Quantifying Specific Ion Effects on the Surface Potential and Charge

Article pubs.acs.org/JPCC

Quantifying Specific Ion Effects on the Surface Potential and Charge Density at Silica Nanoparticle−Aqueous Electrolyte Interfaces Tobias A. Gmür, Alok Goel, and Matthew A. Brown* Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, CH-8093 Zurich, Switzerland S Supporting Information *

ABSTRACT: The surface charge density and surface potential of oxide surfaces are to a large extent regulated by the co- and counterion distributions in the electrical double layer. Here we study the effect of different anions (Cl−, Br−, I−, HCOO−, and NO3−) in sodium electrolytes and different cations (Li+, Na+, K+, and Cs+) in chloride electrolytes on the surface charge density and relative surface potential of colloidal nanometer sized silica (SiO2) as a function of electrolyte concentration and bulk pH using potentiometric titrations, attenuated total reflection−Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) from a liquid microjet. Our results reveal that the identity of the anion has no significant effect on the nanoparticles surface charge density and relative surface potential at bulk pH where silica is negatively charged, consistent with textbook arguments of electrostatics and their exclusion from the region of the electrical double layer that regulates these properties. By contrast, the identity of the cation in solution is significant on both the surface charge density and surface potential. Specific cation effects are rationalized by their respective distance of closest approach to the nanoparticles surfacethe hydrated cation diameter sets the outer Helmholtz plane and in turn the capacitance of the Stern layer. properties at water−silica interfaces.18,25,27 In 1998, Kosmulski published the first research paper studying colloidal silica in which salts with a common cation (H+, Li+, Na+, Cs+, or Ba2+) and different anions (Cl−, ClO4−, NO3−, and I−) were investigated.18 His electrophoretic (EP) mobility measurements (EP mobility is used to calculate zeta-potentials, which are most commonly interpreted as the charge at the start of the diffuse layer45) were none too surprising and yielded a conclusion coherent with basic electrostatics:3 Anion effects are substantial only at very low pH values. At pH > 2−3 (the isoelectric point of silica4,5) the interfacial region is depleted from anions due to electrostatic repulsion, and anion effects become insignificant.18 More recently, however, nonlinear optical experiments (vibrational sum-frequency generation (VSFG) and second harmonic generation (SHG)) at negatively charged silica−water interfaces have been interpreted to suggest that different anions play a significant role in regulating the surface structure and electrostatics of system (even at concentrations of anions where Kosmulski showed there to be no effect).25−27 These interpretations question to what extent Kosmulski’s zetapotential results can be generalized to other electrostatic properties of aqueous electrolyte−silica interfaces, such as those postulated to be measured by the nonlinear optical methods surface potential and surface charge density in the case of SHG25 and the structure of local water molecules at the silica

1. INTRODUCTION In aqueous electrolyte solutions all oxide surfaces develop an electric charge. This creates an electrical double layer (EDL) as ions in solution rearrange to shield (screen) the surface charge.1,2 The microscopic structure of this EDL is a topic of considerable interest to a broad chemical community as it plays a vital role in directing/regulating surface chemistry, reactivity, (mineral) dissolution, and surface structure. It is known that by simply changing the identity of the electrolyte ions in aqueous solution both the structure and reactivity of oxide−water interfaces can be greatly influenced. This general observation is referred to as specific ion effects and is a topic of utmost importance to the physical, biological, geochemical, and nanoscience communities. Following basic arguments of electrostatics,3 it becomes obvious why the vast majority of research in this field has focused on the specific role of the counterion (electrolyte ion in solution with opposite charge to the excess charge of the surface) in the EDL. Indeed, for negatively charged oxides such as silica (SiO2) at neutral pH,4,5 the specific roles of different cations are established.6−42 Specific ion effects manifest themselves in observables that influence, for example, coagulation/colloid stability6,7 and (mineral) dissolution rates.41,43 These processes, while observed macroscopically, are ultimately governed by microscopic properties: surface potential,23,37,44 zeta potential,17 and surface charge density (protonation state of surface bound hydroxyl groups).11,13,18 Naturally, much less attention has been given to the role of the co-ion in regulating both microscopic and macroscopic © 2016 American Chemical Society

Received: March 9, 2016 Revised: June 21, 2016 Published: June 23, 2016 16617

DOI: 10.1021/acs.jpcc.6b02476 J. Phys. Chem. C 2016, 120, 16617−16625

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The Journal of Physical Chemistry C interface in VSFG.27 Could it be that the presence of different anions in solution has no influence on the potentials at the start of the diffuse layer (zeta-potentials) but an overwhelming effect on the surface potentials and surface charge densities? In this study we set out to try and resolve the discrepancy between the zeta-potential results of Kosmulski and the interpretations of nonlinear optical SHG23,25 and VSFG27 measurements. We quantitatively measure the relative surface potential and the absolute surface charge density of colloidal silica nanoparticles in different aqueous electrolyte solutions using a combination of analytical techniques: X-ray photoelectron spectroscopy for the NPs surface potentials, potentiometric titrations, and attenuated total reflection− Fourier-transform infrared (ATR-FTIR) spectroscopy for the NPs surface charge densities. Our results focus primarily on specific anion effects, Cl−, Br−, I−, HCOO−, and NO3−, in sodium (Na+) electrolytes, but for completeness we also include a brief discussion of specific cation effects, Li+, Na+, K+, and Cs+, in chloride (Cl−) electrolytes. These electrolytes are chosen primarily on the basis that they coincide with the salts used in the above for mentioned nonlinear optical SHG23,25 and VSFG27 measurements. Formate (HCOO−) was not studied in refs 23, 25, or 27 but offers a particular challenge to quantifying the interface structure because its pKa prevents quantitative potentiometric titrations of the NPs surface charge density. Our results, at pH where colloidal silica is negatively charged, are consistent with the anions residing beyond the outer Helmholtz plane of the EDL. This relatively large separation between the surface of the NPs and the anions precludes them from having any measurable influence over the most fundamental properties of this interface, the surface potential, and the surface charge density. By contrast, the identity of the cation, which resides at the outer Helmholtz plane, sets the capacitance of the Stern layer and thereby has a direct effect on the surface potential and surface charge density.

measured using a four-point (2.00, 4.01, 7.00, and 10.00, Technical Buffer Solutions, Mettler-Toledo) calibrated Mettler Toledo Expert Pro electrode. This experimental approach means that there is always a finite amount of Cl− in solution, which also contributed to the apparent anion effects, in addition to the added electrolyte anion (Cl−, Br−, I−, HCOO−, NO3−). Since the concentration of Cl− from the adjustment of the pH (HCl) is constant for a given pH, any differences between experiments reflect specific anion effects for the different salts. This approach is analogous to that employed in a previous vibrational sum-frequency generation study on the effects of cations at the water−silica and water−titania (TiO2) interfaces.28 Potentiometric Titrations. Surface charge densities (SCDs) are determined by potentiometric titration using a Mettler-Toledo G20 Compact Titrator equipped with a Mettler-Toledo Expert electrode and an electronic controlled rod stirrer (70% polypropylene and 30% fiberglass). Polypropylene sample beakers of 100 mL are used. Experiments are performed using 5 wt % silica (Ludox SM) in NaCl, NaBr, NaI, NaNO3, LiCl, KCl, or CsCl at 295 K. The electrode is calibrated using a four-point curve (2.00, 4.01, 7.00, and 10.00, Technical Buffer Solutions, Mettler-Toledo) immediately prior to every experiment. Suspensions of 25 mL volume are titrated from high pH to low in 10 or 50 mM electrolyte using 0.1 N HCl (Acros Organics). Electrolyte is added to the HCl solution to ensure the concentration of salt in solution remains constant at 10 or 50 mM throughout the titration. Experiments are performed under an inert atmosphere of nitrogen (N2) gas that is bubbled through Milli-Q water. The drop volume of the HCl is set at 0.2 mL per step for the silica sample and 0.005 mL per step for the blank (electrolyte but no silica NPs). The stir rate (electronic stir bar) of the suspension and blank samples is 700 rpm. The end point is set at the point of zero net charge, pH 3.0.16 SCDs are calculated following the procedure described by Lützenkirchen et al.47 with a specific surface area of 282 m2/g. Measurements are performed in triplicate to ensure reproducibility, and the error bars correspond to the standard deviation of these measurements. Attenuated Total Reflection−Fourier Transform Infrared (ATR-FTIR) Spectroscopy. Experiments are carried out at 295 K on a Cary 670 FTIR spectrometer (Agilent Technologies) equipped with a SPECAC ATR diamond accessory using 5 wt % silica (Ludox SM) in 10, 50, and 100 mM NaCl, NaBr, NaI, NaCOOH, NaNO3, LiCl, KCl, or CsCl. A ceramic air-cooled light source, an extended range potassium bromide (KBr) beamsplitter, and a liquid nitrogen cooled MCT detector are used. Spectra are recorded from 700 to 4000 cm−1. The spectrometer cutoff is 700 cm−1 on the low energy side, and therefore spectra intensities below ∼750 cm−1 are weak and show small intensity fluctuations between measurements. These variations, while subtle, are evident in the spectra. The results presented herein focus exclusively on the spectral region 710−1420 cm−1, which comprise the major absorption bands of silica. The displayed spectra are normalized to the absorption intensity of the asymmetric O−Si−O mode at 1121 cm−1. The interferometer is scanned with an acquisition rate of 37.5 kHz at 2 cm−1 resolution. 1024 scans are averaged to produce one spectrum, and experiments are performed in duplicate. Error bars correspond to the standard deviation of these repeat measurements. Single channel spectra are obtained by performing a Fourier transform of the interferogram after apodization with a Blackman-Harris 4-Term function without a filling

2. EXPERIMENTAL SECTION Chemicals and Suspension Preparation. Sodium chloride (NaCl, ≥99.8%, ACS Reagent, Sigma-Aldrich), sodium bromide (NaBr, ≥99.0%, ACS Reagent, Sigma-Aldrich), sodium iodide (NaI, ≥99.5%, ACS Reagent, Sigma-Aldrich), sodium formate (NaCOOH, 99.998%, Sigma-Aldrich), sodium nitrate (NaNO3, ≥99.0%, ReagentPlus, Sigma-Aldrich), lithium chloride (LiCl, ≥99%, ACS reagent, Sigma-Aldrich), potassium chloride (KCl, ≥99.5%, puriss. p.a., Sigma-Aldrich), and cesium chloride (CsCl, 99+%, Acros Organics) are used as-received. Milli-Q water is used throughout. All experiments are performed using commercially available Ludox SM colloidal silica (Sigma-Aldrich). The particles are 9 nm in diameter (±1.5) as determined by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS).46 The relatively poor contrast of silicon in TEM does not permit quantification of the surface roughness of these particles. The suspension is sold at 30 wt % silica in an aqueous solution of pH 10 and has a reported shelf life of 1 year that the manufacturer claims originates from their particles charge stabilization. The surface structure of these particles has been previously studied by in situ 29Si NMR, and no evidence of surface bound polymeric brushes was seen,36 providing some external verification of the manufacturer’s claims and evidence that these particles have a excellent stability toward dissolution at high pH. The pH is adjusted by the addition of concentrated HCl (fuming ≥37%, ACS Reagent, Sigma-Aldrich) and 16618


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Figure 1. Surface charge density of 5 wt % colloidal silica in (a) 10 mM and (b) 50 mM sodium electrolytes as a function of bulk pH. Red: NaCl. Black: NaBr. Green: NaI. Blue: NaNO3. (c, d) Comparison of the surface charge density at pH 8.0 and 9.6 in the different electrolytes.

factor. The phase correction is set to Auto in the Agilent Resolutions Pro software. Absorption spectra of the nanoparticle suspensions are calculated using Milli-Q water as the reference. Liquid Microjet X-ray Photoelectron Spectroscopy (LJ-XPS). LJ-XPS experiments are carried out at the Surface/ Interface: Microscopy beamline (SIM) beamline48 of the Swiss Light Source (SLS) using either a 19 or 25 μm quartz liquid jet operating at 279 K and a flow rate of 0.3 mL/min. The X-ray spot size of the SIM beamline is 100 (w) × 250 (h) μm.48 Experiments are performed using 5 wt % silica (Ludox SM) in 10 and 50 mM NaCl, NaBr, NaI, NaCOOH, NaNO3, LiCl, KCl, or CsCl. The SLS near ambient pressure photoemission (NAPP) endstation is used.49 The entrance cone of the hemispherical energy analyzer (NAPP) has an aperture diameter of 500 μm with a working distance to the surface of the LJ of 500 μm. A complete description of in situ XPS at the three-way interface of air−water−colloid is given elsewhere.50 The primary photon energy of the beamline is set to 420 eV to ionize the Si 2p orbital, and second order light, 840 eV, with ∼10% the intensity of the primary beam simultaneously ionizes the O 1s orbital.46 This approach for liquid-based XPS ensures uniform probe depth into solution for both orbitals. It also guarantees constant overlap of the incident X-rays and the LJ for both spectral regions and thereby greatly improves the reliability and reproducibility of the reported Si 2p peak positions. Measurements are repeated in triplicate for the specific anion effects and in quadruplicate for the cation series, and the error bars correspond to the standard deviation of these repeat measurements. The signals of the ions are not collected due to their low concentration (10 or 50 mM). Spectra are fit following a standard linear background subtraction using Voigt functions.

of surface bound hydroxyl groups. Figures 1c and 1d quantify the surface charge density at bulk pH of 8.0 and 9.6 (pH where SiO2 has noticeable negative charge (>∼−0.05 C/m2) and are further investigated by ATR-FTIR and LJ-XPS; see below). It becomes immediately evident that the surface charge density of colloidal silica is independent of the identity of the anion in solution, within the reproducibility of the experiments. In 50 mM electrolyte the surface charge density is (slightly) higher than in 10 mM (the ordinate of Figures 1c and 1d have the same scale), a result that is attributed to enhanced screening of the deprotonation site in the presence of higher concentration of the sodium counterions.11,51 ATR-FTIR. The region of the ATR-FTIR spectra (710−1420 cm−1) that comprise the major absorption bands of silica are shown in Figure 2a for a 5 wt % sample of colloidal silica in 50 mM NaCl at pH of 9.6 (black), 8.7 (red), 7.8 (green), and 6.9 (blue). The different absorption bands are depicted schematically in the figure and include three bulk modes: the symmetric O−Si−O stretch (∼1200 cm−1),52 the asymmetric O−Si−O stretch (1121 cm−1),52,53 and the Si−O−Si bend (793 cm−1).52 Two surface modes are also evident in the spectra: the Si−O stretch vibration of neutral silanols, ≡Si−OH (∼960−980 cm−1),54−57 and the Si−O stretch vibration of deprotonated silanols, Si−O− (960−984 cm−1).56 The shift of the deprotonated site, ≡Si−O−, to higher wavenumber than the neutral surface site, ≡Si−OH, is a result of the shortening of the Si−O bond36 and an increase in its force constant.56,58 While the relative intensities and absolute band positions of the three bulk absorptions are indifferent to pH (consistent with bulk structures that are not in contact with the electrolyte), the two surface modes follow a clear trend as the pH is decreased (Figure 2b): the shoulder-like feature near 1040 cm−1 decreases in intensity, whereas the band near 970 cm−1 increases in intensity and shifts to higher energy. Figure 2c shows the ATR-FTIR spectra for 5 wt % colloidal silica at pH 9.6 in 50 mM NaCl (red), NaBr (black), NaI (green), NaCOOH (light blue), and NaNO3 (dark blue). The weak intensities that appear near 1350 cm−1 are assigned to absorption bands of the molecular anions, HCOO− (light blue) and NO3− (blue). Figure 2d shows the surface modes region in the different sodium electrolytes. By comparing Figure 2d with Figure 2b, it becomes immediately clear that changing the identity of the anion in the electrolyte, at pH 9.6, has no obvious effect on the position or intensity of the Si−O stretch vibrations of neutral or deprotonated surface bound hydroxyl groups.

3. RESULTS Specific Anion Effects. Potentiometric Titrations. The surface charge densities of 5 wt % colloidal silica are shown in Figures 1a and 1b for 10 and 50 mM electrolytes, respectively. The overall shapes of the curves are consistent with the wellknown behavior of colloidal silica13a point of zero net surface charge (pHPZC) near pH 3 and an increasing surface charge density as the pH is increased. The latter is ascribed to the deprotonation reaction ≡Si−OH + OH− ⇔ ≡Si−O− + H 2O

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Figure 2. (a) ATR-FTIR spectra of 5 wt % colloidal silica in 50 mM NaCl at pH 9.6 (black), 8.7 (red), 7.8 (green), and 6.9 (blue). The different vibrational modes are depicted schematically. (b) Region corresponding to the two surface vibrational modes: the Si−O stretch vibration of neutral silanols (Si−OH, ∼975 cm−1) and the Si−O stretch vibration of deprotonated silanols (≡Si−O−, ∼1040 cm−1). (c) ATR-FTIR spectra of 5 wt % colloidal silica at pH 9.6 in 50 mM NaCl (red), NaBr (black), NaI (green), NaCOOH (light blue), and NaNO3 (blue). (d) Analogous to (b) except for the anion series of electrolytes at pH 9.6.

increases, resulting in a band shift to higher wavenumber.56,58 The position of the neutral Si−(OH) mode is quantified in Figure 3a (10 mM electrolyte) and Figure 3b (50 mM electrolyte) for pH between 7 and 10. Across this 4-unit pH range the band position is independent of the identity of the anion in the electrolyte (generalizing the observation made at pH 9.6 based on the spectra of Figure 2c). The position of the band does, however, depend on the concentration of electrolyte in solution, a result supported by the potentiometric titrations of Figure 1. The results of the ATR-FTIR measurements show that at pH where colloidal silica is negatively charged the identity of the anion plays no significant role, within the reproducibility of the experiments, in dictating the NPs surface

The increase in intensity of the Si−O stretch vibration of neutral surface bound hydroxyl groups and the simultaneous decrease in the mode associated with the deprotonated site as the pH is decreased (Figure 2b) are inline with the results of the potentiometric titrations (Figure 1) and a shift in the equilibrium of reaction 1 to the left. The shift in band position of the neutral Si−(OH) stretch vibration with bulk pH has been attributed previously for this colloidal silica sample to the significant ionic character of the Si−(OH) bond59 and its response to the local protonation state at the NPs surface. As the pH is decreased, the concentration of deprotonated silanol groups on the NPs surface decreases (see Figure 1) and the force constant (f R) of the neighboring Si−(OH) groups 16620


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Figure 3. ATR-FTIR band position of the Si−O stretch vibration of neutral silanols, ≡Si−OH, for 5 wt % colloidal silica in (a) 10 mM and (b) 50 mM NaCl, NaBr, NaI, NaCOOH, and NaNO3.

vibrational structure. Taken together with the results of the potentiometric titrations (Figure 1) a picture begins to emerge about the SiO2−electrolyte interface that suggests the anion has no measurable influence on the NPs SCD or on its surface vibrational spectrum. The strong overlap of the deprotonated silanol band with the bulk asymmetric Si−O−Si mode makes quantifying its position nontrivial and prevents plotting its position as is done for the neutral silanol mode. X-ray Photoelectron Spectroscopy (XPS). XPS is used to quantify any change in surface potential of the NPs that results from a change in chemical potential of the suspension (by changing the electrolyte and/or its concentration) by measuring the binding energy (BE) shift of a core level (CL). The relation between a change in surface potential and the recorded BE is quantitative and given by60 salt reference ΔΦ0e = BESi2p − BESi2p

Figure 4. (a) In liquid microjet X-ray photoelectron spectroscopy (XPS) the reference level of the experiment is the vacuum level (Ev) of the near ambient pressure photoemission analyzer. The physical observable of the experiment is the kinetic energy (KE) of the outgoing photoelectron. In the case of a grounded sample (blue) the Fermi (EF) levels of the analyzer and the sample are aligned, but the vacuum levels might differ depending on the work functions (Φ). As the particle develops a (surface) potential all levels of the sample, including the core level (CL), shift down for positive potentials (red, +V) or up for negative (−V, green) relative to those of the analyzer. (b) The observed effect of surface potential is a shift in the apparent binding energy of the core level (BE = hυ − KE). The observed shift in binding energy is quantitative with surface potential and given by eq 2.

(2)

where ΔΦ0 is the change in surface potential [V] (the subscript 0 indicating the surface plane), e is the elementary charge, and BESi 2p is the measured BE of the Si 2p orbital [eV]. The superscript corresponds to a change in chemical potential brought about by a change in electrolyte. In what follows NaCl is used as the reference electrolyte. As was shown previously for extended surfaces of SiO2, eq 2 remains quantitative to potentials of at least ±5 V.60 The principle of the measurement is shown in Figure 4a based on the energy level alignment of a sample with that of the analyzer (typically a hemispherical kinetic energy detector). It is imperative to recall that in a liquid microjet XPS experiment the reference level remains at all times the vacuum level (Ev) of the analyzer. Binding energies of CLs are recorded relative to this reference level. In the case of a grounded sample (in colloid terminology this would correspond to a particle with zero potential) the Fermi levels (EF) of the sample and the analyzer are aligned and the apparent relative BE of the CL is assigned as zero (Figure 4b, blue spectrum). With positive surface potential the energy levels of the colloid shift down relative to those of the analyzer (Figure 4a) and the apparent BE of the CL increases (Figure 4b, red spectrum). In the case of negative potential the energy levels of the sample increase in energy relative to the reference state of the analyzer and the apparent BE decreases (Figure 4b, green spectrum). It is important to reiterate that the recorded apparent BE shift exactly matches the change in surface potential according to eq 2 and that XPS

is unique in its ability to directly measure changes in NPs surface potential. Here, we use liquid microjet XPS to quantify the change in surface potential of colloidal silica NPs brought about by specific anion and cation effects. Figure 5a shows oxygen 1s (O 1s) and silicon 2p (Si 2p) photoelectron spectra collected from a 25 μm liquid jet (Figure 5b) of 5 wt % colloidal silica at pH 8.0 in 10 mM NaCOOH (red, top spectrum) and in 10 mM NaCl (blue, bottom spectrum). There are two peaks in the O 1s spectral region. The one at lower photoelectron kinetic energy is assigned to gas phase water from the background vapor in the ionization chamber, O 1s(gas), while the one at higher photoelectron kinetic energy is from condensed water of the liquid microjet, O 1s(liq).48 Spectral overlap with liquid water and its relatively weak intensity prevent the oxygen component of the NPs from being resolved.36,61 The position of the Si 2p peak provides information on the change in surface potential of the NPs relative to that in NaCl electrolyte (see Figure 4). We have performed experiments in the five different electrolytes of the anion series, and the relative positions of the Si 2p peaks, expressed as ΔΦ0, are shown in Figure 5c. The ordinate of Figure 5c is chosen such that one scale can be used to plot both the specific anion and cation effects (the cations are discussed below). Within the reproducibility of the XPS experiments 16621


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Figure 6. Change in surface potential of colloidal silica in 50 mM of different sodium electrolytes relative to that in NaCl as determined by the Si 2p binding energy at (a) pH 8.0 and (b) pH 9.8 (see eq 2 for the relation between ΔΦ0 and binding energy). (c) O 1s and Si 2p photoelectron spectra from 5 wt % colloidal silica in 50 mM NaI at pH 9.8 (red, top spectrum) and pH 8.0 (blue, bottom spectrum). The shift in Si 2p is a result of the change in bulk solution pH.60 The liquid jet has a diameter of 19 μm. The smaller diameter of liquid jet relative to that used in the collection of the spectra in Figure 5a (25 μm) increases the relative contribution of gas phase water. Figure 5. (a) O 1s and Si 2p photoelectron spectra from 5 wt % colloidal silica in 10 mM NaCOOH (red, top spectrum) and 10 mM NaCl (blue, middle spectrum) at pH 8.0. The liquid jet has a diameter of 25 μm. The difference spectrum is scaled by a factor of 10. The abscissa is in kinetic energy (the physical observable of an XPS experiment) but can, in principle, be converted to the more conventional binding energy using BE = hυ − KE, after recalling that two photons are used simultaneously to ionize the O 1s (840 eV) and Si 2p (420 eV) orbitals. (b) A photograph of a 25 μm liquid jet running in the measurement position ∼500 μm in front of the entrance aperture (500 μm in diameter) of the NAPP spectrometer. (c) Change in surface potential of 5 wt % colloidal silica in different sodium electrolytes relative to that in NaCl as determined by the Si 2p binding energy (see eq 2).

the reproducibility of the measurements, is independent of the identity of the anion in solution. Specific Cation Effects. The effect of different alkali cations on the surface charge density and surface potential of silica has been studied in detail previously by us and others.6−42 Here, simply for comparison, we report the surface charge density and the relative surface potential in 50 mM LiCl, NaCl, KCl, and CsCl.60 The point of this comparison is to show that the analytical techniques employed herein are, in fact, sensitive to changes in surface charge density and relative surface potential, if they exist. In contrast to the negligible effect different anions play on the surface charge density and relative surface potential at the silica NP−aqueous electrolyte interface the identity of the cation is significant. Figure 7a shows the surface charge densities measured by potentiometric titrations at pH 10 in 50 mM LiCl, NaCl, KCl, and CsCl, while Figure 7b shows the

there are no shifts in the recorded positions of the Si 2p peaks in the different sodium electrolytes (also see the difference spectrum in Figure 5a which confirms the O 1s and Si 2p regions of the spectra are independent of electrolyte). These XPS results provide direct evidence that the identity of the anion does not play any significant role in regulating surface potential at the water−silica NP interface in 10 mM electrolyte at pH 8.0. We have performed additional XPS experiments in 50 mM electrolyte, which like 10 mM has no effect on the relative surface potentials of the NPs, using a 19 μm liquid jet. Figure 6a shows the surface potentials relative to that in NaCl electrolyte for the different electrolytes of the anion series at pH 8.0, while Figure 6b shows the results at pH 9.8. In both cases the measured surface potentials are independent of the anion in solution. Figure 6c shows two of the spectra used to create the results of Figures 6a and 6b. There is a non-negligible shift in the kinetic energy of the Si 2p peak as the pH is changed. The pH dependence of the absolute surface potential has been explored in detail in an earlier study60 and is beyond the scope of the present investigation. The present study concerns the effect of different ions on the NPs relative surface potentials, for which the ΔΦ0 reported in Figures 5c, 6a, and 6b are sufficient to conclude that the surface potential of colloidal silica, at pH where the surface is negatively charged and within

Figure 7. (a) Surface charge density of 5 wt % colloidal silica in 50 mM LiCl, NaCl, KCl, or CsCl at pH 10. (b) Change in surface potential of 5 wt % colloidal silica in different chloride electrolytes relative to that in NaCl as determined by the Si 2p binding energy (see eq 2). 16622


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zeta-potential18 and AFM62 measurements of others to the radius of curvature of the NP; however, this remains to be confirmed.17 Our XPS measurements reveal pronounced specific cation effects on the nanoparticles relative surface potentials, in agreement with our earlier results60 and consistent with zetapotential measurements available in the literature.17 These effects are rationalized by the increasing hydrated-ion size going from Cs+hyd to Li+hyd.64 The hydrated-cesium ion can approach closer to the silica surface than Li+hyd, which results in a larger capacitance of the Stern layer and a corresponding decrease in the magnitude of the surface potential. In 2012, Cremer and co-workers used VSFS to show that under the very specific conditions of pH 10 and 0.1 mM sodium electrolyte specific anion (Cl−, Br−, NO3−, ClO4−, and SCN−) effects become important for the interpretation of the attenuation of the OH stretch signal at planar silica interfaces.27 Anions that resulted in reduced spectral intensity, such as Cl−, were interpreted as being more excluded from the EDL than say SCN − , which gave the highest spectral intensity. Interestingly, at concentrations either above or below 0.1 mM Cremer and co-workers concluded that anion-specific differences could not be discerned.27 It is difficult to rationalize the findings of this study, in particular the peculiar concentration dependence, in light of earlier VSFG measurements by Chou and co-workers (NaCl, NaBr, and NaI) that over the same concentration range found no anion specific effects,26 except to ascribe it to a pH effect. Cremer and co-workers performed all of their measurements at pH 10,27 whereas Chou and coworkers worked exclusively at pH 5.7.26 That being said, one would expect specific anion effects to be more prominent at lower pH, where electrostatic (negative−negative) repulsion is reduced. It is important to note here that the probe volume of the VSFG experiment is unclear and the changes in OH spectral intensity that lead to Cremer and co-workers27 conclusions could originate from water molecules far from the surface. This is particularly true at 0.1 mM where the Debye length is ∼30 nm. Our XPS measurements are insensitive to these large distances from the oxide surface as the inelastic mean free path of the outgoing photoelectrons limits the probe depth of the experiment. At the kinetic energy used herein, ∼312 eV for Si 2p, the information depth is limited to 4.5 nm.65

relative surface potentials determined from the Si 2p BE under identical conditions. In both cases pronounced specific cation effects are evident. The distance of closest approach of the hydrated cations to the silica surface explains the antipodal extremes in surface charge density and relative surface potential. Cesium has been shown to approach the surface more closely than lithium, which results in a higher capacitance of the Stern layer and lower magnitude negative surface potential (Figure 7b).60 In addition, the closer approach of Cs+ offers enhanced screening of the charged site and a decrease in the Coulomb potential between deprotonated surface sites,12,51 which allows for more deprotonation for the same energy penalty (Figure 7a).

4. DISCUSSION Both the potentiometric titrations and the ATR-FTIR measurements are consistent with the identity of the anion being insignificant in determining the surface charge density and surface vibrational structure at the silica NP−aqueous electrolyte interface. The surface vibrational structure for this colloid is known to be an indirect spectroscopic measure of surface charge density56 and thereby provides independent support for the results of the potentiometric titrations. The surface charge density varies by less than 2% between samples for the five different anions investigated herein. The lack of response to the specific anions of this study is, however, not a result of a lack in sensitivity of our analytical methods. As demonstrated by the surface charge density for the cation series (Figure 7a), the potentiometric titrations are sensitive (and reproducible) to subtle changes in surface charge density. In an effort to bridge the concentration gap between the present colloid based experiments and second harmonic generation studies from silica substrates,23 we have performed additional ATR-FTIR experiments in 100 mM for the anion series of electrolytes (see Supporting Information Figure S1) and our conclusions holdno specific anion effects are observed. It is worth noting the striking differences between the present colloid based experiments and the substrates used in SHG, where a factor of more than two difference in SCD for NaCl and NaI electrolytes was reported.23 These differences may originate from the large difference in surface areas of the two types of samples (they may also have different roughnesses) or from the requirement in SHG to a priori assume an electrical double layer structure in order to be able to interpret the results (the constant capacitance model was invoked).23 X-ray photoelectron spectroscopy, which requires no a priori knowledge of the structure of the electrical double layer, measures directly the relative surface potential of the NPs in the different electrolyte solutions. The results of our measurements are clear: there is no change in the nanoparticles surface potential for the five different anions studied herein. This result is consistent with the original zeta-potential work of Kosmulski,18 who concluded anion effects are insignificant, and the AFM work of Sivan and co-workers, who noted no difference in the force distance curves between NaCl and NaBr electrolytes up to 200 mM.62 All of these results are, however, at odds with the pronounced specific anion effects on the relative surface potentials of silica substrates reported by SHG.25 Recently, Haber and co-workers postulated that SHG might be a measure of the zeta-potential and not the surface potential.63 It would be interesting to ascribe the differences in the interpretations of SHG experiments with those of the present potentiometric titrations, ATR-FTIR, XPS, and the

5. CONCLUSIONS The results of our potentiometric titrations, ATR-FTIR, and Xray photoelectron spectroscopy experiments are internally consistent and reveal that the identity of the anion in sodium electrolytes has no significant influence on the surface charge density (in 10, 50, and 100 mM) and surface potential (in 10 and 50 mM) of colloidal silica at pH where the surface is negatively charged. By contrast, the identity of the cation (in chloride electrolytes) has a pronounced effect. We ascribe this behavior to the close approach of the cation to the NPs surfacethe hydrated-ion radius sets the outer Helmholtz plane distance and in turn the capacitance of the Stern layer, whereas the anions are (essentially) excluded from the interface region. There remain significant inconsistencies between the interpretations of SHG experiments with those of potentiometric titrations, ATR-FTIR, XPS from a liquid microjet, and zeta-potentials concerning the role of anions in regulating the surface charge density and surface potential of silica−aqueous electrolyte interfaces. We remain optimistic these differences 16623


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The Journal of Physical Chemistry C may be reconciled by SHG scattering experiments,66 a variant of the technique used for extended substrates, but that can be applied to dispersed colloidal systems. Given, however, the general agreement of our measurements with the low-surfacearea AFM results of Sivan and co-workers,62 the surface area change alone may not be enough to realize the satisfactory level of agreement between the techniques sought by the community.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02476. ATR-FTIR results for specific anion effects in 100 mM electrolyte (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Ph +41 44 632 3048 (M.A.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Portions of this work were performed at the SIM beamline of the Swiss Light Source, Paul Scherrer Institute, Villigen PSI, Switzerland. A.G. and M.A.B. are supported by the Swiss National Science Foundation (no. 153578). N. D. Spencer (ETH Zürich) is acknowledged for his continued support, A. Kleibert (PSI) for his assistance during the beamtime, and A. Rossi (ETH Zürich) for her help in the laboratory.

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Quantifying Specific Ion Effects on the Surface Potential and Charge

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