Surface Vibrational Structure of Colloidal Silica and Its Direct

Article pubs.acs.org/Langmuir

Surface Vibrational Structure of Colloidal Silica and Its Direct Correlation with Surface Charge Density Tove Lagström,† Tobias A. Gmür,‡ Luca Quaroni,§ Alok Goel,‡ and Matthew A. Brown*,‡ †

Department of Chemistry and Applied Biosciences and ‡Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, CH-8093 Zurich, Switzerland § Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen, Switzerland S Supporting Information *

ABSTRACT: We show that attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy can be used to determine the surface charge density (SCD) of colloidal silica nanoparticles (NPs) in aqueous solution. We identify the Si−O stretch vibrations of neutral surface bound silanol, Si−OH, and of the deprotonated group, Si−O−. The position of the Si−(OH) stretch vibration is shown to directly correlate with the NPs SCD as determined by traditional potentiometric titrations, shifting to lower wavenumber (cm−1) with increasing density of Si−O−. The origin of this shift is discussed in terms of inductive effects that reduce the ionic character of the Si−(OH) bond after delocalization of the negative charge left on a terminal Si−O− group across the atoms within ∼1 nm of the charged site. Using this new methodology, we quantitatively determine the SCD of 9, 14, and 25 nm diameter colloidal silica in varying concentrations of NaCl electrolyte at different bulk pH. This novel spectroscopic approach to investigate SCDs provides several opportunities for in situ coupling, for example, in microfluidic channels or with liquid microjets, and requires only very little sampleall potential advantages over a traditional potentiometric titration.

1. INTRODUCTION The structure ofand chemistry atliquid−solid interfaces is a topic of tremendous interest as it plays a prominent role in fields as diverse as biology, geology, atmospheric chemistry, catalysis, electrochemistry, and materials science. In contrast to gas−solid1 and air (vacuum)−liquid2 interfaces relatively little is known about the microscopic structures of the liquid (solvent or solute) adjacent to or of the solid (extended surface or nanoparticle (NP)) at liquid−solid interfaces. In large part this stems from our inability to interrogate buried liquid−solid interfaces with molecular resolution that has become commonplace in modern surface science.3 In fact, the next big challenge to the surface-science community was recently identified as the ability to provide molecular-level detail at liquid−solid interfaces.4 In response to this need, powerful analytical techniques such as X-ray photoelectron spectroscopy (XPS) have recently been extended to the measure of liquid− nanoparticle interfaces (with synchrotron radiation and a liquid microjet),5−7 while younger nonlinear optical methods such as second harmonic generation (SHG)8,9 and sum-frequency generation (SFG),9,10 and their scattering analogues11 have quickly matured and now contribute regularly to a growing molecular-level picture of liquid−solid interfaces. Here we use a simple linear spectroscopic approach, attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy, to interrogate the microscopic surface structures of colloidal silica (SiO2) NPs in aqueous solutions of NaCl. © 2015 American Chemical Society

The abundance of silica on earth, its technological importance, and the common belief that the water−silica system is relatively simple10 (it is not) has made it one of the most studied liquid−solid interfaces. In aqueous solution the water−silica interface is charged, a result of protonation/ deprotonation reactions of surface bound hydroxyl groups (−OH) to yield charged species (mainly Si−O− and Si− OH2+).12,13 However, because the isoelectric point of silica is pH ∼2−3,12,14 the surface is nearly always negatively charged under common conditions. The speciation, distribution, and quantity of these charged Si−O− species are of specific interest to the chemical community as they create an electric field that controls local structure of the liquid solvent15 and the silica surface,16 which ultimately governs the chemistry of silica.17 In this regard, developing a microscopic description of the water−silica interface structure under varying experimental conditions (e.g., pH, electrolyte/strength, NP diameter) will provide benefit to the environmental (geochemical),18−20 nanotechnology,21,22 catalysis,23 pharmaceutical,24 separation,25 and biological/nanomedical21,24,26−28 sciences communities. Here we perform ATR-FTIR spectroscopy experiments as a function of bulk pH, SiO2 NP diameter, and NaCl electrolyte strength to identify the Si−O stretch vibrations of the neutral, Received: November 19, 2014 Revised: March 11, 2015 Published: March 12, 2015 3621

DOI: 10.1021/acs.langmuir.5b00418 Langmuir 2015, 31, 3621−3626

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Langmuir Si−OH, and deprotonated, Si−O−, silanol groups at the surfaces of the NPs. The position of the neutral Si−(OH) stretch vibration depends on bulk pH, NP diameter, and NaCl electrolyte strength and is shown to directly correlate with the NPs SCD as determined by traditional potentiometric titrations.

2. EXPERIMENTAL SECTION Chemicals. Commercially available Ludox colloidal silica (W.R. Grace and Company, Sigma-Aldrich) was used as received for all experiments. Three different variants, Ludox SM (9 nm particle diameter), Ludox HS (14 nm), and Ludox TM (25 nm) were studied. The nomenclature SM, HS, and TM refer to trademark names of the Ludox products. Sodium chloride (≥99.8%, ACS Reagent, SigmaAldrich) was used as-received. Milli-Q water was used for all dilutions. Vibrational Spectroscopy. All experiments were carried out at 295 K on a Cary 670 Fourier-transform infrared (FTIR) spectrometer (Agilent Technologies) equipped with a SPECAC attenuated total reflection (ATR) diamond accessory. A ceramic air-cooled light source, an extended range potassium bromide (KBr) beamsplitter, and a liquid nitrogen cooled MCT detector were used. Spectra were recorded from 700 to 4000 cm−1. The results presented herein focus exclusively on the spectral region 700−1450 cm−1 (see Figure S1 for the full region). The interferometer was scanned with an acquisition rate of 37.5 kHz at 2 cm−1 resolution. A total of 1024 scans were averaged to produce one spectrum, and experiments were repeated in triplicate to ensure reproducibility. Single channel spectra were obtained by performing a Fourier transform of the interferogram after apodization with a Blackman-Harris 4-Term function without zero filling. The phase correction was set to Auto in the Agilent Resolutions Pro software. Absorption spectra of the nanoparticle suspensions were calculated using water as the reference. FTIR experiments were carried out in 5 and 10 wt % suspensions with identical results. Only the results for 10 wt % spectra are shown (which have roughly twice the signal-to-noise of the 5 wt % samples). pH was adjusted by the addition of concentrated HCl (fuming ≥37%, ACS Reagent, Sigma-Aldrich) and measured using a four-point calibrated (2.00, 4.01, 7.00, and 10.00, Technical Buffer Solutions, Mettler-Toledo) Expert Pro electrode (Mettler-Toledo). Potentiometric Titrations. Experiments were performed for 5 wt % silica (Ludox SM) in 40 mM NaCl at 295 K on a Mettler-Toledo G20 automatic titrator equipped with an Expert electrode (MettlerToledo). The electrode was 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 were titrated from high pH to low using 40 mM NaCl in 0.1 M HCl (Acros Organics). The addition of 40 mM NaCl to the HCl solution ensured that the concentration of NaCl remained at 40 mM throughout the entire titration. Experiments were perfomed in an inert atmosphere of nitrogen (N2) gas that was bubbled through MilliQ water. The drop volume of the HCl was set as 0.2 mL per step for the silica sample and 0.005 mL per step for the blank (40 mM NaCl, no SiO2). The stir rate (electronic stir bar) of the suspension and blank samples were 700 rpm. The end point was set at pH 3.0.29 SCDs were calculated following the procedure described by Lützenkirchen et al.30 with a specific surface area of 282 m2/g. Measurements were performed in triplicate to ensure reproducibility.

Figure 1. (a) ATR-FTIR spectra of 10 wt % 9 nm Ludox SM colloidal silica in 40 mM NaCl at pH 1.4 (blue), 8.2 (green), and 9.8 (red). The well-known vibrational modes of bulk silica are shown schematically. Surface modes (box) correspond to the Si−O stretch vibrations of deprotonated and neutral silanol groups as discussed in the text. (b) ATR-FTIR spectra showing the surface modes region for 9, 14, and 25 nm silica particles at pH 9.8 (red), 8.2 (green, only 9 nm), and 1.4 (blue). The size-dependent response confirms these modes originate from the surface of the nanoparticles.

O−Si−O stretch (∼ 1200 cm−1),31 the antisymmetric O−Si−O stretch (1121 cm−1),31,32 and the Si−O−Si bend (793 cm−1).31 The relative intensities and absolute band positions of these three absorptions do not change with pH, providing evidence that they originate from the bulk of the NPs that are not in direct contact with the changing pH.32 This result also provides evidence that there are no bulk structural changes in the NPs with changes in suspension pH, a result that is consistent with the nonporous nature of Ludox silica.16 Since the focus of this study is to identify surface structures at the water−silica NP interface, these three bulk bands will not be discussed further. Surface Vibrational Structures of Colloidal SiO2. There are pronounced changes with suspension pH between 920 and 1020 cm−1 (Figure 1a), which suggests that this region of the spectra originates from the surface of the NPs (surface modes) that are in direct contact with aqueous solution. The response of these surface modes follow a clear trend with decreasing pH (see box labeled surface modes in Figure 1a): the shoulder-like feature near 1040 cm−1 decreases in intensity, whereas the band at 961.5 cm−1 increases in intensity and shifts to higher wavenumber (983.1 cm−1 by pH 1.4, see lower panel of Figure 1b). The latter has been assigned previously to the Si−O stretch vibration of neutral silanols, Si−OH.33−37 Specifically, this mode is observed at 980 cm−1 in pyrogenic36 and aerosil type colloidal silica,34,36 at ∼970 cm−1 in amorphous silica slabs (at 980 cm−1 by the same authors in α-quartz (0001)),35 at 960 cm−1 in nujol emulsions of gel-type silica,33 and at 940 cm−1 in silica glasses.37 We assign the feature at ∼1040 cm−1 to the Si− O stretch vibration of deprotonated silanols, Si−O−. This assignment is qualitatively supported by (i) its decrease in intensity as the pH is lowered from 10.0 (we expect less Si− O− as the pH approaches the isoelectric point of silica,38,39 pH ∼ 2−312,14) and (ii) by its position at higher wavenumber than the Si−O stretch vibration of neutral silanols, Si−OH.33−37 A shift to higher wavenumber with deprotonation is consistent with a shortening of the Si−O bond (1.55 Å in deprotonated Si−O− compared with 1.63 Å in neutral Si−OH16) and an increase in its oscillator force constant (vide inf ra). Sizedependent measurements provide further confirmation these

3. RESULTS AND DISCUSSION Bulk Vibrational Structures of Colloidal SiO2. The 700− 1450 cm−1 region of the ATR-FTIR spectra for 10 wt % 9 nm Ludox SM colloidal silica in 40 mM NaCl are shown in Figure 1a for bulk suspension pH of 9.8, 8.2, and 1.4. Measurements were also performed at bulk pH of 10.0, 9.0, 8.0, 7.3, 6.4, 6.0, 4.5, and 2.0 (not shown for clarity, see Figure S2). This region of the FTIR spectrum comprises the major absorption bands of silica (schematically shown in Figure 1a), namely the symmetric 3622

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the Si−O stretch vibration of the neutral Si−OH group to higher wavenumber. In the presence of electron-releasing groups, for instance the −O− of Si−O−, f R decreases and the band shifts to lower wavenumber. In fully relaxed models of the amorphous SiO2−water interface it has been shown that the negative charge on a terminal Si−O− group is partly delocalized across all the atoms within 1 nm of the deprotonation site.42 While the absolute values of the charge differences are small,41 this additional negative charge (which decreases the positive charge on the silicon atom) is sufficient to reduce the ionic character of the surrounding Si−OH bonds (recall this decreases f R) and shift the stretch vibration to the red. This is exactly the behavior shown in Figure 2 as a function of pH. At pH 10 the SCD of colloidal silica is ∼16 μC/cm2 (see right axis of Figure 2), equivalent to ∼1.0 e/nm2. Since charge delocalization extends up to ∼1 nm,42 this density of deprotonated sites is sufficient to affect the force constants of approximately all remaining surface bound neutral silanol groups. The results of Figure 2, which can be considered a calibration curve (0.8 μC/cm2/cm−1) for Ludox silica, reveals the ability to evaluate SCDs by FTIR spectroscopy based solely on the position of the stretch vibration of neutral surface bound silanol groups, Si−OH. In contrast to neighboring Si−(OH) bonds the Si−O bond of the deprotonated site, Si−O−, has increased ionic character due to the negative charge of the oxygen atom. This results in a shift of the Si−O stretch vibration to the blue of the neutral site and a characteristic energy ∼1040 cm−1. We note that this stretch vibration is more of a shoulder-like feature in the spectra (see Figure 1a), overlapping in part with the asymmetric O−Si−O stretch at 1121 cm−1, and therefore its exact energy and intensity are difficult to quantify. However, it is qualitatively clear from Figure 1 that the intensity of this mode increases with an increase in pH, consistent with the results of potentiometric titrations that show more deprotonated silanol sites, Si−O−, at higher pH. Nanoparticle Size Dependence. Under otherwise equivalent conditions the SCD of nanometer sized colloidal silica in aqueous solution varies as a function of particle size,6,43,44 decreasing with increasing diameter before reaching bulk-like

modes originate from the surface of the NPs by showing that the intensity of the surface modes increases with the surface to volume ratio of the NPs (Figure 1b). pH-Dependent Shift of the Si−O Stretch Vibration of Neutral Si−OH. The position of the Si−O stretch vibration from neutral silanol, Si−OH, is shown in Figure 2 (left axis) as a function of pH for 10 wt % 9 nm Ludox SM colloidal silica in 40 mM NaCl. The position of this absorption band is a strong function of pH, shifting from 962.0 ± 1 cm−1 at pH 10.0 to 983.1 ± 1 cm−1 at pH 1.4. On the right axis of Figure 2 is plotted the SCD of 5 wt % 9 nm Ludox SM colloidal silica in 40 mM NaCl electrolyte as determined by potentiometric titrations (sigmoid fit with a confidence interval of 95 is shown, see Figure S3 and Table S1 for individual data points). It becomes immediately clear from Figure 2 that the ATR-FTIR measured band position of the Si−O stretch vibration of neutral surface bound hydroxyl groups, Si−OH, is directly correlated with the number of deprotonated silanol sites, Si−O−. The shift in band position of the neutral Si−OH stretch vibration with bulk pH can be attributed to the significant ionic

Figure 2. Left axis, black triangles: ATR-FTIR band position of the Si−O stretch vibration of neutral silanols, Si−OH, from 10 wt % 9 nm Ludox SM colloidal silica as a function of pH in 40 mM NaCl. Right axis, red solid line: surface charge density of 5 wt % 9 nm Ludox SM colloidal silica in 40 mM NaCl as determined by potentiometric titrations. The line represents a sigmoid fit to the experimental data with a confidence interval of 95. See Figure S3 and Table S1 for the individual data points.

character of the Si−(OH) bond40 and its response to the local environment of the NPs surface. In molecules of the form X3SiOH inductive effects of electron-withdrawing substituent groups (e.g., X = F, O, ...) have been shown to increase the positive charge on the silicon atom, whereas electron-releasing groups (e.g., X = Me, Et, SiH3, ...) decrease it.41 A change in the charge density on the silicon atom increases (with increased positive charge) or decreases (with decreased positive charge) the ionic character of the Si−(OH) bond, which is known to have minimal effect on the structural parameter of the Si−O bond (for molecules where X = Et, Me, H, F, OH, MeO, H3SiO, F3SiO, and (OH)3SiO the Si−(OH) bond length varies by maximum 4%), but a significant effect (up to 31% for the same series of molecules) on its force constant ( f R).41 f R for a classical oscillator is the proportionality constant between the applied force and its displacement, as defined by Hooke’s law. In the case of a molecular oscillator, it is used to describe the stiffness of the bond connecting the atoms. f R increases when substituent groups of high electron-withdrawing character are present, which in FTIR spectroscopy has the effect of shifting

Figure 3. (a) ATR-FTIR spectra of 10 wt % colloidal 9 (red, Ludox SM), 14 (blue, Ludox HS), and 25 nm (black, Ludox TM) silica in 40 mM NaCl at pH 9.0. Only the region corresponding to the Si−O stretch vibration of neutral Si−OH is shown. (b) As in (a), except at pH 1.4. HCl was used to acidify the suspensions. 3623

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theoretical predictions of SCD for NPs of this size regime.6,44 Only with SiO2 particle diameters less than ∼5−10 nm are the SCDs predicted to sharply increase.6,44 In Figure 3b, the same spectral region is shown for the three different sized NPs at pH 1.4. The band position (solid marker in Figure 3b) is centered at 983−984 cm−1 independent of NP diameter. Using the calibration curve of Figure 2, we find (very small) positive SCDs at pH 1.4 (Table 1), consistent with the isoelectric point of silica at pH ∼2−3,12,14 and the presence of (a few) protonated silanol groups, Si−OH2+, on the surfaces of the NPs.13,45 Sodium Chloride (NaCl) Concentration Dependence. The SCD of colloidal silica at fixed pH is known to increase with an increase of NaCl electrolyte strength.19,29,38,47 Figure 4 shows the position of the Si−O stretch vibration from neutral silanol, Si−OH, as a function of NaCl concentration for 10 wt % 9 nm Ludox SM colloidal silica at pH 9.9 ± 0.2 (the gray band of Figure 4 is a fit using an exponential function with a confidence interval of 90). The position of the band shifts continuously to lower wavenumber with increasing NaCl concentration, e.g., 964.0 cm−1 in 10 mM and 958.5 cm−1 in 100 mM NaCl. These results are consistent with a higher negative SCD in 100 mM compared with 10 mM NaCl. Quantifying the result allows for comparison to SCDs derived from potentiometric titrations.29,38,47 The difference in band position in 10 and 100 mM NaCl is 5.5 cm−1, which corresponds, using the results of Figure 2, to a change in SCD of 4.4 μC/cm2. Given the slightly different experimental conditions (pH and particle diameter), this value is in very good agreement with 3.85 μC/cm2 found by Milonjić for 9 nm SiO2 at pH 9.529 and qualitatively agrees with the 2.5 μC/cm2 determined by Sonnefeld for 10 nm SiO2 at pH 8.047 and the 7.0 μC/cm2 reported by Bolt for 14 nm SiO2 at pH 10.0.38 Experiments in 10 and 100 mM NaCl have also been carried out as a function of bulk pH. Figure 5a shows the position of the Si−O stretch vibration from neutral silanol, Si−OH, as a function of pH for 10 wt % 9 nm Ludox SM colloidal silica in 10 (blue, closed markers) and 100 mM (black, open markers) NaCl. The shift to lower wavenumber in 100 mM NaCl at all pH greater than ∼7 is consistent with increased SCD. The growing difference of SCD (the curves start to diverge from another at higher pH) in the two concentrations of NaCl is well reproduced from the results of potentiometric titrations (Figure 5b).38

Table 1. Nanoparticle Size Dependence on the Position of the Si−(OH) Stretch Vibration in 40 mM NaCl Electrolyte sample Ludox SM Ludox HS Ludox TM

TEM diam (nm)

pH [±0.1]

νSi−(OH) (cm−1) [±1]

9.0 ± 1.6a 8.0 ± 1.6b 14.4 ± 2.9c 14.1 ± 2.4c 25.2 ± 3.9c,d 25.2 ± 4.3c

9.0 1.4 9.0 1.4 9.0 1.4

967.0 983.1 968.5 984.3 969.2 984.0

SCDe,f (μC/cm2) −12.2 −0.3 −11.0 0.9 −10.5 0.3

± ± ± ± ± ±

0.8 0.8 0.8 0.8 0.8 0.8

a

From ref 46. bFrom ref 5. cSee Figure S5. dFrom ref 6. eDetermined from the calibration curve of Figure 2. fError estimated using ±1 cm−1 (0.8 μC/cm2/cm−1).

Figure 4. Band position of the Si−O stretch vibration of neutral silanols, Si−OH, as a function of NaCl electrolyte strength for 9 nm Ludox SM colloidal silica at pH 9.9 ± 0.2. The gray band is a fit using an exponential function with a confidence interval of 90.

properties by ∼100 nm. Figure 3a shows the ATR-FTIR spectra for 10 wt % Ludox silica NP suspensions of 9, 14, and 25 nm diameter in 40 mM NaCl electrolyte at pH 9.0 (only the neutral Si−(OH) stretch region is shown for emphasis, see Figure S4 for the 700−1450 cm−1 region). There is a dependence of the band position on the diameter of the NP (Table 1), shifting to higher wavenumber with increasing NP diameter. This result is consistent with increased SCD on the smaller NPs and can be quantified using the calibration curve of Figure 2 under the assumption that the total number of silanol sites (neutral plus charged, 4.6−4.9 OH/nm2 for amorphous silica17) is independent of NP diameter. In this case, the SCD of colloidal silica in 40 mM NaCl at pH 9.0 is −10.5 ± 0.8 μC/ cm2 (25 nm), −11.0 ± 0.8 μC/cm2 (14 nm), and −12.2 ± 0.8 μC/cm2 (9 nm). These relatively small, but reproducible, shifts in band position with NP diameter are consistent with

Figure 5. (a) Band position of the Si−O stretch vibration of neutral silanols, Si−OH, as a function of bulk pH for 9 nm Ludox SM colloidal silica in 10 and 100 mM NaCl. The solid lines are fits using a Sigmoid function. (b) Surface charge density of 14 nm Ludox colloidal silica in 10 and 100 mM NaCl as measured by potentiometric titration and reported by Bolt.38 3624

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Salmeron, M. Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides. Science 2005, 307, 563−566. (3) Zaera, F. Probing Liquid/Solid Interfaces at the Molecular Level. Chem. Rev. 2012, 112, 2920−2986. (4) Zaera, F. Surface Chemistry at the Liquid/Solid Interface. Surf. Sci. 2011, 605, 1141−1145. (5) Brown, M. A.; Beloqui Redondo, A.; Sterrer, M.; Winter, B.; Pacchioni, G.; Abbas, Z.; van Bokhoven, J. A. Measure of Surface Potential at the Aqueous-Oxide Nanoparticle Interface by XPS from a Liquid Microjet. Nano Lett. 2013, 13, 5403−5407. (6) Brown, M. A.; Duyckaerts, N.; Beloqui Redondo, A.; Jordan, I.; Nolting, F.; Kleibert, A.; Ammann, M.; Wörner, H. J.; van Bokhoven, J. A.; Abbas, Z. Effect of Surface Charge Density on the Affinity of Oxide Nanoparticles for the Vapor-Water Interface. Langmuir 2013, 29, 5023−5029. (7) Brown, M. A.; Jordan, I.; Beloqui Redondo, A.; Kleibert, A.; Wörner, H. J.; van Bokhoven, J. A. In Situ Photoelectron Spectroscopy at the Liquid/Nanoparticle Interface. Surf. Sci. 2013, 610, 1−6. (8) Azam, M. S.; Darlington, A.; Gibbs-Davis, J. M. The Influence of Concentration on Specific Ion Effects at the Silica/Water Interface. J. Phys.: Condens. Matter 2014, 26, 244107. (9) Geiger, F. M. Second Harmonic Generation, Sum Frequency Generation, and chi((3)): Dissecting Environmental Interfaces with a Nonlinear Optical Swiss Army Knife. Annu. Rev. Phys. Chem. 2009, 60, 61−83. (10) Flores, S. C.; Kherb, J.; Konelick, N.; Chen, X.; Cremer, P. S. The Effects of Hofmeister Cations at Negatively Charged Hydrophilic Surfaces. J. Phys. Chem. C 2012, 116, 5730−5734. (11) Roke, S.; Gonella, G. Nonlinear Light Scattering and Spectroscopy of Particles and Droplets in Liquids. Annu. Rev. Phys. Chem. 2012, 63, 353−378. (12) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica; Wiley: New York, 1979. (13) Duval, Y.; Mielczarski, J. A.; Pokrovsky, O. S.; Mielczarski, E.; Ehrhardt, J. J. Evidence of the Existence of Three Types of Species at the Quartz-Aqueous Solution Interface at pH 0−10: XPS Surface Group Quantification and Surface Complexation Modeling. J. Phys. Chem. B 2002, 106, 2937−2945. (14) Parks, G. A. The Isoelectric Points of Solid Oxides Solid Hydroxides and Aqueous Hydroxo Complex Systems. Chem. Rev. 1965, 65, 177−198. (15) Ostroverkhov, V.; Waychunas, G. A.; Shen, Y. R. Vibrational Spectra of Water at Water/Alpha-Quartz (0001) Interface. Chem. Phys. Lett. 2004, 386, 144−148. (16) Brown, M. A.; Arrigoni, M.; Heroguel, F.; Beloqui Redondo, A.; Giordano, L.; van Bokhoven, J. A.; Pacchioni, G. pH Dependent Electronic and Geometric Structures at the Water-Silica Nanoparticle Interface. J. Phys. Chem. C 2014, 118, 29007−29016. (17) Zhuravlev, L. T. The Surface Chemistry of Amorphous Silica. Zhuravlev Model. Colloids Surf., A 2000, 173, 1−38. (18) Nelson, D. M.; Treguer, P.; Brzezinski, M. A.; Leynaert, A.; Queguiner, B. Production and Dissolution of Biogenic Silica in the Ocean - Revised Global Estimates, Comparison with Regional Data and Relationship to Biogenic Sedimentation. Global Biogeochem. Cycles 1995, 9, 359−372. (19) Dove, P. M.; Craven, C. M. Surface Charge Density on Silica in Alkali and Alkaline Earth Chloride Electrolyte Solutions. Geochim. Cosmochim. Acta 2005, 69, 4963−4970. (20) Sposito, G.; Skipper, N. T.; Sutton, R.; Park, S. H.; Soper, A. K.; Greathouse, J. A. Surface Geochemistry of the Clay Minerals. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 3358−3364. (21) Tan, W. H.; Wang, K. M.; He, X. X.; Zhao, X. J.; Drake, T.; Wang, L.; Bagwe, R. P. Bionanotechnology Based on Silica Nanoparticles. Med. Res. Rev. 2004, 24, 621−638. (22) Cruz-Chu, E. R.; Aksimentiev, A.; Schulten, K. Water-Silica Force Field for Simulating Nanodevices. J. Phys. Chem. B 2006, 110, 21497−21508.

4. CONCLUSION This study presents a detailed investigation of the surface charge density of colloidal silica in aqueous solutions using ATR-FTIR spectroscopy. The position of the Si−O stretch vibration from neutral surface bound silanol groups, Si−OH, is shown to correlate with the density of deprotonated sites, Si−O−, shifting to lower wavenumber with an increase in surface charge. The origin of this shift is a reduction in the ionic character of the Si−O bond on neighboring Si−OH groups after delocalization of the negative charge left on the terminal Si−O− group across the atoms within ∼1 nm of the charged site. The reduced ionic character of this bond decreases its force constant, which results in a shift of the band position to the red. We use this new methodology to quantitatively determine the surface charge density of 9, 14, and 25 nm (diameter) colloidal silica as a function of NaCl electrolyte strength and bulk pH. Our measurements reproduce the predicted increase in surface charge density as the diameter of the particle is decreased (under otherwise equivalent conditions) and the well-known increase of surface charge density with an increase in electrolyte strength. The use of an ATR optical configuration is convenient but is not a requirement for the determination of silica SCD. Measurements can easily be extended to a transmission configuration whenever an ATR geometry is not feasible, such as for samples in a microfluidic channel or in a microjet, provided that the sample is sufficiently thin. For thick aqueous samples Raman spectroscopy would prove more practical due to its increased penetration depth.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S5, Table S1, and the experimental procedure for transmission electron microscopy (TEM) measurements. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

L.Q.: Functional Genomics Center Zurich, CH-8057 Zurich, Switzerland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Agilent Technologies Schweiz AG (Basel, Switzerland) for the provision of the FTIR spectrometer and technical support during the time of investigation. The FTIR spectrometer is maintained by Portmann Instruments AG (Biel-Benken, Switzerland). A.G. and M.A.B. are supported by the Swiss National Science Foundation (no. 153578). Professor Nicholas D. Spencer and the LSST are acknowledged for continued support.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b00418 Langmuir 2015, 31, 3621−3626

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DOI: 10.1021/acs.langmuir.5b00418 Langmuir 2015, 31, 3621−3626