Structures of Water Molecules at the Interfaces of Aqueous Salt

Apr 20, 2009 - Structures of water molecules at water/silica interfaces, in the presence of alkali chloride, were investigated using infrared−visibl...
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J. Phys. Chem. C 2009, 113, 8201–8205

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Structures of Water Molecules at the Interfaces of Aqueous Salt Solutions and Silica: Cation Effects Zheng Yang, Qifeng Li, and Keng C. Chou* Department of Chemistry, UniVersity of British Columbia, VancouVer, BC V6T 1Z1, Canada ReceiVed: December 31, 2008; ReVised Manuscript ReceiVed: February 19, 2009

Structures of water molecules at water/silica interfaces, in the presence of alkali chloride, were investigated using infrared-visible sum frequency vibrational spectroscopy. Significant perturbations of the interfacial water structure were observed on silica surfaces with the NaCl concentration as low as 1 × 10-4 M. The cations, which interact with the silica surface via electrostatic interaction, play key roles in perturbing the hydrogen-bond network of water molecules at the water/silica interface. This cation effect becomes saturated at concentrations around 10-2 to 10-1 M, where the sum frequency generation peaks at 3200 and 3400 cm-1 decrease by 75%. Different alkali cation species (Li+, Na+, and K+) produce different magnitudes of perturbation, with K+ > Li+ > Na+. This order can be explained by considering the effective ionic radii of the hydrated cations and the electrostatic interactions between the hydrated cations and silica surfaces. The interfacial water structure associated with the 3200 cm-1 band is more vulnerable to the cation perturbation, suggesting that the more ordered water structure on silica is likely associated with the vincinal silanol groups, which create a higher local surface electrical field on silica. I. Introduction Buried aqueous interfaces play an important role in many natural and industrial processes. Among the various aqueous interfaces, water/silica interfaces, which affect contaminant migration, ice nucleation, soil formation, and micro-organism growth, have been of great interest.1-8 Current understanding of buried aqueous interfaces remains limited because it is experimentally challenging to probe a buried interface. For this reason, a large number of theoretical simulations of water interfaces have been carried out to provide microscopic information, such as the density profiles and orientations of water molecules at interfaces.9-18 These calculations have provided qualitative information, but the simulation results are often limited by the capacity of computers, which can be insufficient for practical conditions involving large numbers of molecules. Previously, the structure of water molecules at buried aqueous interfaces were studied using X-ray spectroscopy,19-21 electron diffraction,22 second harmonic generation (SHG),23 and sum frequency generation (SFG) vibrational spectroscopy.2,24,25 Generally, water structures at a hydrophilic solid surface, such as silica, are understood to be a mixture of a more ordered and a less ordered hydrogen-bond network, which are associated with vibrational peaks at 3200 and 3400 cm-1, respectively.26 In any case, the detailed structure of water molecules on silica is not yet completely understood. Experimentally, the structure of water on silica has been shown to depend on the surface charges. Silica surfaces possess negative charges because of the deprotonation of surface silanol groups (SiOH). The surface charges, which create a surface electric field, induce polar ordering of interfacial water molecules. Ong et al. studied water/silica interfaces with various pH values using SHG.23 They concluded that water molecules near the silica interface were polarized by the interfacial electric field and responsible for the observed SHG. A few years later, * To whom correspondence [email protected].

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Du et al. studied OH vibrations of water molecules at water/ fused quartz interfaces using IR-visible SFG vibrational spectroscopy.25 Their results showed that the orientation of the OH bonds and the ordering of interfacial water molecules are strongly affected by their electrostatic interaction with the deprotonated surface silanol groups. In a recent study on water/ quartz interfaces, Shen and his co-workers showed that two different surface sites exist with different deprotonation pK values on crystalline quartz.27 The peak at 3200 cm-1 seemed to be associated with surface sites that have higher pK values, and the peak at 3400 cm-1 was closely associated with surface sites having lower pK values. The detailed mechanism for the formation of these two different sites is not yet understood. Little is known about the structure of interfacial water molecules under perturbations by cations. Alkali cations are particularly important, as they are the most abundant cations in natural water. Previous studies at air/water interfaces with NaCl solutions showed that a reduced ion density was present near the surface, and the ions produced little effect on the surface water structure, for molar fractions up to 0.036 (∼2 M).28,29 The environment of an air/water interface is very different from that of a water/silica interface, because cations can interact with the silica surface via electrostatic interactions.30 This paper presents studies of water structures on silica surfaces using IRvisible SFG vibrational spectroscopy. Significant perturbation of the water structures was observed with a relatively low concentration of NaCl in water. Further, different alkali cation species (Na+, Li+, and K+) showed different degrees of impact on the interfacial water structures. The electrostatic interactions between the hydrated cations and the silica surface as well as the effective ionic radii of the cations need to be considered to explain the observed phenomena. II. Experimental Section The visible and tunable IR laser beams for SFG vibrational spectroscopy were obtained from a Nd:YAG (yttrium aluminum

10.1021/jp811517p CCC: $40.75  2009 American Chemical Society Published on Web 04/20/2009

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Figure 1. Schematic layout of the SFG vibrational spectroscopy. The frequency of the visible beam was fixed at 532 nm, and the frequency of the IR beam was scanned from 2800-3900 cm-1. The 532-nm and IR beams were overlapped both spatially and temporally on the bottom surface of the solution.

garnet) laser with an output wavelength of 1064 nm (30 ps, 40 mJ/pulse, and 10 Hz). The laser was used to generate a second harmonic beam at 532 nm in a KTiOPO4 (KTP) crystal. The tunable IR beam was produced by difference frequency mixing of the 1064 nm beam with the output of a homemade optical parametric generator/amplifier (OPG/OPA) pumped by the 532nm beam. The 532 nm and IR beams were overlapped, both spatially and temporally, on the sample, as shown in Figure 1. The laser fluence was approximately 2 mJ/cm2 per pulse for the visible beam and 5 mJ/cm2 per pulse for the IR beam. The polarizations of the beams were s-, s-, and p-polarized for SFG, visible, and IR, respectively. The SFG intensity was detected by a photomultiplier tube after spatial filtering by an aperture and spectral filtering by a bandpass filter. The SFG intensity was normalized against that from a z-cut quartz. Each spectrum shown in the current study was an average of 10 scans in a 10 cm-1 step, and each scan was obtained by averaging the SFG intensity of 40 laser shots at each step. Fused silica plates, with a thickness of 3 mm, were cleaned with a commercial cleaning agent (Extran AP12) for 3 min. They were immersed in a 50/50 (v/v) HNO3/H2SO4 solution for ∼12 h, followed by rinsing in pure water (resistivity >18.2 MΩ · cm, Millipore). Alkali chloride salts purchased from Sigma Aldrich (>99.8%, certified ACS reagents) were used to prepare solutions with various concentrations. No organic contamination in the salt solutions was observed in the SFG spectra between 2700 and 3000 cm-1. All data presented in the current study were collected within a period of two weeks. During the experimental period, the silica substrates were mostly kept either in air or acidic solutions to avoid surface quality changes because of prolonged exposures to pure water, as previously reported by Li et al.31 The SFG spectrum of the pure water/ silica interface was also monitored at the beginning and the end of the experiment for each electrolyte to ensure that the quality of the silica surface stayed consistent during the experimental period. After the pure water/silica SFG spectrum was measured, the SFG spectra of a series of NaCl solutions with concentrations of 1 × 10-4 M, 5 × 10-4 M, 1 × 10-3 M, 1 × 10-2 M, and 1 × 10-1 M were measured in the sequence from the lowest concentration to the highest concentration. For each solution with a particular concentration, the cell and silica plates were rinsed thoroughly with the solution before spectroscopic measurements. The rinsing process ensured that the bulk solution in the cell had the desired electrolyte concentration. For each concentration, five scans were collected in a time period of ∼1 h, during which no

Yang et al. observable change of the SFG spectrum was observed. Then the cell and silica plates were cleaned with acids as described above for measurements with a different electrolyte solution (LiCl and KCl). The measurements of different electrolytes were carried out using the same silica substrate within a period of one week, and the experiments were repeated under the same condition in the second week with freshly prepared solutions and the same silica substrate. There was no observable difference in the SFG spectra in comparison to those collected in the previous week. Finally, the 10 spectral scans of the same electrolyte with the same concentration (five scans in the first experiment and five scans in the repeated experiment) were averaged to improve the signalto-noise ratio. As the pH values of solutions can affect the surface water structure, great attention has been made to monitor the pH values of the solutions to ensure that the observed changes of SFG spectra were not a pH effect. The pH value of water was 7 when freshly obtained from the Milli-Q system. However, it is known that water exposed to air is mildly acidic because water readily absorbs carbon dioxide from the air. It ultimately leads to a pH of ∼5.7.32,33 All salts studied in this paper are known as neutral salts, which cause little change of the pH values in aqueous solutions. The pH values of all solutions used in the current study were around 5.74 with a standard deviation of 0.05. A table presenting the pH value for each individual solution was provided in the supporting document. III. Results and Analysis Figure 2A shows the SFG vibrational spectra of water/silica interfaces with pure water and aqueous NaCl solutions of 1 × 10-4 M, 5 × 10-4 M, 1 × 10-3 M, 1 × 10-2 M, and 1 × 10-1 M. The spectra of water exhibit two peaks at 3200 and 3400 cm-1.24,25 As the concentration of NaCl increased, the intensity of both peaks decreased. It has been interpreted that the peak at 3200 cm-1 represents OH in a more ordered hydrogen-bond network, and the peak at 3400 cm-1 represents a less ordered hydrogen-bond network.26,34,35 It is known that silanol groups on silica play a critical role in determining the molecule adsorption on the surface in an aqueous solution.30 The dissociation of protons from the surface silanol groups creates negative charges on a silica surface. The reaction can be described as follows

-SiOH + mH2O ) -SiO- · · · mH2O + H+

(1)

where -SiOH is the surface silanol group and m describes the number of water molecules associated with the -SiO-.36,37 The silanol groups are estimated to have a surface density of ∼5 × 1014 cm-2 on the silica surface, which is equivalent to one silanol group per 20 Å2.30,38 When the pH value of the aqueous solution increases, the silanol groups become deprotonated, and the surface charge increases. Experimentally, both the 3200 and 3400 cm-1 peaks were observed to increase with increasing pH value, suggesting that a larger surface electrical field induces a better ordered hydrogen-bond network on silica.25,27,39 Previous studies by Ong et al., using SHG, showed that there were two types of silanol groups at a water/silica interface, with pK values of 4.9 and 8.5, populating 19 and 81% of the surface area, respectively.23 Further studies by Ostroverkhov et al., using a phase-sensitive SFG technique, indicated that the 3200 cm-1 peak is associated with surface sites that have a higher pK value and that the 3400 cm-1 peak is associated with surface sites with a lower pK value.27

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Figure 3. Amplitudes (Aq) as functions of concentrations of alkali chloride solutions for water/silica interfaces obtained by fitting the spectra in Figure 2 with eq 2.

The interaction of a hydrated cation with a silica surface has been shown to promote negative charge development.30 The reaction can be described as Figure 2. SFG spectra from the water/silica interfaces with different concentrations of three alkali chloride solutions: (A) NaCl, (B) LiCl, and (C) KCl. The solid lines are fitting curves derived using two Lorentzian peaks at 3200 and 3400 cm-1.

To obtain quantitative information, the spectra in Figure 2 were fitted by two Lorentzian peaks at 3200 and 3400 cm-1

|

(2) I(ωSFG) ∝ χNR +

∑

q)1,2

Aq ωIR-ωq+iΓq

|

2

(2)

(2) where χNR is the nonresonant contribution, ωIR is the frequency of the input infrared laser beam, q is the qth vibrational mode, Aq is the amplitude, Γq is the width, and ωq is the resonant frequency. The fitting curves are shown in Figure 2 as solid lines, and the fitted amplitudes Aq are plotted in Figure 3. Parts B and C of Figure 2 show the SFG spectra collected with LiCl and KCl solutions. The magnitudes of the decreases are different when the cation species are changed. Similar experiments with different anions, such as NaBr and NaI, were also carried out, but, within measurement error, the spectra of NaBr and NaI were the same as those of the NaCl solutions. (Spectra of NaBr and NaI solutions are not shown.) Therefore, the cation, which interacts with the silica surface via electrostatic interactions, is the key factor in perturbing the hydrogen-bond network at the water/silica interface.

-SiOH + nH2O · M+ ) -SiO- · · · nH2O · M+ + H+ (3) where M+ denotes a cation and n describes the number of water molecules solvating M+. The notation, -SiO- · · · nH2O · M+, indicates that cations are located at a small but finite distance from the silica surface. Overall, the surface charges created by the cations are mostly neutralized by the cations themselves. Therefore, the surface charges developed by the cations are not expected to enhance the ordering of surface water molecules, or the observed SFG intensity. The pH of the point of zero charge for amorphous silica is about 2-3.40 With a solution pH value of 5.7, the silica surface is negatively charged. The surface -SiO- groups, described in eq 1, interact with the cations via electrostatic interaction30

-SiO- · · · mH2O + nH2O · M+ ) -SiO- · · · nH2O · M+ + mH2O

Kassc

(4)

where Kassc is the equilibrium constant. As a result of the electrostatic interaction, the cations reduce the surface electrical field. Overall, the ordering of the original hydrogen-bond network is perturbed because of the decrease in the surface electrical field and the replacement of the more ordered water molecules by the hydrated cations. Qualitatively, this mechanism explains that the SFG intensity of interfacial water peaks decreases as the salt concentration increases.

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As shown in Figures 2 and 3, for the same concentration, the SFG intensities of interfacial water in alkali chloride solutions are in the order: Na+ > Li+ > K+. To explain the different behaviors for Li+, Na+, and K+, both the equilibrium constant Kassc in eq 4 and the effective ionic radii of the hydrated cations need to be considered. First, the equilibrium constant Kassc describes the interaction between the surface -SiO- groups and the hydrated cation nH2O · M+. Previous studies by Dove et al. showed that the equilibrium constant Kassc for alkali chloride solutions is in the order: Kassc,KCl > Kassc,NaCl > Kassc,LiCl.41 A larger equilibrium constant Kassc indicates a higher density of “-SiO- · · · nH2O · M+” on the silica surface; thus, a larger perturbation of the water structure and, consequently, a smaller SFG intensity. Therefore, if only the equilibrium constant Kassc is considered, the SFG peak intensity would be in the order: Li+ > Na+ > K+. Second, the sizes of the hydrated cations should be considered. Ions in aqueous solution are hydrated. Generally, the smaller and higher-charged ions attract more water molecules. The hydrated ionic radii of Li+, Na+, and K+ are approximately 0.6, 0.4, and 0.3 nm, respectively.42 In an aqueous solution, the radius of a hydrated Li+ is roughly twice that of a hydrated K+. Therefore, a hydrated Li+ displaces more ordered H2O at the surface. Consequently, if only the effective ionic radii are considered, one would expect that the SFG intensity of water is in the order: K+ > Na+ > Li+. Overall, the balance between these two effects gives the observed SFG intensity in the order: Na+ > Li+ > K+. The SFG intensity is lower for K+ and Li+ solutions because K+ has a stronger interaction with the surface -SiO- groups and Li+ has a larger effective ionic radius. These two effects, even though they perturb the interfacial water ordering in different ways, are not totally independent. The size of a hydrated cation is determined by the electrostatic interaction between the cation and water molecules, and the strength of the interaction between the hydrated cation and the silica surface depends on the size of the hydrated cation.43-46 The structures of the water of hydration remain as an active research area, and many theoretical studies of the detailed structures can be found in the references.47-52 As shown in Figure 3, the peak at 3200 cm-1 is more vulnerable to the cation perturbation. The amplitude of the 3200 cm-1 peak experiences an ∼20% decrease with a concentration of 1 × 10-4 M, while the 3400 cm-1 peak does not have an significant decrease until 1 × 10-2 M. Additionally, the perturbation for the 3200 cm-1 peak reaches its saturation at a concentration of 0.01 M, while the perturbation for the peak at 3400 cm-1 is not saturated until 0.1 M. Eventually, both peaks have a 50% decrease in their amplitudes Aq, which is equivalent to a 75% decrease in the measured SFG intensity. Since the peak at 3200 cm-1 is more vulnerable to the cation perturbation, the water structure associated with this peak is more likely to exist in the region where the surface electrical field is higher. As described above, cations interact with the silica surface via electrostatic interaction, and therefore, the surface density of cations is expected to be higher in the region where the surface electrostatic field is strong. Previous studies have suggested that the silanol groups with lower pK values are isolated silanol groups because they are relatively easier to dissociate.23,30,53 Vicinal silanol groups, which locate closely and can be coupled to each other through hydrogen bonds, have higher pK values. The vincinal silanol groups are likely to create a higher local surface charge density because of their higher local number density. Therefore, vincinal silanol groups are more likely to create a higher local electrical field on the silica surface and, consequently, a better ordered structure and attract

Yang et al. more cations to the surface. This model is consistent with previous observations by Ostroverkhov et al., indicating that the peak at 3200 cm-1 is associated with surface sites that have a higher pK value.27 IV. Conclusions IR-visible sum frequency vibrational spectroscopy was applied to study the structure of water molecules at water/silica interfaces with the presence of alkali chloride in the solutions. Significant perturbations of the interfacial water structures were observed with 1 × 10-4 M of NaCl in the solution. The cations play a key role in perturbing the hydrogen-bond network at the water/silica interfaces as they interact with the silica surface via electrostatic interaction. Different alkali cation species produce different degrees of perturbation in the order: K+ > Li+ > Na+. This order can be explained by considering the effective ionic radii of the cations and the electrostatic interaction between the cations and the silica surface. The peak at 3200 cm-1 experiences lager perturbation suggesting that the more ordered structure at 3200 cm-1 is associated with the vincinal silanol groups, which produce a higher surface electrical field and have a higher pK value compared to isolated silanol groups. Acknowledgment. This work was financially supported by the Natural Sciences and Engineering Research Council of Canada. Supporting Information Available: A table presenting the pH values for all solutions used in the current study is given in the supporting document. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kim, J.; Kim, G.; Cremer, P. S. J. Am. Chem. Soc. 2002, 124, 8751. (2) Gurau, M. C.; Kim, G.; Lim, S. M.; Albertorio, F.; Fleisher, H. C.; Cremer, P. S. Chemphyschem 2003, 4, 1231. (3) Fisk, J. D.; Batten, R.; Jones, G.; O’Reilly, J. P.; Shaw, A. M. J. Phys. Chem. B 2005, 109, 14475. (4) Kim, J.; Kim, G.; Cremer, P. S. Langmuir 2001, 17, 7255. (5) O’Reilly, J. P.; Butts, C. P.; I’Anson, I. A.; Shaw, A. M. J. Am. Chem. Soc. 2005, 127, 1632. (6) Konek, C. T.; Musorrafiti, M. J.; Al-Abadleh, H. A.; Bertin, P. A.; Nguyen, S. T.; Geiger, F. M. J. Am. Chem. Soc. 2004, 126, 11754. (7) Hassanali, A. A.; Singer, S. J. J. Phys. Chem. B 2007, 111, 11181. (8) Helms, C. R.; Deal, B. E.; Electrochemical Society. Electronics Division.; Electrochemical Society. Dielectric Science and Technology Division. The Physics and chemistry of SiO b2 s and the Si-SiO b2 s interface 2; Plenum Press: New York, 1993. (9) Frenkel, J. Kinetic Theory of Liquids; Dover: New York, 1955. (10) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. J. Chem. Phys. 1984, 80, 4448. (11) Rossky, P. J.; Lee, S. H. Chemica Scripta 1989, 29A, 93. (12) Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. (13) Jedlovszky, P.; Vincze, A.; Horvai, G. J. Mol. Liq. 2004, 109, 99. (14) Jedlovszky, P.; Vincze, A.; Horvai, G. PCCP 2004, 6, 1874. (15) Feibelman, P. J. Chem. Phys. Lett. 2004, 389, 92. (16) Puibasset, J.; Pellenq, R. J. M. J. Chem. Phys. 2003, 119, 9226. (17) Puibasset, J.; Pellenq, R. J. M. PCCP 2004, 6, 1933. (18) Benjamin, I. J. Chem. Phys. 2004, 121, 10223. (19) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Nature 1994, 368, 444. (20) Reedijk, M. F.; Arsic, J.; Hollander, F. F. A.; de Vries, S. A.; Vlieg, E. Phys. ReV. Lett. 2003, 90. (21) Chu, Y. S.; Lister, T. E.; Cullen, W. G.; You, H.; Nagy, Z. Phys. ReV. Lett. 2001, 86, 3364. (22) Ruan, C. Y.; Lobastov, V. A.; Vigliotti, F.; Chen, S. Y.; Zewail, A. H. Science 2004, 304, 80. (23) Ong, S. W.; Zhao, X. L.; Eisenthal, K. B. Chem. Phys. Lett. 1992, 191, 327. (24) Du, Q.; Freysz, E.; Shen, Y. R. Science 1994, 264, 826. (25) Du, Q.; Freysz, E.; Shen, Y. R. Chem. Phys. Lett. 1994, 72, 238. (26) Shen, Y. R.; Ostroverkhov, V. Chem. ReV. 2006, 106, 1140.

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