Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces

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Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-Ray Lu Lin, Jakub Husek, Somnath Biswas, Stephen M. Baumler, Tehseen Adel, Ka Chon Ng, L. Robert Baker, and Heather C. Allen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05231 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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Journal of the American Chemical Society

Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-Ray Lu Lin, Jakub Husek, Somnath Biswas, Stephen M. Baumler, Tehseen Adel, Ka Chon Ng, L. Robert Baker, and Heather C. Allen* Department of Chemistry & Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States

ABSTRACT Aqueous solutions of FeCl3 have been widely studied to shed light on a number of processes from dissolution, mineralization, biology, electrocatalysis, corrosion, to microbial biomineralization. Yet there is little to no molecular level studies of the airliquid FeCl3 interface. Here, both aqueous and glycerol FeCl3 solution surfaces are investigated with polarized vibrational sum frequency generation (SFG) spectroscopy. We also present the first ever extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy measurements of solvated ions and complexes at a solution interface, and observe with both X-Ray photoelectron spectroscopy (XPS) and XUV-RA, existence of Fe(III) at the surface and in the near surface regions of glycerol FeCl3 solutions, where glycerol is used as a high vacuum compatible proxy for water. XPS showed Cland Fe(III) species with significant Fe(III) interfacial enrichment. In aqueous solutions, an electrical double layer (EDL) of Cl- and Fe(III) species at 0.5 molal FeCl3 concentration is observed as evidenced from an enhancement of molecular ordering of water dipoles, consistent with the observed behavior at the glycerol surface. At higher concentrations in water, the EDL appears to be substantially repressed indicative of further Fe(III) complex enrichment and dominance of a centrosymmetric Fe(III) species that is surface active. In addition, a significant vibrational red-shift of the dangling OH from the water molecules that straddle the air-water interface reveals that the second solvation shell of the surface active Fe(III) complex permeates the topmost layer of the aqueous interface.

INTRODUCTION Iron is the most abundant transition metal on Earth and plays a pivotal role in a number of natural processes including corrosion, rock weathering and biogeochemical processing. It is also a common element on the other so-called rocky planets of Mercury, Venus, and Mars. Iron-rich minerals serve as indicators of geochemical processing on Mars as well.1-4 Iron is also critical to biology, serving as a key element for oxygen transport in vertebrates, and important in proper functioning of redox enzymes in many natural systems. Consistent with the variety of key roles that iron plays in both living and nonliving systems, its chemistry and speciation is also highly variable and complex. Fe(II) and Fe(III) form a redox couple and interconvert, although Fe(III) is the most stable oxidation state of iron under atmospheric conditions. Salts of Fe(III) do not simply 1 ACS Paragon Plus Environment

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dissociate in aqueous environments. They form numerous Fe(III)-complex species that are dependent on initial concentrations in addition to many other factors. Fe(III) ions are also highly acidic. The solution phase chemistry of all Fe(III) salts are also complicated, albeit well-studied in aqueous systems.5 Fe(III)/Fe(II) redox processes are responsible for widely studied Fenton chemistry.6-8 Given the general importance of the chemistry of electrolytes at the aqueous interface,9 Fe(III)-complexation at the airaqueous interface has not been investigated with molecular-level spectroscopic methods. A direct observation of any Fe(III)-complex at an aqueous surface is difficult; thus creative approaches are required to provide insight. Aqueous interfaces play a fundamental role in many chemical, biological and environmental processes including geo-mineralization/dissolution and atmospheric aerosol chemistry.10-11 To advance our understanding of these processes, a molecular level picture of the aqueous interface itself is required. So far, water structure and ion distributions for mostly simple mono- and divalent salts at air/aqueous interfaces have been investigated by a variety of techniques including vibrational sum frequency generation (SFG) spectroscopy,12-25 second harmonic generation,26-34 photoelectron spectroscopy,35-39 X-ray reflectivity measurements,40-41 as well as theoretical calculation and simulation.42-49 Glycerol interfaces have been used as proxy systems for air/aqueous interfaces due to its similar dielectric properties yet very low vapor pressure, which makes it an excellent water proxy for use in high vacuum.50-51 Glycerol’s OH groups play a central role in its water-like hydrogen bonding behavior. The surface vibrational spectra of the air/glycerol and the air/water interface are comparable with respect to the OH stretching region, yet the OH stretching bandwidths are slightly narrower than that of neat water signifying a narrower distribution of hydrogen bonding strengths and lengths.52 Interfacial density profiles and surface propensity of a number of inorganic ions have been both qualitatively and quantitatively estimated.53 Briefly, the monovalent and large polarizable halide anions are accommodated at the aqueous surface, whereas the smaller alkali and alkaline earth cations are repelled from the topmost surface. Additionally, solvent-shared ion pairs at the aqueous surface have been recently observed.12 Despite differences in surface propensity, both anions and cations affect interfacial water structure by altering the hydrogen-bonding character and reorienting water molecules based on the ions surface and near surface residence. Note that salts in mixtures with glycerol show similar surface behavior to that in aqueous solutions.52 It is generally accepted that ions perturb the hydrogen-bonding environment of water, in the bulk and at the aqueous surface.54-57 Yet, revealing water organization detail in the solvation shells of ions is nontrivial. Understanding the spatial extent to which the ionic effects are important is under debate.58-64 Recent studies demonstrate ion-induced long-range orientational order of bulk water molecules, and the long-range effects show specificity associated with the size and charge.62-63 Smaller ions with larger charges tend to perturb water structures more effectively. While specific ionic effects have been observed at the air/aqueous interface,13, 20, 26, 43, 65 those studies have focused mainly on alkali and alkaline earth metal ions. 2 ACS Paragon Plus Environment

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Given what is currently known about surface water organization, one might expect multivalent cations to have a large impact on interfacial water structure relative to simple mono- and divalent cations. The surface water structure is related to the identity of the surface-active species due to surface solvation effects; for multivalent cations, this is not well understood. Of the few published studies, Antonio and coworkers provide evidence of a bridging-polynuclear motif between two aqua lanthanide metal centers at the air/aqueous interface in both Er(III) and Eu(III) solutions; these surface motifs are unique from the mononuclear complexes observed in the bulk.66-67 However, unlike the lanthanide cations, Fe(III) can also complex with its counter ions forming multiple species dependent on concentration and pH.68-69 It is thus difficult to predict precisely which of these Fe(III)-complex species exist at the aqueous surface, if at all. In the present work, we employ SFG spectroscopy to investigate the interfacial water structure of aqueous FeCl3 solutions to elucidate Fe(III) complex surface propensity and speciation at the solution surface. Here, SFG studies of glycerol compared to aqueous surface FeCl3 measurements verify similar enhancement and suppression of signal with concentration, low to high concentration respectively, confirming that glycerol is an excellent low vapor pressure proxy in water for the high vacuum XPS experiments. XPS measurements provide element specific detection to directly identify the Fe oxidation state and concentrations of Fe and Cl ions. To further confirm the XPS results, we also measured the corresponding XUV-RA spectra at the Fe M2,3-edge. These measurements are the first reported XUV spectra of solvated ions measured in a reflectance geometry. Traditionally XUV spectroscopy of transition metal complexes is challenging owing to the very short penetration depth of XUV light in solution.70-71 The ability to obtain these spectra at good signal-to-noise in a reflection geometry opens future opportunities for measuring the dynamics of solvated ions at interfaces. Both the XPS and the XUV-RA reveal the existence of Fe(III) species in the surface and near surface regions of the water proxy solutions. These results then provide confidence that the SFG spectral suppression of the OH stretching region in the high concentration regime is likely due to centrosymmetric Fe(III) complexation structures within the air/aqueous interface. We observe that the surface of FeCl3 solutions exhibit dramatically distinct spectral features dependent on bulk concentration. We primarily attribute the spectral features at the higher concentrations to the solvating waters of centrosymmetric Fe(III) complexes ([FeX2(H2O)4]n , X = Cl, OH, or H2O; n is the complex charge). Some of these hydrating water molecules straddle the air-aqueous interface; by contrast, the four water molecules that are covalently coordinated to Fe(III), forming a complex, have negligible SFG response because of their centrosymmetric internal arrangement, noting that to be a SFG-active vibrational mode, there must be lack of inversion symmetry. We conclude, based on the SFG, XPS, and XUV-RA measurements, that [FeX2(H2O)4]n species exist near and just below the topmost layer of the surface of aqueous FeCl3 solutions for concentrations above 1 molal; and below one molal, at 0.5 molal, Fe(III) complexes and chloride ions exist within an EDL at their air/aqueous and air/glycerol interfaces. Involvement of surface H3O+ is not completely ruled out. 3 ACS Paragon Plus Environment


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EXPERIMENTAL SECTION Materials and Sample Preparation Iron(III) chloride hexahydrate (FeCl3∙6H2O, purity > 99%) was purchased from ACROS and used as received. Solution purity was evaluated using surface tension and SFG spectroscopy. Contamination was below detection limits for all FeCl3 solutions (see Figure S1 in Supporting Information).72 Solutions were prepared to exact concentrations (mole Fe(III) per kilogram water; defined as molal or m) by mixing FeCl3∙6H2O and water (>18.0 MΩ, MilliQ) in a glass beaker. The solution was then stirred until the solid dissolved completely, which took 1 - 5 min, depending on the concentration. Glycerol (Sigma Aldrich, purity >99.5%), was used as a water proxy for SFG, XPS, and XUV-RA measurements. Note that water is introduced into the glycerol solution as FeCl3 is hydrated, i.e. FeCl3∙6H2O and therefore speciation exists, albeit, to a much lesser extent. The solutions of FeCl3 hexahydrate and glycerol were stirred until the solid was completely dissolved. This procedure took approximately 15 - 30 min depending on the concentration. The solutions were kept in dark for 3 hrs before Raman measurements. In SFG experiments, each FeCl3 solution was poured into a Petri dish immediately after stirring, then covered and kept in the dark for greater than 3 h before measurements to allow for equilibration, as the SFG intensity of aqueous solutions gradually decreased over 2 hrs. Control experiments showed that the spectral intensity could be partially recovered by slightly stirring the solution in the Petri dish after it reached equilibrium, and then it decreased again to the final state. This is suggestive of slow Fe(III) speciation at the surface. For XPS and XUV-RA measurements, solutions of FeCl3 hexahydrate and glycerol were mixed as described above. Samples were prepared by treating a Si wafer in UV ozone for 10 min to render the surface hydrophilic followed by placing a drop of glycerol solution on the Si substrate. Following UV treatment, the glycerol solution uniformly wets the Si substrate creating a thin film of solution, which is stable by eye for days. No XPS signal from Si can be detected through this layer, indicating that the film is continuous. Substrates were covered and kept in dark for 12 hrs before XPS and XUV-RA measurements. To determine to what extent evaporation of glycerol in vacuum alters the FeCl3 concentration, substrates with FeCl3 glycerol solution were prepared and weighed prior to being loaded into a vacuum chamber having a base pressure lower than 10-8 Torr. During these experiments no change to the chamber base pressure was observed. The substrates were periodically removed and weighed at time intervals up to 12 hrs to check for mass loss due to evaporation. Results show that for 2.5 molal FeCl3 solutions, less than 5% evaporation is detectable after 12 hrs. For 0.5 molal FeCl3 solutions, 37% evaporation is observed after 1 hr and 71% evaporation is observed after 12 hrs. Consequently, collection of XPS spectra for 0.5 molal FeCl3 in glycerol was completed within 1.5 hrs following sample exposure to vacuum. Based on the measured evaporation rate, the concentration of this sample was maintained between 0.5 and 0.7 molal during XPS measurements. Plots of evaporation versus time in vacuum for FeCl3 4 ACS Paragon Plus Environment

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glycerol solutions are provided in the Supporting Information (Figure S2). These results are consistent with previous studies showing that the vapor pressure of glycerol solutions are salt concentration dependent.73-74 SFG Instrumentation For the aqueous-, and separately the glycerol-, interfaces, SFG spectroscopic measurements were obtained. The broadband SFG spectrometer is similar to that which was previously reported,75-76 with slight modifications. Briefly, a regenerative Ti:sapphire amplifier (Spitfire Ace, Spectra-Physics) seeded with a sub-50 fs 800 nm pulse from a Ti:sapphire oscillator provides a ~4 W beam of 75 fs pulses and 1 kHz repetition rate. The output beam is divided by a 50:50 beam splitter. One-half is directed to an optical parametric amplifier (TOPAS-C, Light Conversion) coupled to a noncollinear difference frequency generator (NDFG, Light Conversion) to generate the broadband infrared beam. The other half is spectrally narrowed to a FWHM of 12 cm1 by an etalon (SLS Optics, United Kingdom) and is used as the visible 797 nm beam. The pulse energy is 10 μJ for the IR and 70 μJ for the visible just before the sample. The IR and visible beams are incident onto the sample surface in a co-propagating geometry at angles from the surface normal of 68° and 52°, respectively. The IR and visible beams are focused onto the sample surface with a CaF2 lens (15 cm FL), approximately 1 cm after the surface with a BK7 lens (25 cm FL), respectively. The sum frequency signal is collected in the reflected direction by a spectrometer (IsoPlane SCT 320, Princeton Instruments) and an LN2 CCD (PyLoN, 1340 × 400 pixels, Princeton Instruments). The optical table between the DFG and the sample is purged with dry N2(g) to lessen atmospheric water vapor absorption of the IR beam. Typical exposure time for one spectrum was 3 min. SSP (for SF, vis, IR) and PPP polarization combinations were used. For orientation analysis, the instrument function was taken into account by using a half-wave plate prior to the monochromator, optimizing (changing) the polarization to horizontal. An average of 3 spectra, backgroundsubtracted and normalized to the non-resonant spectrum of a z-cut quartz crystal, are presented. Raman Instrumentation A confocal Raman microscope (Renishaw InVia) was employed to obtain bulk unpolarized Raman spectra using a 632.8 nm He-Ne laser (100 mW, Renishaw RL633) with a 3 cm-1 resolution using an 1800 lines/mm grating. An internal piece of silicon was used for instrument calibration with the peak centered at 520.5 cm-1. Spectra were collected with 10 s/pixel exposure time in scanning mode. The aqueous samples were measured within a 1 × 1 cm quartz cuvette; no background subtraction was performed. XPS Instrumentation High resolution XPS analysis was performed using a Kratos Axis Ultra x-ray photoelectron spectrometer (monochromatic Al Kα X-ray source, Ephoton = 1486.6 eV). XPS spectra were measured for the Fe 2p, Cl 1s, O 1s, and C 1s lines of high 2.5 molal 5 ACS Paragon Plus Environment


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and low (0.5 m) concentrations of FeCl3 in glycerol. Atomic fractions were determined by fitting the spectra using Casa XPS software followed by normalization to the atomic sensitivity factors. The binding energy of all photoelectron spectra were referenced to adventitious carbon at 284.5 eV. XUV-RA Instrumentation To further probe the element-specific composition of the glycerol interface under vacuum, we also measured the XUV-RA spectrum at the Fe M2,3-edge. Details of the instrument and experimental method have been described previously.77-78 In short, 2.0 mJ of 800 nm light from a Spectra-Physics Spitfire ACE is focused into a semi-infinite gas cell (SIGC) filled with neon gas to generate XUV probe pulses (XUV; 36–72 eV) by a process called high harmonic generation (HHG). No even order harmonics can be generated in a centrosymmetric medium. An additional symmetry breaking field of 40 mJ pulse at 400 nm is overlapped with the 800 nm driving field in the gas interaction region to produce both even and odd harmonics. The XUV beam is focused onto the sample using a toroidal mirror with an incidence angle of 82° relative to surface normal. The XUV beam reflected from the sample is subsequently spectrally dispersed onto a CCD detector using an aberration corrected concave variable line spaced grating. The reflectance of a sample (Rsample) is measured with respect to a Si reference as given by 𝑅𝑠𝑎𝑚𝑝𝑙𝑒 =

𝐼𝑠𝑎𝑚𝑝𝑙𝑒 𝐼𝑆𝑖

where Isample and ISi are the reflected XUV flux from the sample of interest and the Si reference, respectively. To put this measured spectrum in log units and correct for the known reflectance of Si, Reflection-Absorption (RA) is calculated as 𝑅𝐴 = ―𝑙𝑜𝑔(𝑅𝑠𝑎𝑚𝑝𝑙𝑒 ∙ 𝑅𝑆𝑖) Si is selected as a reference because it has a flat reflectance with no resonant features in the spectral range of interest, and the absolute value of RSi is obtained from the Henke tables.79 The probe depth of XUV-RA spectroscopy at this near-grazing incidence angle has been previously characterized to be on the order of 3 nm.77

RESULTS AND DISCUSSION Several types of Fe(III)-complexes can be formed in aqueous FeCl3 solutions in the bulk. Using formation constants for Fe(III)-ligands, we calculated the relative abundance of all possible Fe(III)-complex species as a function of Fe(III) concentration and pH (Supporting Information, Table S1). The bulk mole fraction and abundance of these Fe(III)-complex species against the total Fe(III) concentration are shown in Figure 1a with Figure 1b showing the mole fraction as a function of molality for further clarity. Consistent with their formation constants, [FeCl2(H2O)4]+1 is the most abundant over other Fe(III)-complex species particularly at >0.5 molal Fe(III). The next most prevalent bulk species are [FeCl(H2O)5]+2 and the [FeCl3(H2O)3], followed by free Fe(III), [FeCl4]-1, [Fe(OH)(H2O)5]+2 and its dimerized form, and the hydrolysate 6 ACS Paragon Plus Environment

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[Fe(OH)2(H2O)4]+1. Free chloride ion is the most abundant ion overall.

(a)

(b)

Figure 1. Fe speciation: (a) Relative abundance as a function of mole fraction of Fe:H2O (b) Bulk mole fraction of Fe(III)-complex species against the Fe(III) molality.

Figure 2 shows the Raman spectra of water and FeCl3 solutions. The existence of chloro Fe(III)-complexes in FeCl3 solutions is confirmed by the Raman spectra at lower frequencies, where a peak at 315 cm-1 appears and its intensity grows along with increasing concentration.80-82 At higher frequencies, all spectra are dominated by a broad continuum spanning from 2800 to 3800 cm-1. In the neat water spectrum, three sub-bands centered at ~3200, ~3400 and 3630 cm-1 can be clearly resolved. The first two bands correspond to the symmetric stretch of tetrahedrally and the asymmetrically hydrogen bonded water, and the least hydrogen bonded water OH corresponds to the 3630 cm-1 Raman band.83-85 Adding FeCl3 causes the continuum to broaden, which is ascribed to the presence of H3O+ from a proton continuum in the bulk solution that is initiated by hydrolysis of coordinated waters in the Fe(III)-complexes.86-88 The 3630 cm-1 band gradually vanishes along with increasing FeCl3 concentration, which correlates with increasing Fe(III)-coordinated water molecules. The 3200 cm-1 band intensity decreases more relative to the 3400 cm-1 band, indicating weakening of the hydrogen bond network, albeit solvation shell water molecules are evident.22 The Raman intensity at 3400 cm-1 of FeCl3 solutions is lower than that of neat water, in contrast to most alkali halides solutions, in which water in the solvation shell has an enhanced Raman cross section.89-90 Decreased Raman intensity has been reported in several other chloride salt solutions and has been attributed to interactions between water and chloro metal-complexes.91 Thus, the presence of the Fe(III)-complexes are evident in both the low and high frequency spectral regions.

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Figure 2. Raman spectra of neat water and FeCl3 solutions in the low and high frequency regions upon equilibration.

We use SFG and element-specific X-Ray spectroscopies to selectively probe the topmost layers of the solution surfaces. SFG spectra of neat water and FeCl3 solutions of various concentrations obtained after 3 h of solution equilibration are shown in Figure 3a, with glycerol solution spectra in Figure 3b. (Note that there is a time evolution of the spectra suggestive of a slow surface process will be discussed in a future publication.) The neat water spectrum in Figure 3a (red, second most intense spectrum) consists of a sharp peak centered at ~ 3700 cm-1 and a broad band below 3600 cm-1, corresponding to the dangling OH at the topmost layer and hydrogen bonded water in the surface region, respectively. It has been generally accepted that water molecules with strong hydrogen bonding contribute to the lower part of the broad region, and the hydrogen bonding strength weakens as one moves to higher frequency. This is also true for the glycerol solution surface spectra. Yet more recently there is convincing evidence that alignment in the z direction of surface and subsurface water (and bulk, χ(3) effect ) from the EDL contributes to the low frequency 3200 cm-1 band for low concentration salt solutions in general.92-95 The SFG spectra from the low to high concentration FeCl3 aqueous solutions in Figure 3a are clearly different than that from the Raman spectra in the same spectral region, indicative of surface water reorganization and potentially different speciation. The spectra of 0.5 molal FeCl3 aqueous and glycerol solution surfaces show similar features to that of the neat liquid although we observe an increase in the overall SFG intensity. The peak at 3700 cm-1 is nearly identical with that of neat water, indicating that the dangling OH is almost unaffected. The stronger 3200 cm-1 band from the 0.5 molal solution is assigned to enhanced molecular ordering of solvent molecules by the EDL, where we expect the larger, more polarizable chloride (and much lower concentration of H3O+ in the aqueous solution pH~1) to have preference for the surface or near-to-surface layer and Fe(III)-complex species to prefer more bulk-phase solvation. This finding is also consistent with the 3200 cm-1 region being attributed to the χ(3) response from water ordering in the diffuse layer.92-93 A similar 3200 cm-1 enhancement has been observed for SFG of other acidic solution surfaces.86, 96-99 Yet 8 ACS Paragon Plus Environment

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the SFG phase could be reversed due to the strong surface activity of hydronium.98, 100 In addition, increase of the 3400 cm-1 band for the surface of aqueous 0.5 molal FeCl3 can be attributed to a larger SFG transition moment strength arising from an increase in the Raman polarizability of water molecules in the solvation shell of chloride.64 The solvation shell band assignment at ~3400 cm-1 is consistent with interpretation of solvation shell water by Allen et al.,22 Geissler and Saykally,64 and Ben Amotz et al.90 In noting the shape of the spectrum for the aqueous 0.5 molal FeCl3, the SFG phase appears to be consistent with several scenarios that arise from acids86, 101 and that from selected salts such as sulfate salts.12, 21 The glycerol solution spectra in Figure 3b show OH stretching mode enhancement at low FeCl3 concentration suggesting similar surface behavior as expected from prior glycerol studies on salts.52 Although the glycerol solution surface speciation of Fe(III) is an open question, we show these spectra to demonstrate the similar character of the hydrogen bonding of the OH stretch region in glycerol relative to that of water, as high vacuum XPS and XUV-RA measurements of FeCl3 in low vapor pressure glycerol solutions are obtained to help elucidate Fe speciation and oxidation state. (a)

(b)

Figure 3. ssp SFG spectra of the surface of (a) neat water and aqueous FeCl3 solution surfaces after 3 hours of solution equilibration. (b) glycerol and FeCl3 glycerol solution surfaces.

The aqueous 1.0 - 2.0 molal FeCl3 SFG spectra (Figure 3a) differ dramatically from that of neat water and the aqueous 0.5 molal FeCl3 solution surfaces. The spectral intensity below 3500 cm-1 is nearly negligible and a weak band spanning from 3500 cm-1 to 3750 cm-1 dominates the spectra. For high salt concentration SFG spectra, we expect a reduction of the EDL; however, SFG spectra in the OH stretch region typically reveal some signal due to solvation, a persistent small separation of anion and cation in the surface region giving rise to some water orientation normal to the surface plane, and in some cases from water molecules taking part in solvent shared ion pairs.12, 18, 22, 56, 102-103 Thus, this significant decrease in SFG intensity is first attributed to a complete reduction in the EDL and a lack of strongly oriented water molecules in any solvent9 ACS Paragon Plus Environment


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shared ion pair within the interface.12 Likewise, the SFG intensity of concentrated glycerol solutions decreases dramatically compared to pure glycerol as shown in Figure 3b. Therefore, we expect similar Fe speciation at aqueous and glycerol solution surfaces. Yet, suppression of the SFG signal is limited in the glycerol solution due to the higher rigidity of glycerol, disallowing exact mirroring of the hydration as that found for the aqueous solutions at higher concentrations. To shed light on this scenario, we set out to prove the hypothesis of Fe(III) speciation in the surface and near surface region of water. To do this, we employed XPS and XUV-RA spectroscopy. Glycerol was used as a proxy for water due to the high vacuum requirement for the XPS and XUV-RA measurements. Previous work has shown glycerol to be a valid water proxy for studying salt solutions. To further justify this comparison, we obtained SFG spectra from FeCl3 glycerol solutions as discussed above. XPS surface and near surface spectra were measured for FeCl3 glycerol solutions prepared at low (0.5 m) and high (2.5 m) concentrations to investigate the two speciation regimes evident from the SFG spectra shown in Figure 3 above. Spectra were collected with the sample positioned at 80° and 50° relative to the energy analyzer. Based on the mean-free path escape depth of photoelectrons at the Fe 2p and Cl 1s edges measured using an Al anode X-ray source, the probe depths for these two angles is approximately 0.7 and 7 nm, respectively. Consequently, we refer to XPS spectra obtained at 80° as surface spectra and those obtained at 50° as near-surface spectra. We identified that Fe(III) and Cl- both exist in the interfacial region at the surface and the near surface regions. Calculating the Cl:Fe atomic fraction shows that Fe(III) is enriched relative to Cl- by a factor of ~2 with slightly greater Fe(III) enrichment observed for low concentration solutions. Note that one would expect a 3:1 ratio based simply on the introduction of a FeCl3 salt into aqueous solution. XPS spectra and associated fits are provided in the Supporting Information, Figure S3, and results are summarized in Table 1. Table 1. Cl:Fe ratios of FeCl3 glycerol solutions measured by XPS. The Fe(III) enrichment factor is defined as 3 divided by the Cl:Fe(III) ratio and represents the degree to which Fe is enriched at the surface/near-surface region relative to the bulk solution. Uncertainty values show standard errors based on fits to the Fe 2p and Cl 1s XPS spectra.

Sample 0.5 m surface 0.5 m near-surface 2.5 m surface 2.5 m near surface

Cl:Fe ratio 1.43 ± 0.03 1.19 ± 0.02 1.49 ± 0.03 1.70 ± 0.03

Fe enrichment 2.10 ± 0.04 2.51 ± 0.03 2.01 ± 0.04 1.77 ± 0.03

Fitting the Fe 2p XPS spectra shows that Fe is uniformly in an Fe(III) oxidation state, and this oxidation state assignment is confirmed by the corresponding XUV-RA 10 ACS Paragon Plus Environment

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spectra at the Fe M2,3-edge. This is illustrated in Figure 4, which compares the normalized XUV-RA ground state spectra of FeCl3 thin film solution with a polycrystalline thin film Fe2O3 sample. The peak position of both the low and high concentration of FeCl3 thin films solution closely matches with a polycrystalline Fe2O3 thin film sample, showing a common resonance at 55.5 eV. This resonance peak is associated with the 3p  3d transition of Fe3+, confirming the presence of Fe3+ at the near surface region.77-78 Although the peak center is the same in all three XUV-RA spectra, there are observable differences in the line width and peak shape. These spectral differences reflect changes in local solvation structure between 0.5 and 2.5 m solutions, which influence the near edge fine structure via bonding with the Fe 3d orbitals. Because these are the first M2,3-edge spectra reported for transition metal complexes in solution, there is no precedence for a detailed interpretation of the respective solvation structures based on fine structure analysis but efforts towards this goal are ongoing. At present we find that both the XPS and the XUV-RA confirm the existence of Fe(III) species in the surface of the glycerol solutions, and based on our prior work showing the comparative nature of the glycerol and water, we then make the claim that the Fe(III) species also exists in the surface and near surface regions of the aqueous FeCl3 solutions.

Figure 4. Comparison of XUV-RA spectra of thin film FeCl3 solution for low (0.5m) and high (2.5m) concentrations with polycrystalline Fe2O3 thin film.

Now that the existence of Fe(III) and Cl- species are established with the surface sensitive X-Ray techniques, we set out to further understand the molecular details of the interfacial water arrangement, that is, the solvation shell water of Fe(III) complexes and chloride ions in the interfacial environment. For the aqueous 1.0 - 2.0 molal FeCl3 11 ACS Paragon Plus Environment


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solutions, we completed and evaluated SFG polarization studies to shed light on the OH bond orientation. In SFG, water orientation and hydrogen bonding strength can be inferred from peak position and vibrational amplitude, which can be obtained by fitting the SFG spectra with the following equation: 2

I SFG   eff

2

  NR   2

q

Aq

IR  q  i q

2

(2)

where χeff(2) is the effective second-order nonlinear susceptibility, χNR(2) is the nonresonant contribution, ωIR is the infrared frequency, and ωq, Aq and Γq are the frequency, transition moment strength, and line width of the q-th vibrational mode, respectively.104 Note that sometimes non intuitive peak positions are apparent from fitting to Equation (1) due to the interference effects. The spectral intensity of the 3600 cm-1 band is weak and several peaks are convoluted. Consequently, fitting of a single spectrum would likely lead to inaccurate results and interpretation. Polarization analysis can help improve the nonlinear optical information and fitting accuracy.105 Hence, we measured SFG spectra under ppp polarization. Then the ssp and ppp spectra of each sample were fitted simultaneously with their peak position and line width linked.106-107 The spectra are shown in Figure 5 and the fitting results are listed in Table S2 (Supporting Information). SFG spectra of neat water contain four peaks centered at 3199, 3483, 3566 and 3698 cm-1. The first two peaks are attributed to symmetric stretch of strongly and weakly hydrogen bonded water molecules, respectively. These two peaks appear only in the ssp spectrum but not in the ppp spectrum, owing to SFG polarization selection rules.106 The 3698 cm-1 peak has been unambiguously assigned to the dangling OH of water at the topmost layer.108-110 The other OH of the topmost straddling water is hydrogen bonded to the second water layer, giving rise to the 3566 cm-1 peak.106 This peak is apparent in the ppp spectrum but is covered by the broad band centered at 3483 cm-1 in the ssp spectrum. Thus, global fitting is imperative to powerfully resolve these convoluted bands.104, 107


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Figure 5. SFG spectra of neat water and concentrated FeCl3 solutions (after 3h). Red and blue data were taken under ssp and ppp polarization combinations, respectively. Solid lines are fits to eq 1 (see Table S1). Each pair of ssp and ppp spectra were fitted simultaneously with their peak position and line width linked.

Compared to neat water, the spectra of the FeCl3 solutions have much weaker intensity for the ssp and ppp polarization, yet there is structure within the spectra. The associated vibrational response, as calculated by Weare et. al., of water molecules coordinated to Fe(III) is 230 cm-1 red shifted relative to the non-coordinated OH stretch frequency.111 Upon exploration, we did not observe spectral features in the 2700-3100 cm-1 region (Supporting Information, Figure S1), indicating that water ligands that coordinate to Fe(III) are SFG inactive or their response is canceled out due to centrosymmetric arrangement. Interestingly, in previous work by Brandes et. al., SFG was used to observe the cyano ligands of the hexacyano Fe(III)-complex.112 Unlike their findings, the weak SFG signals observed in our study do not appear to be from the coordinated waters (aqua ligands) of the Fe(III)-complex species, but from the solvating water molecules (SFG active modes) as discussed below. In other words, the Fe(III)complexes are buried underneath the topmost water layer. Due to the arrangement of the solvating waters of the Fe(III)-complex, the hydrogen bonding environment on either side of the complex (i.e. near and away from the surface) is different from each other. Because solvating water molecules orient oppositely on either side of the Fe(III)-complex, the SFG response should have a 180° phase difference. We observe this phase shift in the 1.0 molal Fe(III) ssp spectrum where the amplitudes of the peaks at 3246 and 3433 cm-1 have opposite signs. This is consistent with observation of solvating water molecules of a centrosymmetric Fe(III) 13 ACS Paragon Plus Environment


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complex in the surface region, although solvating water molecules of Cl- and tetrahedral FeCl4- are not ruled out by this analysis. The band in the ssp spectra between 3500 and 3750 cm-1 are deconvoluted into two peaks centered at ~3600 and ~3660 cm-1 (3630 and 3672 cm-1 for aqueous 2.0 molal FeCl3) respectively. Since the peak position of the OH stretch serves as an indicator of the hydrogen-bonding strength, a decreasing hydrogen-bonding strength moves the OH stretch peak position toward higher frequency. Therefore, these two peaks are attributed to weakly and/or non-hydrogen bonded OH groups. At the topmost layer of neat water, water molecules that straddle the surface have one dangling OH pointing up and one hydrogen bonded OH pointing down, of which the vibrational frequencies are 3698 and 3566 cm-1 respectively. In comparing the 3600 cm-1 band from the FeCl3 solutions to the 3566 cm-1 band of neat water (Figure 5), we observe a blue shift due to a decrease in hydrogen bonding strength between the straddling water molecule and the Fe(III)complex. This is consistent with a lowering of the electron density of the ligands in the Fe(III) complex. Similar spectral features have also been observed by Uysal and coworkers, which were ascribed to weak interactions between water and PtCl6complexes.113 The 3660 cm-1 peak is assigned to the dangling OH of the same straddling water molecule. This peak from the FeCl3 solutions is red-shifted from the dangling OH peak of 3698 cm-1 at the neat water surface due to the perturbation imposed by interaction with the Fe(III)-complex, which causes reorientation of the dangling OH groups to become more parallel to the surface (Supporting Information, Figure S4). A similar red shift of the dangling OH stretch has been reported for water interacting with alkyl chains.114-117 The SFG spectra of concentrated FeCl3 solution exhibit distinctly different features compared to other inorganic salt solutions. In simple salt solutions, Cl– adsorbs at the surface yet cations such as Na+ and especially Mg2+ are repelled, giving rise to ordered water layers between oppositely charged ions, as well as solvent separated and/or solvent shared ion pairs.12 In FeCl3 solutions, however, Fe(III) and its ligands are not only separated by solvation shells but form complexes, mononuclear and polynuclear. In addition, hexaaquo Fe(III) ions likely have low surface propensity due to relatively high surface charge density; that is, perturbing the second hydration layer is energetically costly based on hydration energies of highly charged ions. Yet, an increased surface presence of complexed Fe(III) is observed here through its second solvation shell waters in which these vibrations are SFG active. As discussed directly above, centrosymmetric configurations of water molecules in the Fe(III) complex will not be SFG active. Therefore, for the high concentration aqueous FeCl3 solutions, the absence of SFG signal relative to the low concentration solutions is attributed to centrosymmetric arrangement of complexed water molecules at the surface, that is, to centrosymmetric Fe(III) complexes. Similarly, at the concentrated glycerol solution surface, the OH groups that coordinate to Fe(III) are also SFG inactive due to centrosymmetric arrangement. However, glycerol has three OH groups and not all of them are coordinated.118 As a result, the non-coordinated OH groups are SFG active and the SFG response of glycerol solution is not completely 14 ACS Paragon Plus Environment

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suppressed (Figure 3b). Based on our XPS and XUV-RA findings of Fe(III) and Cl- in the interfacial region and that Fe(III) is significantly enriched over chloride, attributing the lack of SFG signal to centrosymmetric water structures is reasonable. Free chloride is still present but does not account for the lack of SFG signal for the high concentration solutions where evidence for an EDL is also lacking. Tetrahedral FeCl4- is also expected to be surface active inferred from its large size and single negative charge;82, 119-121 however, solely attributing the SFG signal reduction to tetrahedral FeCl4- surface activity does not fully explain the dramatic drop in SFG signal. Additionally, the Cl:Fe ratio less than 3 observed by XPS is not consistent with a high surface concentration of FeCl4-. One might expect that hydronium could also be present at the surface because concentrated FeCl3 solutions have pH less than 1. However, Allen et al., and Baldelli and Shultz have shown strong SFG intensity from HCl solutions,86, 96 as is observed for our 0.5 molal solution, yet is contrary to what is observed for the aqueous 1.0 molal FeCl3 and higher concentrations here. Of the Fe(III) species in the bulk, [Fe(H2O)6]+3, [FeCl2(H2O)4]+1, and [Fe(OH)2(H2O)4]+1 are the only centrosymmetric complexed species (see Supporting Information, Figure S5). Although the [FeCl2(H2O)4]+1 complex is most abundant (next to free Cl-) in the bulk for 1.0 molal FeCl3 and higher concentration solutions, we cannot expect that speciation at the surface will be at all similar to bulk speciation due to the asymmetric hydration environment of the interface. Considering the enrichment of Fe(III) in the surface region, one might expect polynuclear Fe(III) species at the surface; however, Winter and co-workers recently found that iron-oxo oligomers are formed only when there is excess OH-.122 Depicted in Figure 6, we show possible scenarios for the low and high concentration aqueous surfaces. For the high concentration, a centrosymmetric Fe(III)complex and the SFG-active hydrating water molecules as proposed from SFG spectral analysis (spectral fit, Supporting Information, Table S2) are depicted. The solvating water molecules observed below ~3500 cm-1 have opposing phase consistent with opposite orientation in differing hydrogen bonding environments. Note that the solvating OH (attached to the dangling OH) and the dangling OH point upward, away from the bulk solution (~80 and 50 degrees from the surface normal, respectively; see Supporting Information, Figure S4) as determined by polarization analysis.



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Figure 6. Schematic illustration of molecular arrangement at the FeCl3 solution surfaces. Low concentration: Water molecules outside of the Fe complexes are not shown for clarity; Fe complexation and electrical double layer exists. High concentration: Centrosymmetric complexation (inside gray circle; X= Cl, OH, or H2O: if H2O the complex charge is +3) dominates at the surface and reduces the SFG intensity, although the chemical formula of the surface complex remains unknown. Water molecules in the topmost layer solvate the Fe(III)complex. The dangling OH groups are perturbed with their vibrational frequency red-shifted relative to the neat water surface.

CONCLUSIONS SFG, XPS, and XUV-RA spectroscopies are used to elucidate the surface of FeCl3 solutions, speciation and electric field effects. Fe(III) is unambiguously identified in the surface and in the near surface regions by XPS. The Fe(III) species is confirmed by the first ever XUV-RA measurements at a solution surface. SFG studies of aqueous solutions indicates that an electrical double layer that strongly orders water molecules persists at the 0.5 molal FeCl3 aqueous solution surface, yet this EDL is dramatically suppressed at higher FeCl3 concentrations. Fe(III) speciation is assigned to centrosymmetric (SFG inactive) Fe(III) species in the surface and near surface regions as indicated by the near complete reduction of SFG signal in the OH stretching region of the aqueous solution surfaces. Glycerol solutions, the low vapor pressure water proxy, shows similar behavior in SFG spectra, albeit to a lower extent, and confirms its use as an excellent water proxy for high vacuum X-Ray experiments. Along with the existence of the Fe(III) surface complexation and identification of surface Cl- ions, interfacial water molecules and hydrogen-bond networks are reorganized. These rearrangements cause significantly reduced spectroscopic intensities in the hydrogen bonded OH stretching spectral region. 16 ACS Paragon Plus Environment

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To summarize, we show that at a lower concentration (0.5 molal FeCl3), molecular ordering of water dipoles at the surface is enhanced by a proposed EDL of Cl–, and Fe(III)-complexation species. At higher FeCl3 concentrations (above 1 molal FeCl3), water at the surface is repressed. Moreover, we observe vibrational signatures from the few non-coordinated water molecules solvating a [FeX2(H2O)4]+ complex (X = Cl, OH, or H2O; if H2O the complex charge is +3). Free chloride ion (and hydronium) also exist in the bulk and likely in the surface region at all concentrations, yet are not responsible for the dramatic spectral changes at and above 1.0 molal concentrations. The Cl:Fe ratio also shows enhancement of Fe(III) over chloride ion in the surface although at low concentration Cl- very likely is most assuredly partly responsible for the water ordering in the EDL. Although the molecular structures of FeCl3 crystal and bulk solution have been extensively investigated, the present work for the first time reveals existence of Fe(III) species at aqueous and glycerol liquid surfaces.

ACKNOWLEDGEMENTS SFG and Raman experiments (L.L., S.M.B., T.A., K.C.N., H.C.A.) are supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, CPIMS program, under Award # DE-SC0016381. XPS and XUV-RA experiments (J.H., S.B., L.R.B.) are supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, CPIMS program, under Award # DE-SC0014051.

SUPPORTING INFORMATION IR profile and SFG spectrum of FeCl3 solution between 2700 - 3100 cm-1, glycerol evaporation rate, details of speciation diagram calculation, Fe 2p and Cl 1s XPS spectra of glycerol solutions, global fitting parameters, simulated SFG intensity against the orientational angle, centrosymmetric and noncentrosymmetric bulk concentration graph.

AUTHOR INFORMATION Corresponding Author [email protected]



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REFERENCES 1. McLennan, S. M.; Anderson, R. B.; Bell, J. F.; Bridges, J. C.; Calef, F.; Campbell, J. L.; Clark, B. C.; Clegg, S.; Conrad, P.; Cousin, A.; Des Marais, D. J., Elemental Geochemistry of Sedimentary Rocks at Yellowknife Bay, Gale Crater, Mars. Science 2013, 343, 1244734. 2. Dyar, M. D.; Jawin, E. R.; Breves, E.; Marchand, G.; Nelms, M.; Lane, M. D.; Mertzman, S. A.; Bish, D. L.; Bishop, J. L., Mossbauer Parameters of Iron in Phosphate Minerals: Implications for Interpretation of Martian Data. Am. Mineral. 2014, 99, 914942. 3. Schröder, C.; Bland, P. A.; Golombek, M. P.; Ashley, J. W.; Warner, N. H.; Grant, J. A., Amazonian Chemical Weathering Rate Derived from Stony Meteorite Finds at Meridiani Planum on Mars. Nat. Commun. 2016, 7, 13459. 4. Grotzinger, J. P.; Hayes, A. G.; Lamb, M. P.; McLennan, S. M., Sedimentary Processes on Earth, Mars, Titan, and Venus. In Comparative Climatology of Terrestrial Planets, Mackwell, S. J.; Simon-Miller, A. A.; Harder, J. W.; Bullock, M. A., Eds. University of Arizona: Tucson, 2014; pp 439-472. 5. Stumm, W.; Morgan, J. J., Aquatic Chemistry : Chemical Equilibria and Rates in Natural Waters. 3rd ed.; Wiley: New York, 1996. 6. Enami, S.; Sakamoto, Y.; Colussi, A. J., Fenton Chemistry at Aqueous Interfaces. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 623-628. 7. Ensing, B.; Buda, F.; Baerends, E. J., Fenton-Like Chemistry in Water: oxidation Catalysis by Fe(III) and H2O2. J. Phys. Chem. A 2003, 107, 5722-5731. 8. Sun, M.; Chu, C.; Geng, F.; Lu, X.; Qu, J.; Crittenden, J.; Elimelech, M.; Kim, J.H., Reinventing Fenton Chemistry: Iron Oxychloride Nanosheet for pH-Insensitive H2O2 Activation. Environ. Sci. Technol. Lett. 2018, 5, 186-191. 9. Durand-Vidal, S.; Simonin, J.-P.; Turq, P., Electrolytes at Interfaces. Kluwer Academic Publishers: New York, 2002. 10. Boily, J. F., Water Structure and Hydrogen Bonding at Goethite/Water Interfaces: Implications for Proton Affinities. J. Phys. Chem. C 2012, 116, 4714-4724. 11. Rubasinghege, G.; Lentz, R. W.; Scherer, M. M.; Grassian, V. H., Simulated Atmospheric Processing of Iron Oxyhydroxide Minerals at Low pH: Roles of Particle Size and Acid Anion in Iron Dissolution. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 66286633. 12. Gotte, L.; Parry, K. M.; Hua, W.; Verreault, D.; Allen, H. C.; Tobias, D. J., SolventShared Ion Pairs at the Air-Solution Interface of Magnesium Chloride and Sulfate Solutions Revealed by Sum Frequency Spectroscopy and Molecular Dynamics Simulations. J. Phys. Chem. A 2017, 121, 6450-6459. 13. Hua, W.; Verreault, D.; Huang, Z. S.; Adams, E. M.; Allen, H. C., Cation Effects on Interfacial Water Organization of Aqueous Chloride Solutions. I. Monovalent Cations: Li+, Na+, K+, and NH4+. J. Phys. Chem. B 2014, 118, 8433-8440. 14. Hua, W.; Verreault, D.; Allen, H. C., Surface Electric Fields of Aqueous Solutions of NH4NO3, Mg(NO3)2, NaNO3, and LiNO3: Implications for Atmospheric Aerosol Chemistry. J. Phys. Chem. C 2014, 118, 24941-24949. 15. Feng, R. R.; Guo, Y.; Wang, H. F., Reorientation of the "Free OH" Group in the 18 ACS Paragon Plus Environment

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Top-Most Layer of Air/Water Interface of Sodium Fluoride Aqueous Solution Probed with Sum-Frequency Generation Vibrational Spectroscopy. J. Chem. Phys. 2014, 141, 18C507. 16. Hua, W.; Jubb, A. M.; Allen, H. C., Electric Field Reversal of Na2SO4, (Nh4)2so4, and Na2CO3 Relative to CaCl2 and NaCl at the Air/Aqueous Interface Revealed by Heterodyne Detected Phase-Sensitive Sum Frequency. J. Phys. Chem. Lett. 2011, 2, 2515-2520. 17. Tian, C. S.; Byrnes, S. J.; Han, H. L.; Shen, Y. R., Surface Propensities of Atmospherically Relevant Ions in Salt Solutions Revealed by Phase-Sensitive Sum Frequency Vibrational Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 1946-1949. 18. Casillas-Ituarte, N. N.; Callahan, K. M.; Tang, C. Y.; Chen, X. K.; Roeselova, M.; Tobias, D. J.; Allen, H. C., Surface Organization of Aqueous MgCl2 and Application to Atmospheric Marine Aerosol Chemistry. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 6616-6621. 19. Xu, M.; Spinney, R.; Allen, H. C., Water Structure at the Air-Aqueous Interface of Divalent Cation and Nitrate Solutions. J. Phys. Chem. B 2009, 113, 4102-4110. 20. Feng, R. R.; Bian, H. T.; Guo, Y.; Wang, H. F., Spectroscopic Evidence for the Specific Na+ and K+ Interactions with the Hydrogen-Bonded Water Molecules at the Electrolyte Aqueous Solution Surfaces. J. Chem. Phys. 2009, 130, 134710. 21. Gopalakrishnan, S.; Jungwirth, P.; Tobias, D. J.; Allen, H. C., Air-Liquid Interfaces of Aqueous Solutions Containing Ammonium and Sulfate: Spectroscopic and Molecular Dynamics Studies. J. Phys. Chem. B 2005, 109, 8861-8872. 22. Liu, D. F.; Ma, G.; Levering, L. M.; Allen, H. C., Vibrational Spectroscopy of Aqueous Sodium Halide Solutions and Air-Liquid Interfaces: Observation of Increased Interfacial Depth. J. Phys. Chem. B 2004, 108, 2252-2260. 23. Schnitzer, C.; Baldelli, S.; Shultz, M. J., Sum Frequency Generation of Water on NaCl, NaNO3, KHSO4, HCl, HNO3, and H2SO4 Aqueous Solutions. J. Phys. Chem. B 2000, 104, 585-590. 24. Raymond, E. A.; Richmond, G. L., Probing the Molecular Structure and Bonding of the Surface of Aqueous Salt Solutions. J. Phys. Chem. B 2004, 108, 5051-5059. 25. Baldelli, S.; Schnitzer, C.; Shultz, M. J.; Campbell, D. J., Sum Frequency Generation Investigation of Water at the Surface of H2O/H2SO4 and H2O/Cs2SO4 Binary Systems. Chem. Phys. Lett. 1998, 287, 143-147. 26. Bian, H. T.; Feng, R. R.; Guo, Y.; Wang, H. F., Specific Na+ and K+ Cation Effects on the Interfacial Water Molecules at the Air/Aqueous Salt Solution Interfaces Probed with Nonresonant Second Harmonic Generation. J. Chem. Phys. 2009, 130, 134709. 27. Bian, H. T.; Feng, R. R.; Xu, Y. Y.; Guo, Y.; Wang, H. F., Increased Interfacial Thickness of the NaF, NaCl and NaBr Salt Aqueous Solutions Probed with NonResonant Surface Second Harmonic Generation (SHG). Phys. Chem. Chem. Phys. 2008, 10, 4920-4931. 28. Otten, D. E.; Petersen, P. B.; Saykally, R. J., Observation of Nitrate Ions at the Air/Water Interface by UV-Second Harmonic Generation. Chem. Phys. Lett. 2007, 449, 261-265. 29. Petersen, P. B.; Saykally, R. J., Probing the Interfacial Structure of Aqueous 19 ACS Paragon Plus Environment


Page 20 of 27

Electrolytes with Femtosecond Second Harmonic Generation Spectroscopy. J. Phys. Chem. B 2006, 110, 14060-14073. 30. Petersen, P. B.; Saykally, R. J.; Mucha, M.; Jungwirth, P., Enhanced Concentration of Polarizable Anions at the Liquid Water Surface: SHG Spectroscopy and MD Simulations of Sodium Thiocyanide. J. Phys. Chem. B 2005, 109, 10915-10921. 31. Petersen, P. B.; Saykally, R. J., Adsorption of Ions to the Surface of Dilute Electroyte Solutions: The Jones-Ray Effect Revisited. J. Am. Chem. Soc. 2005, 127, 15446-15452. 32. Petersen, P. B.; Saykally, R. J., Evidence for an Enhanced Hydronium Concentration at the Liquid Water Surface. J. Phys. Chem. B 2005, 109, 7976-7980. 33. Petersen, P. B.; Saykally, R. J., Confirmation of Enhanced Anion Concentration at the Liquid Water Surface. Chem. Phys. Lett. 2004, 397, 51-55. 34. Petersen, P. B.; Johnson, J. C.; Knutsen, K. P.; Saykally, R. J., Direct Experimental Validation of the Jones-Ray Effect. Chem. Phys. Lett. 2004, 397, 46-50. 35. Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, E. L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M., Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides. Science 2005, 307, 563-566. 36. Perrine, K. A.; Parry, K. M.; Stern, A. C.; Van Spyk, M. H. C.; Makowski, M. J.; Freites, J. A.; Winter, B.; Tobias, D. J.; Hemminger, J. C., Specific Cation Effects at Aqueous Solution-Vapor Interfaces: Surfactant-Like Behavior of Li+ Revealed by Experiments and Simulations. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 13363-13368. 37. Olivieri, G.; Parry, K. M.; D'Auria, R.; Tobias, D. J.; Brown, M. A., Specific Anion Effects on Na+ Adsorption at the Aqueous Solution-Air Interface: MD Simulations, SESSA Calculations, and Photoelectron Spectroscopy Experiments. J. Phys. Chem. B 2018, 122, 910-918. 38. Tissot, H.; Oivieri, G.; Gallet, J. J.; Bournel, F.; Silly, M. G.; Sirotti, F.; Rochet, F., Cation Depth-Distribution at Alkali Halide Aqueous Solution Surfaces. J. Phys. Chem. C 2015, 119, 9253-9259. 39. Brown, M. A.; Lee, M. T.; Kleibert, A.; Ammann, M.; Giorgi, J. B., Ion Spatial Distributions at the Air- and Vacuum-Aqueous K2CO3 Interfaces. J. Phys. Chem. C 2015, 119, 4976-4982. 40. Luo, G. M.; Bu, W.; Mihaylov, M.; Kuzmenko, I.; Schlossman, M. L.; Soderholm, L., X-Ray Reflectivity Reveals a Nonmonotonic Ion-Density Profile Perpendicular to the Surface of ErCl3 Aqueous Solutions. J. Phys. Chem. C 2013, 117, 19082-19090. 41. Sloutskin, E.; Baumert, J.; Ocko, B. M.; Kuzmenko, I.; Checco, A.; Tamam, L.; Ofer, E.; Gog, T.; Gang, O.; Deutsch, M., The Surface Structure of Concentrated Aqueous Salt Solutions. J. Chem. Phys. 2007, 126, 054704. 42. Brown, E. C.; Mucha, M.; Jungwirth, P.; Tobias, D. J., Structure and Vibrational Spectroscopy of Salt Water/Air Interfaces: Predictions from Classical Molecular Dynamics Simulations. J. Phys. Chem. B 2005, 109, 7934-7940. 43. Imamura, T.; Mizukoshi, Y.; Ishiyama, T.; Morita, A., Surface Structures of NaF and Na2SO4 Aqueous Solutions: Specific Effects of Hard Ions on Surface Vibrational Spectra. J. Phys. Chem. C 2012, 116, 11082-11090. 20 ACS Paragon Plus Environment

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44. Ishiyama, T.; Morita, A., Molecular Dynamics Study of Gas-Liquid Aqueous Sodium Halide Interfaces. II. Analysis of Vibrational Sum Frequency Generation Spectra. J. Phys. Chem. C 2007, 111, 738-748. 45. Ishiyama, T.; Morita, A., Molecular Dynamics Study of Gas-Liquid Aqueous Sodium Halide Interfaces. I. Flexible and Polarizable Molecular Modeling and Interfacial Properties. J. Phys. Chem. C 2007, 111, 721-737. 46. Ishiyama, T.; Morita, A., Molecular Dynamics Analysis of Interfacial Structures and Sum Frequency Generation Spectra of Aqueous Hydrogen Halide Solutions. J. Phys. Chem. A 2007, 111, 9277-9285. 47. D'Auria, R.; Tobias, D. J., Relation between Surface Tension and Ion Adsorption at the Air-Water Interface: A Molecular Dynamics Simulation Study. J. Phys. Chem. A 2009, 113, 7286-7293. 48. Thomas, J. L.; Roeselova, M.; Dang, L. X.; Tobias, D. J., Molecular Dynamics Simulations of the Solution-Air Interface of Aqueous Sodium Nitrate. J. Phys. Chem. A 2007, 111, 3091-3098. 49. Jungwirth, P.; Tobias, D. J., Molecular Structure of Salt Solutions: A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry. J. Phys. Chem. B 2001, 105, 10468-10472. 50. Muenter, A. H.; DeZwaan, J. L.; Nathanson, G. M., Collisions of Dcl with Pure and Salty Glycerol: Enhancement of Interfacial D → H Exchange by Dissolved NaI. J. Phys. Chem. B 2006, 110, 4881-4891. 51. Brastad, S. M.; Nathanson, G. M., Molecular Beam Studies of HCl Dissolution and Dissociation in Cold Salty Water. Phys. Chem. Chem. Phys. 2011, 13, 8284-8295. 52. Huang, Z. S.; Hua, W.; Verreault, D.; Allen, H. C., Salty Glycerol Versus Salty Water Surface Organization: Bromide and Iodide Surface Propensities. J. Phys. Chem. A 2013, 117, 6346-6353. 53. Tobias, D. J.; Hemminger, J. C., Getting Specific About Specific Ion Effects. Science 2008, 319, 1197-1198. 54. Jungwirth, P.; Tobias, D. J., Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259-1281. 55. Jubb, A. M.; Hua, W.; Allen, H. C., Organization of Water and Atmospherically Relevant Ions and Solutes: Vibrational Sum Frequency Spectroscopy at the Vapor/Liquid and Liquid/Solid Interfaces. Acc. Chem. Res. 2012, 45, 110-119. 56. Gopalakrishnan, S.; Liu, D. F.; Allen, H. C.; Kuo, M.; Shultz, M. J., Vibrational Spectroscopic Studies of Aqueous Interfaces: Salts, Acids, Bases, and Nanodrops. Chem. Rev. 2006, 106, 1155-1175. 57. Marcus, Y., Effect of Ions on the Structure of Water: Structure Making and Breaking. Chem. Rev. 2009, 109, 1346-1370. 58. Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J., Negligible Effect of Ions on the Hydrogen-Bond Structure in Liquid Water. Science 2003, 301, 347-349. 59. Tielrooij, K. J.; Garcia-Araez, N.; Bonn, M.; Bakker, H. J., Cooperativity in Ion Hydration. Science 2010, 328, 1006-1009. 60. Schienbein, P.; Schwaab, G.; Forbert, H.; Havenith, M.; Marx, D., Correlations in the Solute-Solvent Dynamics Reach Beyond the First Hydration Shell of Ions. J. Phys. 21 ACS Paragon Plus Environment


Page 22 of 27

Chem. Lett. 2017, 8, 2373-2380. 61. Sharma, V.; Bohm, F.; Seitz, M.; Schwaab, G.; Havenith, M., From Solvated Ions to Ion-Pairing: A THz Study of Lanthanum(III) Hydration. Phys. Chem. Chem. Phys. 2013, 15, 8383-8391. 62. Chen, Y. X.; Okur, H. I.; Liang, C. W.; Roke, S., Orientational Ordering of Water in Extended Hydration Shells of Cations Is Ion-Specific and Is Correlated Directly with Viscosity and Hydration Free Energy. Phys. Chem. Chem. Phys. 2017, 19, 2467824688. 63. Chen, Y. X.; Okur, H. I.; Gomopoulos, N.; Macias-Romero, C.; Cremer, P. S.; Petersen, P. B.; Tocci, G.; Wilkins, D. M.; Liang, C. W.; Ceriotti, M.; Roke, S., Electrolytes Induce Long-Range Orientational Order and Free Energy Changes in the H-Bond Network of Bulk Water. Sci. Adv. 2016, 2, e1501891. 64. Smith, J. D.; Saykally, R. J.; Geissler, P. L., The Effects of Dissolved Halide Anions on Hydrogen Bonding in Liquid Water. J. Am. Chem. Soc. 2007, 129, 1384713856. 65. Padmanabhan, V.; Daillant, J.; Belloni, L.; Mora, S.; Alba, M.; Konovalov, O., Specific Ion Adsorption and Short-Range Interactions at the Air Aqueous Solution Interface. Phys. Rev. Lett. 2007, 99, 086105. 66. Bera, M. K.; Luo, G. M.; Schlossman, M. L.; Soderholm, L.; Lee, S.; Antonio, M. R., Erbium(III) Coordination at the Surface of an Aqueous Electrolyte. J. Phys. Chem. B 2015, 119, 8734-8745. 67. Bera, M. K.; Antonio, M. R., Polynuclear Speciation of Trivalent Cations near the Surface of an Electrolyte Solution. Langmuir 2015, 31, 5432-5439. 68. Flynn, C. M., Hydrolysis of Inorganic Iron(III) Salts. Chem. Rev. 1984, 84, 31-41. 69. Burgess, J., Metal Ions in Solution. Halsted Press: New York, 1978. 70. Koralek, J. D.; Kim, J. B.; Brůža, P.; Curry, C. B.; Chen, Z.; Bechtel, H. A.; Cordones, A. A.; Sperling, P.; Toleikis, S.; Kern, J. F.; Moeller, S. P.; Glenzer, S. H.; DePonte, D. P., Generation and Characterization of Ultrathin Free-Flowing Liquid Sheets. Nat. Commun. 2018, 9, 1353. 71. Kleine, C.; Ekimova, M.; Goldsztejn, G.; Raabe, S.; Strüber, C.; Ludwig, J.; Yarlagadda, S.; Eisebitt, S.; Vrakking, M. J. J.; Elsaesser, T.; Nibbering, E. T. J.; Rouzée, A., Soft X-Ray Absorption Spectroscopy of Aqueous Solutions Using a TableTop Femtosecond Soft X-Ray Source. J. Phys. Chem. Lett. 2019, 10, 52-58. 72. Hua, W.; Verreault, D.; Adams, E. M.; Huang, Z. S.; Allen, H. C., Impact of Salt Purity on Interfacial Water Organization Revealed by Conventional and HeterodyneDetected Vibrational Sum Frequency Generation Spectroscopy. J. Phys. Chem. C 2013, 117, 19577-19585. 73. Chen, D. H. T.; Thompson, A. R., Isobaric Vapor-Liquid Equilibriums for the Systems Glycerol-Water and Glycerol-Water Saturated with Sodium Chloride. J. Chem. Eng. Data 1970, 15, 471-474. 74. Carr, A. R.; Townsend, R. E.; Badger, W. L., Vapor Pressures of Glycerol-Water and Glycerol-Water-Sodium Chloride Systems. Ind. Eng. Chem. 1925, 17, 643-646. 75. Adams, E. M.; Wellen, B. A.; Thiraux, R.; Reddy, S. K.; Vidalis, A. S.; Paesani, F.; Allen, H. C., Sodium-Carboxylate Contact Ion Pair Formation Induces Stabilization 22 ACS Paragon Plus Environment

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of Palmitic Acid Monolayers at High pH. Phys. Chem. Chem. Phys. 2017, 19, 1048110490. 76. Adams, E. M.; Verreault, D.; Jayarathne, T.; Cochran, R. E.; Stone, E. A.; Allen, H. C., Surface Organization of a DPPC Monolayer on Concentrated SrCl2 and ZnCl2 Solutions. Phys. Chem. Chem. Phys. 2016, 18, 32345-32357. 77. Cirri, A.; Husek, J.; Biswas, S.; Baker, L. R., Achieving Surface Sensitivity in Ultrafast Xuv Spectroscopy: M2,3-Edge Reflection–Absorption of Transition Metal Oxides. J. Phys. Chem. C 2017, 121, 15861-15869. 78. Biswas, S.; Husek, J.; Baker, L. R., Elucidating Ultrafast Electron Dynamics at Surfaces Using Extreme Ultraviolet (Xuv) Reflection–Absorption Spectroscopy. Chem. Commun. 2018, 54, 4216-4230. 79. Henke, B. L.; Gullikson, E. M.; Davis, J. C., X-Ray Interactions: Photoabsorption, Scattering, Transmission, and Reflection at E = 50-30,000 Ev, Z = 1-92. At. Data Nucl. Data Tables 1993, 54, 181-342. 80. Bohm, F.; Sharma, V.; Schwaab, G.; Havenith, M., The Low Frequency Modes of Solvated Ions and Ion Pairs in Aqueous Electrolyte Solutions: Iron(II) and Iron(III) Chloride. Phys. Chem. Chem. Phys. 2015, 17, 19582-19591. 81. Sharma, S. K., Raman Study of Ferric-Chloride Solutions and Hydrated Melts. J. Chem. Phys. 1974, 60, 1368-1375. 82. Marston, A. L.; Bush, S. F., Raman Spectral Investigation of Complex Species of Ferric Chloride in Concentrated Aqueous-Solution. Appl. Spectrosc. 1972, 26, 579-584. 83. Murphy, W. F.; Bernstein, H. J., Raman Spectra and an Assignment of Vibrational Stretching Region of Water. J. Phys. Chem. 1972, 76, 1147-1152. 84. Carey, D. M.; Korenowski, G. M., Measurement of the Raman Spectrum of Liquid Water. J. Chem. Phys. 1998, 108, 2669-2675. 85. Walrafen, G. E., Raman Spectral Studies of Effects of Temperature on Water Structure. J. Chem. Phys. 1967, 47, 114-126. 86. Levering, L. M.; Sierra-Hernandez, M. R.; Allen, H. C., Observation of Hydronium Ions at the Air - Aqueous Acid Interface: Vibrational Spectroscopic Studies of Aqueous HCl, HBr, and HI. J. Phys. Chem. C 2007, 111, 8814-8826. 87. Thamer, M.; De Marco, L.; Ramasesha, K.; Mandal, A.; Tokmakoff, A., Ultrafast 2D IR Spectroscopy of the Excess Proton in Liquid Water. Science 2015, 350, 78-82. 88. Kim, J.; Schmitt, U. W.; Gruetzmacher, J. A.; Voth, G. A.; Scherer, N. E., The Vibrational Spectrum of the Hydrated Proton: Comparison of Experiment, Simulation, and Normal Mode Analysis. J. Chem. Phys. 2002, 116, 737-746. 89. Ahmed, M.; Singh, A. K.; Mondal, J. A.; Sarkar, S. K., Water in the Hydration Shell of Halide Ions Has Significantly Reduced Fermi Resonance and Moderately Enhanced Raman Cross Section in the OH Stretch Regions. J. Phys. Chem. B 2013, 117, 9728-9733. 90. Perera, P. N.; Browder, B.; Ben-Amotz, D., Perturbations of Water by Alkali Halide Ions Measured Using Multivariate Raman Curve Resolution. J. Phys. Chem. B 2009, 113, 1805-1809. 91. Wall, T. T.; Hornig, D. F., Raman Spectra of Water in Concentrated Ionic Solutions. J. Chem. Phys. 1967, 47, 784-792. 23 ACS Paragon Plus Environment


Page 24 of 27

92. Ohno, P. E.; Wang, H. F.; Paesani, F.; Skinner, J. L.; Geiger, F. M., Second-Order Vibrational Lineshapes from the Air/Water Interface. J. Phys. Chem. A 2018, 122, 4457-4464. 93. Pezzotti, S.; Galimberti, D. R.; Shen, Y. R.; Gaigeot, M. P., Structural Definition of the BIL and DL: A New Universal Methodology to Rationalize Non-Linear Chi((2))(Omega) SFG Signals at Charged Interfaces, Including Chi((3))(Omega) Contributions. Phys. Chem. Chem. Phys. 2018, 20, 5190-5199. 94. Reddy, S. K.; Thiraux, R.; Rudd, B. A. W.; Lin, L.; Adel, T.; Joutsuka, T.; Geiger, F. M.; Allen, H. C.; Morita, A.; Paesani, F., Bulk Contributions Modulate the SumFrequency Generation Spectra of Water on Model Sea-Spray Aerosols. Chem 2018, 4, 1629-1644. 95. Ishiyama, T.; Shirai, S.; Okumura, T.; Morita, A., Molecular Dynamics Study of Structure and Vibrational Spectra at Zwitterionoic Lipid/Aqueous KCl, NaCl, and CaCl2 Solution Interfaces. J. Chem. Phys. 2018, 148, 222801. 96. Baldelli, S.; Schnitzer, C.; Shultz, M. J., The Structure of Water on HCl Solutions Studied with Sum Frequency Generation. Chem. Phys. Lett. 1999, 302, 157-163. 97. Baldelli, S.; Schnitzer, C.; Shultz, M. J.; Campbell, D. J., Sum Frequency Generation Investigation of Water at the Surface of H2O/H2SO4 Binary Systems. J. Phys. Chem. B 1997, 101, 10435-10441. 98. Tian, C. S.; Ji, N.; Waychunas, G. A.; Shen, Y. R., Interfacial Structures of Acidic and Basic Aqueous Solutions. J. Am. Chem. Soc. 2008, 130, 13033-13039. 99. Raduge, C.; Pflumio, V.; Shen, Y. R., Surface Vibrational Spectroscopy of Sulfuric Acid-Water Mixtures at the Liquid-Vapor Interface. Chem. Phys. Lett. 1997, 274, 140144. 100. Hua, W.; Verreault, D.; Allen, H. C., Relative Order of Sulfuric Acid, Bisulfate, Hydronium, and Cations at the Air-Water Interface. J. Am. Chem. Soc. 2015, 137, 13920-13926. 101. Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P., Unified Molecular Picture of the Surfaces of Aqueous Acid, Base, and Salt Solutions. J. Phys. Chem. B 2005, 109, 7617-7623. 102. Callahan, K. M.; Casillas-Ituarte, N. N.; Xu, M.; Roeselova, M.; Allen, H. C.; Tobias, D. J., Effect of Magnesium Cation on the Interfacial Properties of Aqueous Salt Solutions. J. Phys. Chem. A 2010, 114, 8359-8368. 103. Xu, M.; Tang, C. Y.; Jubb, A. M.; Chen, X. K.; Allen, H. C., Nitrate Anions and Ion Pairing at the Air-Aqueous Interface. J. Phys. Chem. C 2009, 113, 2082-2087. 104. Wang, H. F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B. H., Quantitative Spectral and Orientational Analysis in Surface Sum Frequency Generation Vibrational Spectroscopy (SFG-Vs). Int. Rev. Phys. Chem. 2005, 24, 191-256. 105. Simpson, G. J., Nonlinear Optical Polarization Analysis in Chemistry and Biology. Cambridge University Press: New York, 2016. 106. Gan, W.; Wu, D.; Zhang, Z.; Feng, R. R.; Wang, H. F., Polarization and Experimental Configuration Analyses of Sum Frequency Generation Vibrational Spectra, Structure, and Orientational Motion of the Air/Water Interface. J. Chem. Phys. 2006, 124, 114705. 24 ACS Paragon Plus Environment

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


107. Dutta, C.; Benderskii, A. V., On the Assignment of the Vibrational Spectrum of the Water Bend at the Air/Water Interface. J. Phys. Chem. Lett. 2017, 8, 801-804. 108. Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R., Vibrational Spectroscopy of Water at the Vapor Water Interface. Phys. Rev. Lett. 1993, 70, 2313-2316. 109. Brown, M. G.; Raymond, E. A.; Allen, H. C.; Scatena, L. F.; Richmond, G. L., The Analysis of Interference Effects in the Sum Frequency Spectra of Water Interfaces. J. Phys. Chem. A 2000, 104, 10220-10226. 110. Medders, G. R.; Paesani, F., Dissecting the Molecular Structure of the Air/Water Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum. J. Am. Chem. Soc. 2016, 138, 3912-3919. 111. Bogatko, S. A.; Bylaska, E. J.; Weare, J. H., First Principles Simulation of the Bonding, Vibrational, and Electronic Properties of the Hydration Shells of the HighSpin Fe3+ Ion in Aqueous Solutions. J. Phys. Chem. A 2010, 114, 2189-2200. 112. Brandes, E.; Karageorgiev, P.; Viswanath, P.; Motschmann, H., Breaking the Symmetry of Ions at the Air-Water Interface. J. Phys. Chem. C 2014, 118, 26629-26633. 113. Rock, W.; Qiao, B. F.; Zhou, T. C.; Clark, A. E.; Uysal, A., Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface. J. Phys. Chem. C 2018, 122, 29228-29236. 114. Ma, G.; Chen, X. K.; Allen, H. C., Dangling OD Confined in a Langmuir Monolayer. J. Am. Chem. Soc. 2007, 129, 14053-14057. 115. Mondal, J. A.; Nihonyanagi, S.; Yamaguchi, S.; Tahara, T., Three Distinct Water Structures at a Zwitterionic Lipid/Water Interface Revealed by HeterodyneDetected Vibrational Sum Frequency Generation. J. Am. Chem. Soc. 2012, 134, 78427850. 116. Scatena, L. F.; Richmond, G. L., Orientation, Hydrogen Bonding, and Penetration of Water at the Organic/Water Interface. J. Phys. Chem. B 2001, 105, 11240-11250. 117. Scatena, L. F.; Brown, M. G.; Richmond, G. L., Water at Hydrophobic Surfaces: Weak Hydrogen Bonding and Strong Orientation Effects. Science 2001, 292, 908-912. 118. Mori, Y.; Yokoi, H., Studies on the Interaction between Iron(III) and Glycerol or Related Polyols over a Wide pH Range. Bull. Chem. Soc. Jpn. 1994, 67, 2724-2730. 119. Liu, W.; Etschmann, B.; Brugger, J.; Spiccia, L.; Foran, G.; McInnes, B., UV– Vis Spectrophotometric and XAFS Studies of Ferric Chloride Complexes in HyperSaline LiCl Solutions at 25–90 °C. Chem. Geol. 2006, 231, 326-349. 120. Murata, K.; Irish, D. E.; Toogood, G. E., Vibrational Spectral Studies of Solutions at Elevated-Temperatures and Pressures .11. A Raman Spectral Study of Aqueous Iron(III) Chloride Solutions between 25-Degrees-C and 300-Degrees-C. Can. J. Chem. 1989, 67, 517-524. 121. Magini, M.; Radnai, T., X-Ray-Diffraction Study of Ferric-Chloride Solutions and Hydrated Melt - Analysis of the Iron (III)-Chloride Complexes Formation. J. Chem. Phys. 1979, 71, 4255-4262. 122. Seidel, R.; Kraffert, K.; Kabelitz, A.; Pohl, M. N.; Kraehnert, R.; Emmerling, F.; Winter, B., Detection of the Electronic Structure of Iron-(III)-Oxo Oligomers 25 ACS Paragon Plus Environment


Page 26 of 27

Forming in Aqueous Solutions. Phys. Chem. Chem. Phys. 2017, 19, 32226-32234.


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Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces

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