pH-Dependent Inversion of Hofmeister Trends in the Water Structure

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Letter

pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer Emma L DeWalt-Kerian, Sun Kim, Md. Shafiul Azam, Hongbo Zeng, Qingxia Liu, and Julianne Montfort Gibbs J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 5, 2017

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The Journal of Physical Chemistry Letters

pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer ‡

Emma L. DeWalt-Kerian†, Sun Kim†, Md. Shafiul Azam , Hongbo Zeng§, Qingxia Liu§, Julianne M. Gibbs†* †

University of Alberta, Department of Chemistry, 11227 Saskatchewan Drive, Edmonton,

Canada ‡ Bangladesh University of Engineering and Technology (BUET), Department of Chemistry, Dhaka, Bangladesh § University of Alberta, Department of Engineering, 11227 Saskatchewan Drive, Edmonton, Canada * [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Specific ion effects (SIEs) are known to influence the acid/base behavior of silica and the interfacial structure of water, yet evidence of the effect of pH on SIEs is lacking. Here broadband vibrational sum frequency generation (SFG) spectroscopy was used to study SIEs on the water structure at the electrical double layer (EDL) of silica as a function of pH and monovalent cation identity from pH 2-12 at 0.5 M salt concentration. SFG results indicate a direct Hofmeister series of cation adsorption at pH 8 (Li+ < Na+ < K+ < Cs+), with an inversion in this series occurring at pH >10. In addition, an inversion in SFG intensity trends also occurred at pH < 6, which was attributed to contributions from asymmetric cation hydration and EDL overcharging. The highly pH-dependent SIEs for silica/ water have implications for EDL models that often assume pH independent parameters.

TOC GRAPHIC:

KEYWORDS: Specific ion effects, silica, interface, sum frequency generation, spectroscopy


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The chemistry of water at the mineral/water interface is complex, largely due to deviations in the behavior of water molecules at the interface from their counterparts in the bulk liquid.1-4 In particular, the solution pH and electrolyte composition are among the most important parameters that affect the behavior of water at the silica/water interface. Many studies have examined the effect of pH and electrolyte composition on dissolution rates of silica and quartz, 513

with possible mechanisms of dissolution related to cation hydration and changes in water

orientation at the silica/aqueous interface.14,15 Consequently, elucidating the structural properties of water at the silica/ aqueous interface is crucial in understanding the interfacial phenomena that play significant roles in chemical separations as well as geochemical, petrochemical, environmental and biological processes.16-23 To understand the effect of ions on the molecular structure at the charged silica/water interface, double-layer and triple-layer models are often used, such as the Gouy-Chapman-SternGrahame (GCSG) model that is most relevant for silica/water interfaces in electrolyte solutions of intermediate ionic strength.24-27 According to the GCSG model, water molecules that lie between the negatively charged silica surface (0-plane) and the first layer of hydrated cations non-specifically adsorbed at the outer Helmholtz plane (OHP) make up the compact region of the electrical double layer (EDL), also known as the Stern layer. Excess cations in the diffuse layer further screen the negative surface charge of the silica until electroneutrality is reached. From a simple electrostatic model of water alignment, the potential at the OHP (ΦOH), which is approximately equal to the experimentally observable zeta potential, aligns water molecules in the diffuse layer. In general, as cations are added charge screening occurs resulting in a smaller |ΦOH|, although the surface charge density typically increases with salt addition.28-32 In pure water the isoelectric point (IEP) of silica (where ΦOH = 0) is the same as its point of zero charge (pH 2-


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3), however cation charge screening can shift the IEP to higher pH. Moreover, adsorption at the surface can differ among ions, leading to IEPs that are dependent on the ionic species present in solution.33 Such specific ion effects (SIEs) are often assessed with respect to the Hofmeister series, which ranks ions according to their ability to disrupt or strengthen hydrogen bonding in bulk water, with poorly hydrated Cs+ and K+ being characterized as chaotropes (“structure breakers”) and the small, strongly hydrated ions, Na+ and Li+ being characterized as kosmotropes (“structure makers”). A wide variety of tools and methodologies have been used to investigate SIEs at the silica/water interface and to understand these effects within the context of the Hofmeister series.32-35,36 ,37-40 Nonlinear optical methods, including second harmonic generation and sum frequency generation (SFG), have been used to study interfacial water structure due to their intrinsic selectivity for the noncentrosymmetry that arises at interfaces. 1,34,41-46 Specifically, vibrational SFG spectroscopy has been used to examine the molecular structure of water at silica interfaces (Scheme 1).1,22,47,48 In pH-dependent measurements of silica, an increase in surface potential is proposed to increase the ordering of water in the EDL, which can be probed with SFG according 2

to I SFG ∝ χ (2) Evis EIR + χ H(3)O Evis E IR Φ + χ (2) Evis E IR where ISFG is SFG intensity, χ (2) is the second order H 2O

2

H 2O

NR

nonlinear susceptibility of water, χ (3) is the third order nonlinear susceptibility of water, Φ is the H 2O

surface potential, χ ( 2) is the nonresonant second order nonlinear susceptibility and Evis and EIR are NR

the electric fields of the visible and IR beams, respectively.30 In this equation, χ (2) is directly H 2O

related to the amount of water aligned by factors other than the surface potential and χ (3) Φ is H 2O

related to the amount of water aligned by the surface potential. SFG measurements of water/silica in the absence of salt have found that as the pH is decreased the overall magnitude of


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SFG signal is decreased47 and at a given pH SFG intensity decreases as cations are added and charge screening occurs.39 SFG studies of SIEs on water at the silica/water interface were first reported by Yang et al., where the authors observed that K+ decreased the SFG water spectrum the most followed by Li+ and finally Na+ at pH 5.7, suggesting adsorption in the following order: Na+ < Li+ < K+.35 This trend was confirmed in a follow up study by Lovering et al.,49 but conflicted with results from Flores et al. that indicated adsorption in the following order at pH 10: Li+ < Cs+ < K+ < Na+.39

Scheme 1. An overview of the GCSG model of the EDL at a charged silica/water interface in the presence of monovalent cations. A broadband IR pulse is overlapped with a narrowband visible pulse at the silica/water interface to generate the SFG water spectrum. In most experiments cation adsorption on negatively charged silica follows a direct Hofmeister series: Li+ < Na+ < K+ < Cs+, which is inversely related to hydrated cation radius.33,50,51 In addition, most models of the mineral oxide/aqueous interface contain a capacitance parameter directly related to the distance of ions from the silica surface (ie. the distance of the OHP from the surface) which has been found to correspond to hydrated ionic radius.28,52 The majority of capacitance values are reported for a single pH and are often assumed


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to be pH independent when used to interpret measurements of the EDL.53 However, there is some experimental evidence to suggest that the ordering of cation adsorption is pH dependent. Specifically, an AFM study by Morag et al. suggested that the monovalent cations adsorbed following a direct Hofmeister series Li+ < Na+ < K+ < Cs+, but noted an inversion in this series at higher pH (pH >7).38 These findings corroborated an early zeta potential study by Tadros and Lyklema that found a reversal in SIE trends in the zeta potential at the silica/water interface at pH > 7.54 Although these studies have provided insight on pH-dependent cation adsorption, they are centered on macroscopic measurements and offer limited understanding on molecular changes within the EDL. Performing SFG measurements over a wide pH range should shed light on the water structure at the molecular level for different electrolytes and offer insight on previous SFG studies that were performed for a single pH point. As such, in this work SFG measurements of the silica/water interface were acquired over a broad pH range (2-12) for 0.5 M LiCl, NaCl, KCl and CsCl, revealing that SIEs on water structure are highly pH dependent. This relatively high salt concentration was chosen for its relevance to the GCSG model, as the Stern layer should be present and non-negligible over the entire pH range studied.


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Figure 1. ssp SFG spectra of water in the presence of 0.5 M CsCl, KCl, NaCl and LiCl at pH values from 2-12.

Figure 2. Integrated (from 2950 to 3450 cm-1) ssp SFG intensities are shown for 0.5 M CsCl, KCl, NaCl and LiCl from pH 2-12 (the mean of three replicate data sets). Low, neutral and high pH regions are shaded in pink, green and blue respectively.


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In the water SFG spectra reported here, two main peaks were observed in the OHstretching region for the ssp polarization combination: one at ~3200 cm-1 and one at 3400 cm-1, consistent with previously reported SFG water spectra (Figure 1).31,35,55,56 The general pHdependent shape in SFG intensity was the same for all four salts, with a minimum observed at pH 7-8 (Figure 2). However, the SIE trend in SFG intensity observed at high pH (blue region in Figure 2) inverted at neutral pH (green region) and again at low pH (pink region). Although the integrated results in Figure 2 provide an overview of trends in SFG intensity for the different monovalent cations, distinguishing differences between the 3200 cm-1 and 3400 cm-1 peak behavior should offer further insight into structural changes arising within the EDL. Previous studies suggested that the two peaks arise from two different water populations, one from waters aligned near the surface within the Stern layer (3400 cm-1) and the other from waters in the diffuse layer (3200 cm-1). 28,57 In recent work, it was proposed that these different water populations may be experiencing different orienting forces from different surface potentials, with the 3400 cm-1 peak arising from waters oriented by the 0-plane potential (Φ0) and the 3200 cm-1 peak arising from waters oriented by the outer Helmholtz potential, ΦOH (Darlington et al., J. Phys Chem C, submitted for publication, 2017). Peak fitting of ssp SFG spectra for LiCl and CsCl revealed that the 3200 cm-1 and 3400 cm-1 peaks exhibited different ion-specific trends in A/ Γ (which is proportional to the amount of aligned water), where A is the peak amplitude and Γ is the homogenous line width obtained from fitting the spectra to the absolute square of summed Lorentzian functions (Figure 3).1 In previous work the overall trends in the 3200 cm-1 peak were largely attributed to changes in the ΦOH, with the minimum at neutral pH corresponding to an ΦOH of 0 V, which shifted to higher pH with increasing salt concentration. Moreover, the increase in SFG intensity at lower pH observed for


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salt concentrations ≥ 0.1 M was attributed to an increasingly positive ΦOH (where there are more cations adsorbed at the surface than there are negative charges on the silica) and the increasing SFG intensity at high pH attributed to a more and more negative ΦOH (Figure 3a) (Darlington et al., J. Phys. Chem. C, submitted for publication, 2017). However, the SFG results presented here indicate contributions from forces in addition to ΦOH, which will be discussed.


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Figure 3. Representative peak fitting results showing A/Γ for ssp SFG water spectra for 0.5 M CsCl and 0.5 M LiCl as a function of pH for a) the 3200 cm-1 peak and b) the ~3400 cm-1 peak are shown. The error bars represent one standard deviation obtained from the fits. The color scale indicates the predicted corresponding positive (blue) and negative (red) surface potentials for the OHP (a) and 0-plane (b). Firstly SIEs in the neutral region will be considered. From Figure 3 it was observed that the 3200 cm-1 peak never reached zero amplitude where the minimum in SFG intensity (SFGmin) occurred, which would be expected if ΦOH was the only contributing factor, indicating an additional source of water-order. At SFGmin the 3200 cm-1 peak for LiCl had a larger A/Γ compared to CsCl, suggesting less adsorption of Li+ and thereby less charge screening and more alignment of water, consistent with the direct Hofmeister series and with previous studies that have reported that Cs+ adsorption is more favored than Li+ at pH < 7.33,38,50 Recent predictions of interfacial cation concentration profiles indicate that the maximum concentration of Li+ located at the OHP is less than that for Cs+,32 consistent with less adsorption on silica for Li+. Moreover, at pH 10 and 0.05 M salt, the position of the OHP was found to be further away for Li+ than Cs+ resulting in a lower capacitance for the Stern layer that contained Li+, which yielded a greater |ΦΟΗ| for a given charge on silica.32 Despite the lower salt concentrations used in their experiments, the trend in A/Γ  the 3200 cm-1 peak observed here at pH 10 with 0.5 M salt is consistent with their findings (Li+ > Cs+).32 In addition a larger 3400 cm-1 peak was observed for Li+ at neutral and low pH, indicating more ordered water within the Stern layer than for Cs+ as a result of less Li+ adsorption (Figure 3). The strongly hydrated and kosmotropic Li+ is expected to reinforce the hydrogen bonding network of water molecules near it,58 thereby inducing water order in its hydration shell


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unrelated to ΦΟΗ. Yet as Li+ adsorbs on the surface its ability to order surrounding water may be limited by the presence of the silica, resulting in more order in the water molecules on the outside of the ion facing the bulk. Indeed, such a situation was recently predicted in molecular dynamics studies of cation adsorption at the neutral quartz/water interface, where inner-sphere complexes were formed with silanols replacing water within the first cation hydration shell.59,60 This asymmetry could be sufficient to generate non-zero SFG signals. The hydration waters of the cation would be aligned as a result of its positive charge, as opposed to water molecules in the diffuse layer that have their water dipoles aligned by negative ΦOH, which would result in deconstructive interference of two oppositely signed peaks of different frequency (the diffuse layer waters at ~3200 cm-1 and the hydration waters at ~3150 cm-1)61 in the SFG spectrum. Moreover a non-zero minimum in the 3200 cm-1 peak would not arise at the true IEP (i.e. when ΦOH = 0), but when the two oppositely signed peaks are equal in amplitude. In line with this interpretation, we found that SFGmin was approximately 2 log units greater than the measured IEP of silica colloids in the presence of LiCl and CsCl.33,62 Simulations of these interfering modes revealed that the larger signal at SFGmin could be described by a larger contribution from water within the cation hydration layer (Figure S5). As such, due to its weak hydration and chaotropic nature, Cs+ was expected to disrupt the H-bonding structure of water near it and therefore have a smaller contribution from asymmetric hydration than Li+. Accordingly Li+ was found to have a larger contribution to the 3200 cm-1 peak than Cs+ at neutral pH (Figure 3). Next, SIEs at low pH are considered. At low pH the 3200 cm-1 peak increased more for CsCl than for LiCl (Figure 3a), likely due to preferential adsorption of Cs+, such that overcharging in the EDL occurred yielding a positive ΦOH, which was the dominant force for aligning water. Such a change in ΦOH from negative to positive upon decreasing the pH from 9 to


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3 has been observed previously for Cs+.33,62 Owing to interactions with the surface potential and the water dipole moment, this change in sign in ΦOH would cause a net flip in water orientation in the diffuse layer, possibly accounting for the red shift in the 3200 cm-1 peak at low pH (Figure 1) as previous studies on CaF2 observed a red shift as the surface changed from negative to positive.63 For LiCl, there is also evidence to indicate small positive ΦOH at low pH.33 However, the presence of asymmetric cation hydration could also lead to the presence of an oppositelysigned and red-shifted peak64 that begins to dominate the low wavenumber region as ΦOH decreases in magnitude (described above and in detail in SI). Finally, for both LiCl and CsCl, the 3400 cm-1 peak decreased slightly as the pH was lowered, corresponding to a decreasing surface charge on silica and decreasing magnitude of Φ0 as the point of zero charge was approached (Figure 3). Lastly, SIEs at high pH are examined. A clear inversion of the direct Hofmeister series was revealed at high pH, where the reverse Hofmeister series was observed (CsCl < KCl 10 as the distance of the OHP from the silica surface increased. As the capacitance is inversely related to |ΦOH| for a given charge on silica, this inversion of the capacitance results in an inversion of ΦOH, where |ΦOH,Li+| < | ΦOH,Cs+|. These results agree with previous experimental studies and molecular dynamics simulations that inferred Cs+ is repelled from the charged surface at high pH while the adsorption of smaller ions (Na+ and Li+) is favored, 38,65,66 attributed


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to displacement of Cs+ at the OHP by water molecules that more strongly hydrogen bonded to the increasingly deprotonated silica.38 Indeed, adsorption of cations at the silica surface has been attributed primarily to surface hydration forces rather than electrostatics, allowing for excess adsorption of cations and resulting charge reversal at low pH.38 The observed changes in the A/Γ of the 3400 cm-1 peak were also consistent with expulsion of Cs+ from the Stern layer. For CsCl, the 3400 cm-1 peak increased from pH 2-11, which was expected as the surface, and with it Φ0, became more negative, leading to more water alignment within the Stern layer. However, the significant increase from pH 8 to 11 could also suggest an increase in the amount of water near the surface, consistent with expulsion of Cs+. Yet in the case of LiCl, the 3400 cm-1 peak increased only slightly, and then began to decrease at pH > 10 (Figure 3b), suggesting that LiCl was specifically adsorbed at high pH, displacing water near the surface (also resulting in a greater capacitance and smaller 3200 cm-1 mode). A schematic of the combination of these factors proposed to account for the observed pH-dependent trends for both the 3200 cm-1 and 3400 cm-1 peaks are shown in Scheme 2 as well as an expanded equation for the source of SFG intensity. It should also be noted, that trends observed in the 3400 cm-1 with the ssp polarization combination were similarly observed using pss polarization, which more selectively probes the 3400 cm-1 peak (Figure S4).


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Scheme 2. Schematic of Li+ (left) and Cs+ ions (right) in the EDL at the silica/water interface and corresponding alignment of water dipoles (arrows, not to scale). The silica surface is shown as a black line, the Stern layer is shown in grey, cation hydration layers in light blue and Clcounter ions in red. The expression describing SFG intensity is expanded to include contributions (2) will also change as the number of water from surface potentials and cation hydration. χ 3400

molecules within the Stern layer changes. The results presented here demonstrate that SIEs for silica are highly pH dependent and indicate that capacitance values used in EDL models are only appropriate for a narrow pH range. Trends in SFG spectra among the four salts studied indicated a direct Hofmeister series of cation adsorption at neutral pH and a reversal of these trends at high pH, which was attributed to


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expulsion of Cs+ from the silica surface and specific adsorption of Li+. In addition, the increase in SFG signal and red shift in the 3200 cm-1 peak at low pH was attributed to contributions from asymmetric cation hydration effects and overcharging of the EDL. Here the behavior of the 3400 cm-1 peak was attributed primarily to changes in water structure within the Stern layer as a result of changes in Φ0, consistent with many nonlinear optical studies that interpret pH-dependent measurements in the context of surface potentials.30,31,36 However, it should be noted that water structure within the Stern layer could also be influenced by hydrogen bonding, which might change as the surface becomes deprotonated and the silanol groups transform from proton donors or acceptors to only proton acceptors (siloxides).47 This hydrogen bonding aligning force would (2) be significant owing to the energy of hydrogen bond formation and would contribute to the χ 3400

term (Scheme 2), revealing the need for advanced molecular dynamics simulations that can disentangle the effects of pH-dependent hydrogen bonding and water alignment driven by the surface potential. Therefore caution should be used in the interpretation of SFG measurements solely in the context of surface potentials. In addition, the role of silica dissolution on water structure has not been considered here, with high salt concentration and high pH conditions having been shown to increase the rate of dissolution.6,10,67 However, dissolution of one silica monolayer (assuming 1014 silanols/ cm2) has been shown to take ~ 7 hours at pH 12 and ~ 12 hours at pH 11 for quartz in 0.2 M NaCl and we therefore reason dissolution is unlikely to effect the SFG measurements here (~ 10 min per pH point). 12,68 Nevertheless, additional time-dependent SFG studies may provide insight on effects of silica dissolution.


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Finally, an AFM study by Zhao et al was used to determine the effect of ions and pH on Φ0 and ΦOH that could aid in the analysis of SFG data, although such analysis requires that the capacitance parameter be known a priori.69 As the results shown here indicate that capacitance is both ion specific and pH dependent, such modeling is not trivial. However, to quantify the effect of SIEs a rigorous comparison of SFG with both experimental surface potential measurements and theoretical modeling is required and is the subject of ongoing studies. Nevertheless, these results provide insight on inconsistencies among previous SFG experiments that examined single pH points and provide further evidence that the trends in SFG spectral shapes and in particular intensities are likely due to a combination of Φ0, ΦOH, interfacial cation hydration and pHdependent changes in cation affinities for silica. Supporting Information Available: experimental methods; replicate raw SFG data sets; simulations of interfering water peaks to demonstrate the effect of ion hydration and charge reversal on SFG intensity; pss SFG data; description of peak fitting AUTHOR INFORMATION: The authors declare no competing financial interests. ACKNOWLEDGMENTS: We gratefully acknowledge support from the Canadian Centre for Clean Coal/Carbon and Mineral Processing Technologies (C5MPT). JMG also acknowledges the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council of Canada for an Accelerator Award, the Sloan Foundation for a Research Fellowship and PetroCanada for a Young Innovator Award. SK acknowledges support from the Alberta/Technical University of Munich International Graduate School for Hybrid Functional Materials (ATUMS) program. We would also like to thank Eric Tyrode (KTH), Matthew Brown (ETH) and Akemi Darlington (University of Alberta) for many helpful discussions.


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pH-Dependent Inversion of Hofmeister Trends in the Water Structure

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