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New Information on the Ion-Identity Dependent Structure of Stern Layer Revealed by Sum Frequency Generation Vibrational Spectroscopy Kaitlin A. Lovering, Allan K. Bertram, and Keng C. Chou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05564 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016

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The Journal of Physical Chemistry

New Information on the Ion-Identity Dependent Structure of Stern Layer Revealed by Sum Frequency Generation Vibrational Spectroscopy Kaitlin A. Lovering, Allan K. Bertram*, and Keng C. Chou* Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada

Corresponding Authors

*

Allan K. Bertram: [email protected];

*

Keng C. Chou: [email protected]

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ABSTRACT

We observed that the structure of water in the Stern layer depends on the identity of the cation, which cannot be predicted with classical double layer models. The ability of a cation to displace the hydration water on silica surface is in the order of Mg2+ > Ca2+ > Li+ > Na+, which is consistent with the trend of the acid dissociation constant (Ka) of the salt. Our data suggests that ions with a high pKa, such as Mg2+ and Ca2+, have a local electrostatic field strong enough to polarize water molecules in the hydration shells of the ions which form linkages with the negative charges on the silica: Si-O− -- H+δ -- OH− δ -- M2+ and displace the hydration water on silica surface. Ions with a low pKa, such as such as Na+ and Li+, are unable to displace the hydration water on silica surface.


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INTRODUCTION The structure and interaction of water and ions at mineral aqueous interfaces is important in a range of fields including atmospheric chemistry1-2, geology3-4, and material science5-7. For example, the presence of ions at the interface may inhibit ice nucleation8-11, increase mineral dissolution12, and change surface reaction rates6. Silica is a common mineral and understanding the interactions between silica, water, and ions is important for atmospheric and geological processes. However, because it is difficult to probe buried interfaces, few techniques are available for studying these interactions. Second-order nonlinear optical techniques, such as second-harmonic generation (SHG) and sum frequency generation (SFG), have been shown to be effective for characterizing buried aqueous mineral interfaces.13 Earlier SHG studies by Ong et al. showed that the silica surface is deprotonated (SiOH ↔ SiO− + H+) at higher pH values.14 The deprotonation of the silanol groups creates negative surface charges on silica, and the surface electric field induces an ordered interfacial water structure, which can be observed by second-order nonlinear spectroscopy.15 Ong's SHG study also inferred that there were two different types of silanol sites with pKa values of 4.5 and 8.5.14 A more recent SFG study by Ostroverkhov et al. suggested that these two deprotonation sites were related to the two water species seen as bands near 3200 and 3400 cm-1, referred to as ‘ice-like’ and ‘liquid-like’ water peaks, respectively.16 The detailed structure of water in contact with a silica surface, however, remains a subject of debate.17-22 Furthermore, the interactions between ions and the silica surface are not well understood. When ions are present in water, the columbic interaction between the negatively charged silica surface and the cations perturbs the ordered water structures.11,14-16 While the intensity of the 3 ACS Paragon Plus Environment


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SFG signal is always reduced in the presence of salts, the extent of reduction depends on the concentration of the salts and the specific effects of each ion (See Supporting Information for examples). As silica has a surface charge,23 double layer models are often used to understand the effects of ions.22 At concentrations less than ~ 0.1 M the Gouy-Chapman (GC) model can be used.24 The GC model predicts that the surface potential decreases exponentially from the surface, and the thickness of the diffuse layer is characterized by the Debye length, which is a function of the ionic strength. However, because it treats ions as point charges, the GC model fails at high ion concentrations and over predicts the number of ions that are capable of approaching the surface. At high concentrations, the Stern-Gouy-Chapman model (SGC), a modification to the GC model, has been used to explicitly account for ion-surface interactions. In the SGC model, ions at the surface are given a finite radius and occupy the Stern Layer, while ions in the diffuse layer are accounted for with the GC model, as shown in figure 1.

Figure 1: Schematic representation of the Stern-Gouy-Chapman model. The Stern layer consists of ions specifically interacting with the surface. Beyond the Stern layer the ions are distributed in a diffuse Gouy-Chapman layer.

The inclusion of the Stern layer, however, does not provide insight into the specificity of ion interactions, the mechanism of these interactions, or the structure of water in this layer. 4 ACS Paragon Plus Environment

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Furthermore, although many SFG studies examine the signal of water at water/mineral interfaces, the mechanism of the ion-solid interaction in the Stern layer has not been clearly understood.3-4, 25 A recent study by Eftekhari-Bafrooei et al. measured the ultrafast vibrational relaxation of water at the water/silica interface and suggested that, for NaCl concentrations greater than 10-2 M, the hydrated cations accumulated near the silica surface with one or more layers of water between the silica and cations. These sandwiched aqueous layers dominated the observed vibrational relaxation dynamics.26 In addition, Dewan et al. carried out molecular dynamics (MD) simulations of alkali chloride solutions to study the structure of water in the solvation shell of the silica surface. Dewan et al. found that, in addition to a diffuse region consistent with the GC model, there was a “compact layer” of water next to the silica surface that was not described by standard electrical double layer theories.27 Interestingly, the depth of the “compact layer” was found to be different for Na+ vs Cs+ while the depth of the diffuse layer was independent of the identity of the cation screening the charge. The MD simulation suggested that the structure of water in the Stern layer (or the “compact layer”) depended on the specific ion properties but direct experimental observation of these specific ion effects in the Stern layer is lacking. To address this deficiency, we carried out a SFG study at the water/silica interface using solutions with high concentrations of ions. At the high concentrations used, ions readily partition to the silica surface, surface charge screening is maximized, and the thickness of the diffuse layer is minimized. Consequently, the SFG signal from ordered water in the diffuse layer is depressed, and the SFG signal from the Stern layer dominates. The studies were carried out with solutions of NaCl, LiCl, CaCl2 and MgCl2, and the effect of the cations was compared. The experiments allowed us to investigate how the structure of water in the Stern layer depends on the identity of 5 ACS Paragon Plus Environment


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the cation and to infer the nature of the mineral-cation interaction. We found that the structure of water in the Stern layer is indeed dependent on the identity of the cation. For example, ordered water was always observed with NaCl solutions, even at a saturation concentration (~6 M), but MgCl2 solutions at the same ionic strength (2M) nearly destroyed the ordered water structures. While our results confirm Dewan’s MD simulations, suggesting that the nature of the “compact layer” is dependent on ion identity, existing models used to explain the surface charge screening observed by SFG could not fully explain our experimental data. However, the ‘localized hydrolysis’ of the water molecules, proposed by Robinson and Harned in 1941 to explain trends in the chemical activities of proton acceptors with different cations, is a plausible mechanism governing the structures of water and ions near the charged mineral surface. EXPERIMENTAL SECTION Silica Surface Preparation. An equilateral IR-grade fused silica prism was washed with Extran AP12 cleaning agents, rinsed with Millipore water (resistivity > 18.2 MΩ·cm), and left to soak in concentrated sulfuric acid containing NOCHROMIX (Godax Laboratories, Inc. USA) for several hours. The prism was subsequently rinsed with Millipore water several times, soaking in the Millipore water for at least thirty minutes between each rinse. To prevent exposure to air during the course of the experiment, the prism was attached to a flow cell that allowed the addition of each solution without removing the prism. Solution Preparation. The solutions used for the experiment were prepared using Millipore water, and the salts were purchased from Sigma Aldrich at the highest available purity: ammonium chloride 99.5%+, sodium chloride ≥99.5 %, calcium chloride ≥ 96 %, and


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magnesium chloride ≥98 %. All glassware used in the preparation of the solutions was cleaned using the method described for the surface preparation. SFG Setup. The visible and IR laser beams for SFG vibrational spectroscopy were obtained from a Nd:YAG (yttrium aluminum garnet) laser with output wavelength of 1064 nm (30 ps, 40 mJ/pulse, and 10 Hz). The laser was used to generate a second harmonic beam at 532 nm in a KTiOPO4 (KTP) crystal. The tunable IR beam was produced by difference frequency mixing of the 1064 nm beam with the output of a KTP optical parametric generator/amplifier pumped by the 532 nm beam. The 532 nm and IR beams were overlapped, both spatially and temporally, on the sample. The energy of the laser beams were both ~ 200 µJ/pulse for the visible and IR beams. The polarizations of the beams were s-, s-, and p-polarized for SFG, visible, and IR, respectively. The incident angles of the IR and the visible beam are 60 and 65 degree, respectively. The SFG intensity was detected by a photomultiplier tube after spatial filtering by an aperture and spectral filtering by a bandpass filter. Each spectrum is an average of 300 laser shots per step. RESULTS AND DISCUSSION To demonstrate that the structure of water near the silica surface is not solely determined by the ionic strength, Figure 2 shows the SFG spectra of aqueous/silica interfaces with NaCl, LiCl, CaCl2, and MgCl2 at an ionic strength of 6. The ionic strength corresponds to salt concentrations of 2 M for the solutions of the divalent cations (MgCl2 and CaCl2) and 6 M for the solution of the monovalent cations (NaCl and LiCl), which is the saturation concentration of NaCl at 20 ºC. A SFG spectrum of pure water at the water-silica interface is included for reference. The peak at 3200 cm-1 in the pure water spectrum has been interpreted as a strongly 7 ACS Paragon Plus Environment


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hydrogen-bonded network and the peak at 3400 cm-1 has been interpreted as a weakly hydrogenbonded network.20 The peak at ~3600 cm-1 has been attributed to OH groups on the surface of the silica that weakly interact with the solution or an asymmetric OH stretch.15, 22 These are likely OH groups sitting on small surface detect sites as this peak is correlated with surface roughness and is consequently less affected by the addition of ions as these water molecules are less accessible to attenuation.

Figure 2. Spectra of pure water and the salt solutions at ionic strength of six. The concentration of the salt solutions of the divalent cations (MgCl2 and CaCl2) is 2 M and the concentration of the salt solutions of the monovalent cation (NaCl and LiCl) is 6 M.

The spectra of NaCl and LiCl in Figure 2 support the prediction by Dewan’s MD simulations showing that the structure of water in the Stern layer near the silica surface is dependent on the identity of the monovalent cation. The spectra of MgCl2 and CaCl2 indicate that specific ion effects also occur with divalent ions. The difference between the spectrum of NaCl and MgCl2 is striking. While an ordered water layer was still observable at 6M of NaCl, 2 M of MgCl2 nearly eliminated the ordered water structure at the surface. In the following we first propose a mechanism to explain the elimination of the ordered water structure at the surface for 8 ACS Paragon Plus Environment

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2 M of MgCl2, and then we show that this mechanism is consistent with the trends observed for the different cations. In one of the few other SFG studies that have been carried out to study water structures in solutions with high ionic strength, Yang et al. observed that a high concentration of sulfuric acid caused the disappearance of the ordered water structure on mica. It was thought that the sulfate ions tie up all the free water molecules at the surface, essentially dehydrating the silica. In Yang’s study, the addition of sulfuric acid neutralized the surface charge of the mineral surface. The neutralized mineral surface binds water less tightly and the sulfate ions are able to fully outcompete the mica for solvating water molecules. In the current study, the addition of MgCl2, or any of the salts studied, does not significantly alter the pH, and the silica surface remains negatively charged. It is consequently less likely that the silica surface would completely give up its solvation water molecules. Another possible but unlikely mechanism to explain the near elimination of the ordered water structure at the surface for 2 M of MgCl2 is the higher charge density cation Mg2+ is able to penetrate the solvation shell of the charged silica surface and directly bind with Si-O−, as shown in Figure 3a. However, direct binding between Si-O− and Mg2+ is unlikely because Mg2+ ions have large enthalpies of hydration.28-29 It is improbable that the associated water molecules would dissociate spontaneously from the ions. Although the interaction between cations and SiOis attractive, experimental evidence indicates that a direct interaction between the ion and surface cannot compensate for the loss of the also favorable solvent-ion interactions. Indeed the retention of the solvation shell and outer sphere interactions, in which the solvation shells of the ion and surface interact rather than the ion and surface directly, has been reported.27, 30-31 Additionally, it has been shown that the dissolution of silica is faster in the presence of Na+ ions than in the 9 ACS Paragon Plus Environment


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presence of Mg2+ ions.12, 32 If Mg2+ ions were forming a chemical bond with the surface, the opposite would be true because the additional charge density of the Mg2+ ion would weaken the Si-O bond more than the Na+ ion, making that bond more vulnerable to nucleophilic attack.32 Indeed, Dove et al. state that ‘the ability of cations to promote negative charge development on silica surfaces is directly correlated with the solvent-structuring character of the cation’.12 For these reasons, it is unlikely that the disappearance of the water peaks can be attributed to direct binding of the ions to the silica surface. While direct binding is unlikely, the structure in the Stern layer in the presence of Mg2+ cannot be as simple as the solvated ions interacting with solvated silica surface, as shown in Figure 3b, since this case should produce oriented water molecules between silica and cations and give substantial signal in the SFG spectrum and cannot explain how the ordered water is eliminated in MgCl2 solutions.

Figure 3. Three possible structures of the Stern layer for the case of Mg cations. (a) Both hydration shells of the silica and ions are lost, and Si-O− - M+ is formed. (b) The hydration shells of both the ion and the silica surface are intact. (c) The hydrating water molecules are shared between Si-O− and M+ and highly polarized O-H bonds are formed.


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A possible explanation for the lack of ordered interfacial water structure in the presence of Mg2+, despite retention of the solvation shell by the cations, is that the surface deprotonated silanol and Mg2+ cation can be considered to be an ion-pair with a water molecule as an intermediary; a water molecule between the surface and the ion undergoes ‘localized hydrolysis’ in which the hydronium ion is shared.33 Robinson and Harned first described this water mediated ion-pair in order to explain the reversal of chemical activity trends for the alkali metals in the presence of different anions.34 In this model, as illustrated in figure 3c, the ability of the cation to polarize water leads to the localized (or partial) hydrolysis of water and concomitant association of the localized hydrogen ion with a strong proton acceptor, schematically depicted for our case as Si-O− -- Hδ+-- OHδ−-- Mg2+. The dipole of water in gas phase is 1.8 D, and its value increases significantly in the hydration shells of ions. With 6 water molecules in the first hydration shell, Bucher et al. reported that the dipole of water is 2.35 D for K+ and 3.06 D for Ca2+.35 Near a neutral pH, 15-20% of the surface silanol groups are deprotonated,36 and this concentration is sufficient for significant sharing of the partially hydrolyzed water between the cations and SiO−. Water in the hydration shell has been shown to be vibrationally decoupled from its neighbors.37 The vibrational resonance of [O−-- Hδ+-- OHδ−] is expected to be low, as previous studies by Diken et al., using argon predissociation spectroscopy, showed; the vibrational mode of a similar system [HO--H--OH]− is located at 697 cm-1,38 which is outside the detection range of SFG. Although the interaction between the surface SiO− and the water in the hydration shell breaks the centrosymmetric structure of the hydration shell and may produce some SFG signal, this SFG signal is negligible compared to that of ordered water structure near a charged surface with incomplete charge screening by monovalent cations.



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The ion-pair-induced ‘localized hydrolysis’ model proposed by Robinson and Harned predicts that the equilibrium of the localized hydrolysis should be determined by the acid dissociation constant (pKa) of the ion. Table 1 shows that the order of pKa for the cations is Mg2+ < Ca2+ < Li+ < Na+. The spectra in Figure 2 are reasonably consistent with this trend with the amount of structured water in the spectrum having a trend of Na+ > Li+ > Ca2+ > Mg2+. This order is different from that seen by Flores et al. using SFG at a low ionic strength, which indicated a SFG intensity in the order of Li+ > Na+ > Ca2+ > Mg2+.39 The order found in the current study exchanges the position of Ca2+ and Mg2+ from the order commonly given for the Hofmeister series.40-41

Na+

Radius42 (pm) 116

Li+

90

14.2 Na+(OH2)n ↔Na(OH2)n-1OH + H+

Ca2+

114

Mg2+

86

13.4 Ca2+(OH2) ↔[Ca(OH2)n-1OH]+ + H+ 12.5 Mg2+(OH2) ↔[Mg(OH2)n-1OH]+ + H+

Ion

Acid dissociation constant, pKa43 14.7 Na+(OH2)n ↔Na(OH2)n-1OH + H+

Table 1. The metal cations facilitate the hydrolysis of water. The ability of the metal cation to perturb surface water structure follows the trend in decreasing pKa for the hydrolysis reaction.

To further test if the localized hydrolysis mechanism is consistent with the SFG spectra, we carried out SFG spectra with 12 M of LiCl (I = 12) and 4 M of CaCl2 (I = 12), shown in Figure 4. Experiments with concentrations of NaCl beyond 6 M were not possible as the solution was saturated at this concentration. Figure 4 illustrates that the amount of ordered water was Li+ > Ca2+ at an ionic strength of 12, consistent with the trends in the pKa values given in Table 1. At 12 M (mole fraction of LiCl ~ 0.18), there are on average only 5-6 water molecules 12 ACS Paragon Plus Environment

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available to solvate one pair of Li+ and Cl−. Even at this extremely high concentration, ordered water structures could still be observed, suggesting that Li+ ions are not able to interact with the solvation water of the silica surface in the way Mg2+ ions can at a much lower concentration. On the other hand, the spectrum of CaCl2 solution with I = 12 (figure 4) shows that higher concentration of Ca2+ was able to nearly destroy the solvation water on silica surface.

Figure 4. The spectra of water with CaCl2 and LiCl at an ionic strength of 12, concentration of 4 M and 12 M, respectively. A spectrum of pure water and a spectrum of CaCl2 at ionic strength (I) of 6 (M=2) are included for reference. At the ionic strength of 12 the spectrum of CaCl2 is seen to be similar to the spectrum of the MgCl2 solution at an ionic strength of 6 (Figure 2). The LiCl solution still shows some ordered water structure, despite the high concentration of LiCl.

A higher concentration of CaCl2 is needed to cause a similar effect as solutions of MgCl2 because the pKa of hydrolysis of water by Ca2+ ions is higher than the pKa of hydrolysis of water by Mg2+ ion. The ability to perturb water structure follows the trend of pKa constants. The ‘localized hydrolysis’ model also successfully explains the SFG spectra of NaCl and LiCl 13 ACS Paragon Plus Environment


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solutions. The monovalent cations, with higher hydrolysis constants, do not polarize the oxygenhydrogen bond of water as drastically and do not promote the formation of the water mediated ion-pairs with the silica surface. Instead both the surface and the monovalent cations maintain relatively independent solvation shells, and the vibrational resonances of these water molecules are seen in the spectra of the NaCl and LiCl solutions. While at lower concentration (0.10 mM), Na+ ions were seen to affect surface water structure more than Li+ ions,39 in our study we found that Li+ ions perturb the water structure more than Na+ ions at high concentration (Figure 2). CONCLUSIONS Studies of the Stern layer with high concentration of ions at water/silica interfaces using sum frequency generation vibrational spectroscopy reveal that the structure of water in the Stern layer depends on the identity of the cation. We found that the observed residual SFG signal of water in the presence of salts is consistent with the acid dissociation constant of the salt. We proposed a new ion-identity dependent model for the electrical double layer based on the hypothesis of “localized hydrolysis” described by Robinson and Harned. In this model, the protons of the water molecules in the hydration shells of the ions form linkages with the negative charges on the silica: Si-O− -- H+ -- OH− -- M2+. Our results complement Dove’s hypothesis that entropy and solvent structure largely determine ion-mineral interactions by providing molecularlevel insight into how the solvent shell is interacting with the surface.44 The data suggested that divalent cations, such as Mg2+ and Ca2+, have a local electrostatic force strong enough to cause localized hydrolysis while the monovalent cations, such as Li+ and Na+, do not.

ASSOCIATED CONTENT


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Supporting Information. Additional SFG spectra of salt solutions. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This work was financially supported by the Natural Sciences and Engineering Research Council of Canada. REFERENCES 1.

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