface. How sure is that, or could it be that it remains a longer range interaction? In turn, to what degree is an carboxylate - K+ ion pair different from the protonated carboxylic acid form in terms of surface propensity? This could also help to explain the differences in the trends between Cl- and K+

3) There are some other studies discussing interactions between halide ions and organics (Krisch et al., 2007; Lee et al., 2016; Lee et al., 2019; Chen et al., 2021) also employing liquid jet XPS, and for instance, molecular beam work (Gil Nathanson's group) that should be included in the references, even if they did not explicitly study pH effects.

Response to Reviewers - The Surface Composition of Amino Acid -
Halide Salt Solutions is pH-Dependent

We uploaded a pdf version of our response, which contains a figure. We recommend using the uploaded answer; it will be easier to read.

General remarks from the authors:
We would like to thank the reviewers for their comments and hope that the revised manuscript
will address any uncertainties or mistakes pointed out.
Referee 1
Referee: 1
Comments to the Author The authors reported some interesting results on the influence of pH on
the relative distribution of inorganic ions in salt solutions. However, I have some concerns regarding
the reliability of experiments results and their significance or application in aerosol chemistry. I
could support its publication if these concerns are fully addressed.
Comments:
It is not clear how the relative humidity (RH) was controlled during the sample droplet measurement.
As is well known, the RH condition has a strong influence on the pH, water content and
phase state of aerosol droplets. The RH may also induce liquid–liquid phase separation in aerosol
droplets (Freedman, 2020). The liquid–liquid phase separation would alter the relative distribution
of inorganic ions and organic components in aerosol droplets. How did the authors judge that their
results were influenced by pH rather than RH. Also, how did the authors determine the pH of sample
droplets under the measurement? I found that they used the bulk pH instead of aerosol pH. It
is my feeling that they have used the droplets with supermicrometer size rather than bulk solutions
for (XPS) measurements. If so, without RH control, the droplet pH may significantly differ from
the bulk data. As a result, the current findings in this MS may not advance our understanding in
aerosol or atmospheric science.
Miriam Arak Freedman. Liquid-Liquid Phase Separation in Supermicrometer and Submicrometer
Aerosol Particles. Accounts of Chemical Research, 2020, 53 (6), 1102-1110.
Our experiments have not been carried out on droplets, but on the laminar phase of a free-flowing
liquid jet injected into a vacuum chamber (base pressure about 10−3 mbar). The measurements
take place before the liquid jet breaks apart into droplets. With regard to most aerosol particles
our sample is very large and thus possesses a bulk region and a surface layer on top. As the
X-rays intersect with the liquid jet about 1−5mm downstream of the jet nozzle, the measurement
takes place roughly 20−100 μs after the sample leaves the quartz glass nozzle. Immediately upon
exposure to the vacuum, water evaporates and creates a gas sheath around the jet, which results
in a rapid cooling of the jet.
Currently, there is no good model for the cooling rate for jets of this size, only for smaller ones [1].
In order to estimate a cooling rate for a jet such as ours, we used the model by Wilson et al. with
an ablation rate of water from the jet as given in their work. The result of a calculation is shown
in figure R 1. According to this estimate, the jet cools down by 1 − 5 ◦C across the interaction
region and does not contain supercooled water. Though this is only an estimate, we have no reason
to assume that these numbers are off by an order of magnitude. Given the larger size of our jet
compared to the ones used in the work by Wilson et al., we assume that the jet cooling as given
in figure R 1 is an overestimation. To the best of our knowledge, this is consistent with the bulk
of work carried out by liquid jet setups.
By using a liquid jet and solutions below saturation concentration, the water loss through evaporation
during the first few microseconds until the measurement does not increase the concentration of
KCl and organics enough that we would have to worry about liquid-liquid phase phase separation
and the possible complications connected to it. Unfortunately, this experimental setup prevents us
to measure the pH at the surface of the sample, as it is situated in a vacuum chamber and moving
For that reason, we use the bulk pH of our samples for orientation and mention for
the reader that the pH at the surface will be different.
In our opinion, the remark by referee has merit as it highlights the complexity of aerosol, in
particular in comparison with lab experiments such as ours. There are many more processes at
play regarding microdroplets when it comes to surface pH or the composition in general. By
investigating a liquid with a large bulk phase, we address a different concentration regime. Our
samples are thus rather a model system to study some of the underlying physico-chemical processes
governing liquid surfaces and the results should only be applied with caution to the surface of
aerosol containing a highly concentrated solution.
It would be desireable to carry out XPS measurements on systems closer to systems we find in
the atmosphere, i.e. free-flying aerosol, and while such measurements are possible [2, 3], they are
currently rather limited due to technical hurdles. Studies such as ours or the ones conducted for
example by the group of Heather Allen [4, 5, 6, 7] are the best we currently have to investigate
liquid surfaces with nm-scale resolution. They may fall short in some aspects considering the
complexity of real aerosol, but they reveal major trends and thus make a contribution to the field,
which, hopefully, will be of use to other scientists in the future.

Referee 2
Comments to the Author This manuscript reports the surface propensity of halides in presence
of amino acids at varying pH. Essentially, this is a demonstration of electrostatic effects being
important in the concentration of ions at the aqueous solution - air interface of sea water or sea
spray aerosol particles. Amino acids are important, mostly surface active species, naturally present
as part of the organic material associated with biologically active ocean surface water and thus
also with particles emitted therefrom. In turn, amino acids have both basic and acidic functional
groups responding to pH changes in opposing ways. X-ray photoelectron spectroscopy as used in
this work is well suited to measure surface chemical composition with high chemical selectivity,
including the protonation states of acids and bases, and also the halide ions, chloride in this case.
It complements macroscopic surface tension measurements, but the latter cannot replace XPS
measurements in terms of chemical detail in complex mixtures, as mentioned by the authors in
their response to previous review comments on this manuscript.
As this manuscript has been extended from a previous submission, and the authors have adequately
responded to the previous comments, there is not much to add. The manuscript is well written,
and within the context and audience addressed for the field of atmospheric chemistry, the results
are relevant and significant. Also, the response to the reviewer comments related to the surface
sensitivity or probing depth of XPS are correct. The additional analysis in terms of the departure
of the local chloride and potassium concentration in presence of the amino acids from that in neat
KCl solution helps nicely to discuss the somewhat unexpected trends for proline and cysteine.
I have only a few points:
1) The authors use terminology of ’surface pKa’ when discussing the degree of protonation at
the aqueous solution - air interface. I would like to caution that the higher abundance of the
protonated acid must not be related to a locally different value of the pKa, as the local proton
concentration is not known. Therefore, I would prefer saying that the degree of protonation is
different in comparison to what is expected from the bulk pK and bulk pH. This avoids the debate
about interfacial acidity.
This is certainly good advice considering the heated debate about the surface pH / surface equilibria.
We revised the sentence in the manuscript (line 112 and following).
2) The authors talk about ion-pair formation as driving force for attracting Cl- or K+ towards the
interface. How sure is that, or could it be that it remains a longer range interaction? In turn, to
what degree is an carboxylate - K+ ion pair different from the protonated carboxylic acid form in
terms of surface propensity? This could also help to explain the differences in the trends between
Cl- and K+
The argument that ion pairing plays a role for the observed surface enrichment refers to observations
other groups have been making with sum frequency generation and surface tension measurements
[4, 5, 6, 7, 8]. They give, in our opinion, compelling evidence for the existence of ion pairs
between carboxylic (and other) organic functional groups and cations. Since our samples do not
differ fundamentally from the compounds used in these works and our data does not contradict
the prior measurements, we have no reason to assume that the interactions in our samples should
be fundamentally different. However, we do not want to imply that ion pairing is the only process
at work, since we cannot observe ion pairs directly with XPS.
Concerning the long-range interactions - two possible interactions would come to mind here: 1) The
formation of an electrical double layer containing e.g. an increased amount of anions at the surface
and and of cations in a layer below. 2) The formation of non-contact ion pairs, like solvent-shared
or solvent-separated ion pairs, possibly in combination with the formation of a double layer.
We suppose that the former case, the formation of an electrical double layer, contributes to our
observations. We propose the formation of such layers in the manuscript. Since these double layers
have been found in pure alkali halide solutions, too, we opted to use the pure KCl solution as a
reference for the mixed solutions, so that we could address the ’excess effect’ introduced by the
amino acids.
The formation of an excess amount of ion pairs formed at the liquid surface compared with the
bulk is hard to measure. The presence of such ion pairs at aqueous surfaces has been observed in
salt solutions [9], but the probing techniques used in these measurements have been very surface
sensitive (XPS and SFG). The formation of an excess of such ion pairs might contribute in the
bigger picture. Tang and Allen discuss the possibility of a disparity in contact ion pairing between
carboxylic acid and alkali metal ions between surface and bulk at the very end of their paper [5].
However, this discussion is based on apparently contradicting evidence and we consider this issue
to remain an open question.
Finding solvent-separated or solvent-shared ion pairs with XPS is difficult, since water is very
efficient in screening the charges of the ions, even in highly concentrated salt solutions. Thus, the
approach to detect ion pairs through a shift of the energy levels of the ions have failed so far with
liquid jets. There is a work by Tissot et al., in which the authors find remarkable energy shifts
[10], but they did not use liquid jets during these measurements and discuss how their data is
seemingly in disagreement with data measured on liquid jets. In our opinion the source for these
differences remains unclear.
Regarding the surface propensity of the [R-COO−      K+] ion pair vs. R-COOH: Two interactions
have to be considered here, which influence the dynamics: The increased stabilization at the surface
of the base solution by ion pairing (and to a lower degree of the acid, too) [8] and the change in
protonation behaviour in the presence of alkali metal cations [5]. At the current state, we do not
believe that the answer to the question which of the two species has a higher surface propensity
is known. MD simulations in the spirit of the ones done by Caleman et al. could probably shed
some light on this question [11].
With regard to our work, this question is only relevant for the high-pH data, which do not show
significant deviations from the reference solution. However, the question is worth addressing when
measurements such as ours are extended into the basic pH region. In fact, we conducted such
measurements, but could not use the data due to an oversight during the preparation of the basic
samples. The old version of the manuscript on ChemRxiv still contains these data points.
3) There are some other studies discussing interactions between halide ions and organics (Krisch
et al., 2007; Lee et al., 2016; Lee et al., 2019; Chen et al., 2021) also employing liquid jet XPS,
and for instance, molecular beam work (Gil Nathanson’s group) that should be included in the
references, even if they did not explicitly study pH effects.
We thank the referee for the reference to these works. Many of them were indeed useful in the
context of the manuscript and interesting.
References
[1] K. R. Wilson, B. S. Rude, J. Smith, C. Cappa, D. T. Co, R. D. Schaller, M. Larsson, T. Catalano,
and R. J. Saykally. Investigation of volatile liquid surfaces by synchrotron x-ray spectroscopy
of liquid microjets. Review of Scientific Instruments, (75):725, 2004.
[2] E. Antonsson, M. Patanen, C. Nicolas, J. J. Neville, S. Benkoula, A. Goel, and C. Miron.
Complete Bromide Surface Segregation in Mixed NaCl/NaBr Aerosols Grown from Droplets.
Physical Review X, 5:011025, 2015.
[3] I. Unger, C.-M. Saak, M. Salter, P. Zieger, M. Patanen, and O. Bj orneholm. Influence of
Organic Acids on the Surface Composition of Sea Spray Aerosol. The Journal of Physical
Chemistry A, 124:422–429, 2020.
[4] Wei Hua, Dominique Verreault, and Heather C. Allen. Solvation of calcium–phosphate headgroup
complexes at the DPPC/aqueous interface. ChemPhysChem, 16:3910–3915, 2015.
[5] Cheng Y. Tang and Heather C. Allen. Ionic binding of Na+ versus K+ to the carboxylic
acid headgroup of palmitic acid monolayers studied by vibrational sum frequency generation
spectroscopy. The Journal of Physical Chemistry A, 113:7383–7393, 2009.
[6] Ellen M. Adams, Bethany A.Wellen, Raphael Thiraux, Sandeep K. Reddy, Andrew S. Vidalis,
Francesco Paesani, and Heather C. Allen. Sodium–carboxylate contact ion pair formation
induces stabilization of palmitic acid monolayers at high pH. Physical Chemistry Chemical
Physics, 19:10481–10490, 2017.
[7] Jennifer F. Neal, Mickey M. Rogers, Kimberly A. Smeltzer, Morgan A.and Carter-Fenk,
Alexander J. Grooms, Mia M. Zerkle, and Heather C. Allen. Sodium Drives Interfacial
Equilibria for Semi-Soluble Phosphoric and Phosphonic Acids of Model Sea Spray Aerosol
Surfaces. ACS Earth and Space Science, 4:1549–1557, 2020.
[8] Man Luo, Nicholas A.Wauer, Kyle J. Angle, Abigail C. Dommer, Meishi Song, Christopher M.
Nowak, Rommie E. Amaro, and Vicki H. Grassian. Insights into the behavior of nonanoic
acid and its conjugate base at the air/water interface through a combined experimental and
theoretical approach. Chemical Science, 11:10647–10656, 2020.
[9] L. G otte, K. M. Parry, W. Hua, D. Verreault, H. C. Allen, and D. J. Tobias. Solvent-Shared Ion
Pairs at the AirSolution Interface of Magnesium Chloride and Sulfate Solutions Revealed by
Sum Frequency Spectroscopy and Molecular Dynamics Simulations. The Journal of Physical
Chemistry A, 121:6450–6459, 2017.
[10] H. Tissot, G. Olivieri, J.-J. Gallet, F. Bournel, M. G. Silly, F. Sirotti, and F. Rochet. Cation
Depth-Distribution at Alkali Halide Aqueous Solution Surfaces. The Journal of Physical
Chemistry C, 119:9253–9259, 2015.
[11] C. Calemana, J. S. Hub, P. J. van Maaren, and D. van der Spoel. Atomistic simulation of ion
solvation in water explains surface preference of halides. PNAS, 108:6838–6842, 2011.

02-Mar-2022

Dear Dr Unger:

Manuscript ID: EA-ART-12-2021-000104.R1
TITLE: The Surface Composition of Amino Acid - Halide Salt Solutions is pH-Dependent

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Dr Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

Reviewer 1

The referee's concerns have been addressed.