Adsorbed Water Promotes Chemically Active Environments on the Surface of Sodium Chloride

Gas–particle interfaces are chemically active environments. This study investigates the reactivity of SO2 on NaCl surfaces using advanced experimental and theoretical methods with a NH4Cl substrate also examined for cation effects. Results show that NaCl surfaces rapidly convert to Na2SO4 with a new chlorine component when exposed to SO2 under low humidity. In contrast, NH4Cl surfaces have limited SO2 uptake and do not change significantly. Depth profiles reveal transformed layers and elemental ratios at the crystal surfaces. The chlorine species detected originates from Cl– expelled from the NaCl crystal structure, as determined by atomistic density functional theory calculations. Molecular dynamics simulations highlight the chemically active NaCl surface environment, driven by a strong interfacial electric field and the presence of sub-monolayer water coverage. These findings underscore the chemical activity of salt surfaces and the unexpected chemistry that arises from their interaction with interfacial water, even under very dry conditions.

reactive than NH4Cl. At low RH, NaCl reacts with SO2 to form (SO4)2-, with a depletion of Cl-. The amount of (SO4)2-increases over time. In contrast, sulfate quickly accumulates on NH4Cl under 4%RH, and the amount remains stable over time and over RH increases. Molecular dynamics simulations show water lowers the energy barrier to replace a Cl-with a sulfate. the MD simulations and BE calculations help assign a low BE shoulder to solvated Cl-.
Immediate impact: The high reactivity of NaCl with SO2 under low RH conditions has several impacts. First, little fundamental chemistry is known about aerosol particles, which often contain salts and interact with SOx pollutants. This study builds fundamental knowledge in atmospheric/environmental chemistry. Secondly, salts are thought to have little reactivity, especially at low RH. This study shows we must rethink this assumption. Finally, the paper's theoretical component shows the importance of the electric field at the solid surface on the surface chemistry. This finding is important as the understanding of the electronic structure of solid/liquid interfaces is evolving. I recommend that the paper be published following consideration of the few minor technical comments.
Technical Recommendations: In Figure 3, please specify the SO2 exposure time used for the data.
page 8, line 56, "stretching of the valence electrons" is a unfamiliar phrase to me, and it is not clear what the authors are intending. Please clarify.

Editor:
As indicated in the enclosed comments, the reviewers' comments were generally positive, but certain improvements are suggested. I would be pleased to consider further for publication a revised manuscript that addresses the reviewers' concerns. Reply: We appreciate the comments from the reviewers and the editor, which have been instructive and illuminated several areas for improvement and clarification within our manuscript. We have prepared a revised manuscript that thoroughly addresses the questions and comments that have been posed. Below we address and/or point to changes in the text for each reviewer comment in an itemized fashion.
The reviewer comments are copied in plain text, while our responses and the actions we have taken are highlighted in blue.
Please also make the following non-scientific changes: 1. TOC Graphic: Please change/reformat the graphic and provide a TOC image per journal guidelines: The graphic should be in the form of a structure, graph, drawing, photograph, or scheme-or a combination. Non-scientific cartoon-like images or caricatures are discouraged. Action: A TOC image is now provided.
7. Supporting Information Statement: A brief, nonsentence description of the actual contents of each supporting information file is required. This description should be labeled Supporting Information and should appear before the Acknowledgement and Reference sections. Action: Supporting Information Statement has been added.

Reviewer: 1
The paper is globally convincing; however, the following points should be better clarified (or corrected) by the authors: 1. Indicate the water pressure and substrate temperature, not only the RH. Reply and Action: The water pressure and surface temperature have been added in a new table (Table S1).
2. Although no ClO -(hypochlorite) forms, the authors should indicate what precautions are taken to avoid radiolysis phenomena. Precise the photon dose rate. maybe add in SI. This is a very useful information for people working in the field. Reply and Action: Although hypochlorite was not observed, the X-ray beam still accelerates the sulfate -chloride replacement. To understand this effect, we compared the two salts, NaCl and NH 4 Cl, and the results show that the different chemical reactivities of the two salts are not because of the X-ray beam. Also, the effect of X-ray beamtime is illustrated in Figure S1, as stated in the text "...the soft X-ray radiation used in the experiments also plays a role and accelerates the displacement of chloride by sulfate, as discussed in Figure S1 in Supporting Information.". To emphasize the difference of the two salts, we now added the following text: "Note that the different chemical reactivities of the two salts are not caused by the X-ray, as the NH 4 Cl received even longer exposure time than NaCl.". The photon dose rate was between 1.1•10 14 to 5.6•10 14 photons•s -1 •cm -2 , depending on the photon energy. This information is now added in Supporting Information.
3. page 3 line 18, line 39. How many water molecules on the surface? Does a thin film form? Reply and Action: Under the conditions used in the cited reference studies, the salt surface was completely covered by water molecules, and the thickness of the coating is most likely of a microscopic/molecular character. However, from the cited studies it is not clear what water coverage is required to trigger an active surface environment.
However, our computer simulations suggest that one sublayer of water is sufficient to activate chemistry, such as the spontaneous dehydration of sulfurous acid into SO 2 . We have added the following sentence in the main manuscript: "This system composition aims to resemble a possible scenario that may have been observed experimentally at low RH, with a water sublayer deposited on the surface of the crystals." 4. Precise how the binding energies are measured? with respect to contamination C 1s? Indicate in the SI the calibration procedure. This is important to convince the reader that the low BE Cl 2p doublet IS NOT Cland the high binding one ClO-.... Reply and Action: The binding energies were aligned using aliphatic carbon as a reference (C1s at 284.8 eV). This is added in the caption of Figure 1, to clarify the binding energy has been carefully aligned.
5. Figure 1. The authors should precise whether the spectra are taken while the sample is exposed to the gas, or whether the hydration is performed in the dark (50-140 min) and then the NAP-XPS measurement is made. Reply and Action: During the measurements of these spectra, the water and SO 2 gases were present. This information has been now added into the caption of Figure  1.
6. Page 5 line 22. There spectrum delta. Correct the English. Reply and Action: Corrected.
7. page 8. line 56. The discussion explaining the calculated BE between expelled Cland in site Clis not convincing. Especially the concept of screening. If the BE diminishes this means that the core-hole is better screened, not the contrary as stated.
Where this expelled Clwill go on the real surface? some kind of adatom? what about its possible hydration that is not taken into account.
Reply: We thank the reviewer for this comment, and we agree that the source of misunderstanding lies in the concept of screening.
Generally, BE of the core electrons increases when the valence electrons are involved in bonds because of the screening of the valence electrons, which are slightly more distant from the nucleus than the core electrons, and the core electrons/nuclei interaction is reduced. Even in an ionic crystal involving elements with distinctly different electronegativities, such as NaCl, in which valence electrons are transferred to complete the Lewis octet of each element, there is always some degree of electron sharing. Thus, BE of the core electrons in the mineral is higher than in isolated Clbecause the screening of the valence electrons on the nuclei/core electrons is less effective in the crystal.
We change the text in the main manuscript as follows: "Generally, BE of the core electrons increases when the valence electrons are involved in bonds, 28 and the higher BE for Clcore electrons in the crystal may be due to a certain degree of electron sharing that is always present also in ionic bonds. This sharing reduces the screening of the valence electrons on the nuclei-core electrons interaction, increasing the BE of core electrons in the crystal. On the other hand, when chloride is isolated, the screening of the valence electrons on the nucleus interaction is more effective and, thus, BE of core electrons is lower than in the mineral case." Regarding the second part of the comment about the location of expelled chloride: Our FPMD shows, indeed, that Clremains adsorbed on the surface and hydrated by few water molecules ( Figure S5b), within the (unavoidable limited) time scale of our simulations (60 ps). For reasons of computational time and methodology (e.g., Nudged Elastic Bands do not work well with too many degrees of freedom) Figure  S4 shows the energy barrier for the chloride -sulfate replacement in absence of water. As stated in the main manuscript, the adsorption of a few water molecules, even at low RH, are very likely to catalyze the interfacial chloride -sulfate substitution.
8. p 9. line 49-50. Do the authors see spectral changes with time? If so, this should be reported in the SI. Reply and Action: The X-ray beam accelerates the transformation of the NaCl surface but does not significantly affect the NH 4 Cl surface. Figure S1 illustrates this in the Supporting Information.
Reviewer: 2 1. Major Advance: The authors use surface sensitive spectroscopy (APXPS & NEXAFS) to determine the interaction between SO 2 and chloride salts under humid