Perturbations of Water by Alkali Halide Ions Measured using

1805

2009, 113, 1805–1809 Published on Web 01/29/2009

Perturbations of Water by Alkali Halide Ions Measured using Multivariate Raman Curve Resolution Pradeep N. Perera, Breanna Browder, and Dor Ben-Amotz* Purdue UniVersity, West Lafayette, Indiana ReceiVed: October 02, 2008; ReVised Manuscript ReceiVed: December 05, 2008

Polarized Raman spectroscopy, combined with multivariate curve resolution (MCR), is used to measure the influence of dilute alkali halide ions on the OH stretch vibrational band of water. The frequency and integrated intensity of the resulting hydration shell OH bands are found to increase with increasing anion size. Comparisons of results obtained from salt solutions in H2O and HOD/D2O imply that ion-water interactions reduce the influence of resonance coupling on the OH stretch band of H2O. Polarized Raman results indicate that the hydration shell of F- gives rise to a nearly perfectly polarized OH stretch band, while large anions produce larger depolarization. I. Introduction Vibrational spectroscopy can provide information pertaining to ion-water interactions which is of relevance to a wide variety of biological and geological processes. However, traditional data analysis methods such as difference spectroscopy1-3 and peak fitting4,5 do not, in general, produce the actual spectral shapes of perturbed water molecules. As an alternative strategy, we have recently shown that multivariate curve resolution (MCR)6,7 can be used to extract spectra of solvent molecules that are perturbed by solutes without making any assumptions regarding the shape of the perturbed solvent spectrum (other than assuming that it has a non-negative concentration and intensity).8 Briefly, we use MCR to decompose a data matrix containing measured mixture spectra (D) into an inner-product of a concentration matrix (C) and a matrix of pure component spectra (S). In alkali halide solutions of sufficiently low concentration, all of the spectra in the measured mixture matrix may be represented as a linear combination of bulk water (W) and perturbed water (PW) spectra, weighted by their respective concentrations, CW and CPW.

D ) CWSW + CPWSPW

(1)

Previous static and dynamics vibrational spectroscopic studies of aqueous salt solutions have often employed HOD/D2O rather than H2O as the solvent9-12 in order to suppress both intramolecular and intermolecular resonance coupling between OH groups. Our MCR-based method is compatible with both normal and isotopically dilute water and therefore may be used to reveal solute-induced perturbations of water and how those perturbations are influenced by both intramolecular and intermolecular resonance coupling, as we will see. Since the water hydrogen atoms point predominantly toward anions and away from cations, one would expect the vibrational spectrum of water to be more strongly perturbed by anions than * To whom correspondence should be addressed.

10.1021/jp808732s CCC: $40.75

cations. This expectation is largely consistent with the results of previous Raman studies of aqueous alkali halide solutions. More specifically, the OH stretch band of water blue shifts (increases in frequency) with increasing anion size,12,13 while alkali cations have relatively little effect on the water OH Raman band shape.13-15 Moreover, the water OH stretch Raman scattering cross section increases by over a factor of 2 in going from F- to I-13 but is little affected by alkali cations.14 Although these general trends hold for the entire alkali halide series, the smallest members of the series are exceptional, as both Li+ and F- produce a red shift of the water OH stretch band (while other halide anions produce a blue shift, and other alkali cations apparently produce little or no shift).12,16,17 Moreover, F- is the only halide anion which decreases the Raman cross section of the water OH stretch band.12,13 A variety of other methods have also been used to investigate aqueous alkali halide solutions. For example, neutron scattering studies suggest that perturbations of the water structure around both anions and cations are largely localized to the first hydration shell.10 Recent time-resolved infrared (pump-probe) studies by Bakker and co-workers indicate that anions slow the vibrational and rotation relaxation times of water in the first hydration shell (while cations have little effect on water dynamics).18-20 The latter studies also imply that ions have little or no effect on the dynamics of water beyond the first hydration shell. A similar conclusion was reached in a recent study by Smith et al.,12 whose combined ion-water cluster density functional theory (DFT) calculations, computer simulations, and Raman studies of aqueous alkali halide solutions indicate that perturbation of the OH stretch band of water by anions arises largely from the first solvation shell, rather than from long-range ordering (or disordering) of water.12 Since the first hydration shells of alkali halide ions contain fewer than 10 water molecules, solutions of ∼1 M or lower concentration (with more than 25 water molecules per ion) are expected to have largely nonoverlapping hydration shells, thus justifying the use of eq 1. In this work, we use Raman spectroscopy and MCR to decompose the spectra of series of low-concentration alkali  2009 American Chemical Society

1806 J. Phys. Chem. B, Vol. 113, No. 7, 2009

Letters

Figure 1. The left-hand panel compares the Raman spectrum of pure water (dashed) with the ion-correlated MCR spectra of NaCl(aq) (solid) and KCl(aq) (dotted). The middle panel compares the ion-correlated MCR spectra of NaCl(aq) in H2O (solid) and HOD/D2O (dashed). The right-hand panel compares the Raman spectra of salt-free H2O (solid) and HOD/D2O (dashed). All spectra are scaled to the same peak intensity and are derived from nonpolarized Raman measurements. Results obtained from different salt solutions and using different detection polarizations are shown in Figure 2.

halide solutions (e1 M) into a linear combination of the spectra of bulk water and water that is perturbed by ions. Our results confirm trends inferred from previous Raman studies of more concentrated solutions and extend these to obtain additional information regarding ion-induced perturbations of the spectral shape, depolarization, and resonance coupling of the OH stretch band of water. II. Experimental Section Ultrapure Millipore water (18.2 Ω) and 99.99% D2O (Cambridge Isotope Laboratory, Inc.) were used for sample preparation. Salts were obtained from various vendors and used as received. For each salt, five solutions ranging in concentration from 0 to 1 M were prepared. Raman spectra of these solutions were obtained and analyzed using MCR (as described below). Raman spectra were collected using a home-built microRaman (back-scattering) system with a 514.5 nm diode laser, fiber-coupled to a 300 mm imaging spectrograph (Acton SpectraPro 300i) with a liquid-N2-cooled CCD detector (Princeton Instruments, 1024 × 256 pixel). A long working distance objective (Olympus, 20×) was focused about 1 mm inside of the glass wall of a 4 mL round glass vial (to avoid scattering from the glass). The excitation laser power at the sample was ∼80 mW, and the integration time for each spectrum was 400 s. For polarization studies, parallel (V-polarized) and perpendicular (H-polarized) spectra were collected for each sample by placing a wire grid polarizer plate (Edmund Optics Inc.) between the objective and the collection fiber bundle which transmits the Raman scattered light to the CCD/spectrograph for detection. The isotropic and anisotropic Raman scattering components were calculated in the usual way, Isotropic ) V - (4/3)H; Anisotropic ) (4/3)H.21 Cosmic spike artifacts were removed using the upper-bound variance minimization method.22 The small residual baseline in each spectrum was removed by subtracting a constant value (near the minimum intensity value of each spectrum). The resulting spectra were then normalized to unit area. MCR analysis was performed using the PLS 2.0 toolbox (Eigenvector Inc.), with the initial (nominal) mole fractions of each component as an initial guess for the concentration matrix (the MCR results are insensitive to the initial conditions, as identical results

are obtained using other initial guess concentrations up to 5 times the nominal salt mole fractions). The predicted concentrations of the bulk water and perturbed water spectra in the pure water sample were constrained to 1 and 0, respectively (to identify this as the zero ion concentration spectrum). III. Results The left-hand panel of Figure 1 shows the ion-correlated OH spectra obtained from NaCl(aq) and KCl(aq). The fact that these two spectra are virtually identical clearly suggests that these cations have little influence on water OH stretch vibrations. Similar results obtained with other alkali halide salts (including LiCl) confirm that water OH spectra are primarily perturbed by the halide anions, as also noted in previous studies.14,15,23,24 However, other recent studies suggest that Li+ produces a red shift in the surrounding water molecules.16,17 Thus, although cations clearly may influence vibrational spectra of surrounding water molecules, it is generally accepted that anions perturb water vibrations more significantly. Moreover, the increased Raman scattering cross section of water around anions (except F-) serves to further enhance the influence on anions on our measured ion-correlated spectra. The middle panel in Figure 1 compares the ion-correlated OH spectra of NaCl(aq) in H2O and HOD/D2O (made by adding ∼10% D2O to H2O). These results clearly reveal the additional spectral intensity on the red side of the OH band in H2O resulting from resonance coupling, as further discussed below. Similar results have been obtained from other aqueous alkali halide salt solutions. Previous experimental and theoretical studies of pure water have suggested that the observed low-frequency OH shoulder in H2O (as opposed to that in HOD/D2O) contains contributions from direct resonance coupling between OH vibrations 25,26 and/ or from intramolecular Fermi resonance coupling between OH bend overtone and OH stretch fundamental vibrations.13,27,28 Note that in HOD/D2O, both intermolecular OH resonance coupling and intramolecular Fermi resonance coupling are reduced (because of the greater mismatch between the bend overtone and OH stretch fundamental frequencies of HOD, although strong HOD Fermi resonance may be induced at very high pressures).29 Thus, the fact that our MCR-derived anion-

Letters

J. Phys. Chem. B, Vol. 113, No. 7, 2009 1807

Figure 2. Nonpolarized (top row), isotropic (middle row), and anisotropic (bottom row) total Raman spectra (dotted) and the resulting ioncorrelated (solid) and pure water (dash) components obtained from 1 M KX(aq) solutions, where X ) F-, Cl-, Br-, and I-. The panels in each row have the same vertical axis scale, so the relative intensities across each row reflect those of the corresponding 1 M solutions.

TABLE 1: Peak Frequency (ω), Width (Γ), and Relative Areas (Σ) of Ion-Correlated Spectraa non-polarized -1

-1

isotropic -1

-1

anisotropic -1

-1

anion

ω (cm )

Γ (cm )

Σ (%)

ω (cm )

Γ (cm )

Σ (cm )

ω (cm )

Γ (cm-1)

Σ (cm-1)

FClBrI-

3458 3450 3463 3480

353 283 253 227

6 15 17 20

3440 3443 3451 3471

328 326 316 265

4 8 9 12

3454 3473 3490

253 212 192

1 4 4 5

a Γ is the full width at half-maximum of the ion-correlated MCR OH band, and Σ is its area, expressed as a percentage of the total OH band area in a 1 M solution of the corresponding alkali halide salt.

correlated OH spectra in H2O and HOD/D2O are quite similar suggests that both intermolecular and intramolecular resonance coupling is suppressed in water molecules that are strongly perturbed by anions. For comparison, the right-hand panel in Figure 1 shows the corresponding OH Raman spectra of the pure (ion-free) solvents, H2O and HOD/D2O, which clearly reveals the much larger influence of resonance coupling on the OH stretch band of bulk H2O. Figure 2 shows the nonpolarized, isoptropic, and anisotropic components of the solute-correlated (perturbed water) spectra of KX(aq), where X ) F-, Cl-, Br-, and I-. In the presence of F- anions, our MCR ion-correlated OH spectra differ quite significantly from those induced by other halides, as further discussed below. For the larger halides, the ion-correlated OH band shifts to higher frequency and decreases in width with increasing anion size (see Table 1), indicating a progressive

weakening of the ion-water hydrogen bond. Moreover, the fact that the peak positions of all ion-correlated bands are higher in frequency than those of bulk water implies that the corresponding ion-water hydrogen bonds are weaker than the average hydrogen bond strength in bulk water. The results shown in the top panels of Figure 2 indicate that the intensities (areas) of the ion-correlated spectra increase with anion size. More specifically, the nonpolarized ion-correlated spectral intensities obtained from 1 M solutions of F-, Cl-, Br-, and I- are 6.4 ( 1, 15.4 ( 2.2, 17 ( 2.4, and 19.8 ( 2.8, respectively (expressed as a percentage of the total OH stretch Raman band intensity). These intensity changes may result from either changes in the number of water molecules in the hydration shell and/or changes in the Raman cross section of water molecules bound to different anions, as further discussed below.

1808 J. Phys. Chem. B, Vol. 113, No. 7, 2009 The width (fwhm) of ion-correlated OH stretch bands induced by Cl-, Br-, and I- are smaller than that of bulk water (for all three polarization configurations). This may appear to conflict with computer simulations, which suggest that the distribution of ion-water distances increases with anion size.12,30-32 However, Smith et al. have suggested that the observed decrease in line width is a simple consequence of the inverse square distance dependence of the electric field strength around an ion.12 In other words, although the width of the hydration shell around I- is larger than that around smaller halide anions, the range of electric field strengths experienced by these water molecules is narrower than that around smaller anions. Our MCR ion-correlated (nonpolarized and isotropic) spectra of water molecules perturbed by fluoride anions show two bands, with a large slightly blue-shifted peak and a much smaller redshifted component which extends down to ∼2800 cm-1. The presence of these two bands is consistent with the slight broadening of the water OH stretch Raman band by F- (before MCR analysis), as also noted in previous IR11 and Raman4,12 studies. We tentatively attribute this pair of bands to F- induced perturbations, as similar bands are obtained from both NaF(aq) and KF(aq) solutions. However, given that F- is the only halide anion which produces a decrease in the OH Raman cross section of water (as further described below), it is perhaps possible that the small red-shifted band which we observe in F- containing solutions is due to cation-induced perturbations. In other words, perhaps the influence of the cation on the water OH stretch vibration is sufficiently weak that it is not readily detectable in the presence of larger halide anions (which produce significantly strongly Raman scattering from water molecules in their hydration shells). Although previous studies of aqueous F- solutions4,11,12 have not resolved the pair of bands that we have found using MCR, it is interesting to note that a somewhat similar pair of broad water OH stretch bands has been observed in a surface sum frequency spectroscopic study of 0.1 M NaF(aq) or KF(aq) in contact with a solid Ag(100) surface.33 Although the latter results were interpreted in terms of water ordering by the Ag surface, it seems possible that the observed bands are in fact due to Finduced perturbations of water and thus may be related to the bands that we have observed in aqueous F- solutions. An additional unique feature of F- containing solutions is the highly polarized character of the resulting ion-correlated OH stretch spectrum. This suggests that the OH stretch vibration of water molecules H-bonded to F- has a highly isotropic Raman tensor, resembling a spherical breathing normal mode. Larger anions, on the other hand, produce ion-correlated spectra with larger depolarization, roughly comparable to that of bulk water (see Table 1). Although Raman cross sections are typically rather insensitive to solvation environment, that is not the case for ionic hydration, as both Raman experiments 23,34 and quantum calculations of ion-water clusters12 imply that the Raman cross sections of hydration shell waters can be significantly modified by anions. Raman cross sections obtained using the latter two methods are only in rough agreement with each other but are consistent in indicating that large anions can enhance the Raman cross section of neighboring waters by a factor of 2 or more. The quantum calculations further indicate that most of the cross section change comes from the OH groups pointing toward the anion, while the other OH groups on the same water molecules are similar to those in bulk water, in both their hydrogen-bonding and Raman scattering properties.12

Letters It is, in principle, possible to deduce the number of water molecules associated with solute ions from the intensities of MCR ion-correlated spectra. More specifically, the integrated area of a solute-correlated spectrum (A) is expected to be proportional to the number of perturbed water molecules (n), A ) Rn, where R is the associated (average) Raman cross section of each perturbed water molecule. If we assume that Raman cross sections of all water molecules are the same as those in bulk water (which is clearly not correct), then our ion-correlated spectra imply that there are approximately 3.6 ( 0.5, 8.6 ( 1.2, 9.4 ( 1.3, and 11 ( 1.5 water molecules round F-, Cl-, Br-, and I-, respectively. Although these numbers are of the right magnitude, they are not entirely consistent with the accepted coordination numbers obtained from X-ray scattering35 and simulations.30-32 If we assume that the coordination numbers of F-, Cl-, Br-, and I- are 5.0, 5.8, 6.2, and 6.6, respectively (as obtained from recent simulations),30-32 and further assume that these coordination numbers represent the number of waters which give rise to our ion-correlated spectra, then we obtain the Raman cross sections (relative to bulk water) of 0.70 ( 0.1, 1.54 ( 0.26, 1.63 ( 0.27, and 1.83 ( 0.32 for hydration shell water molecules around F-, Cl-, Br-, and I-, respectively. The above Raman cross section error limits reflect both our experimental reproducibility and variations associated with employing alternative experimental data analysis pathways. The latter Raman cross sections are in reasonably good agreement with those inferred from preivous Raman studies (0.7, 1.1, 1.5, and 1.7, respectively)23,34 but are significantly smaller than some of the corresponding cross sections obtained from recent ion-water cluster quantum calculations, (0.70, 1.88, 2.40, and 2.96).12 IV. Summary and Discussion We report the first use of MCR to extract ion-correlated contributions to the OH stretch Raman spectra of aqueous alkali halide solutions. The results are used to obtain the frequency, width, and area of the perturbed OH features arising from interactions of water with F-, Cl-, Br-, and I- anions (while the corresponding alkali cations apparently have little influence on water’s OH vibrational band). We find that the OH peak shifts to higher frequency and decreases in width with increasing halide anion size, in agreement with previous Raman studies of higher concentration salt solutions. We have also compared the ion-correlated spectra in H2O with those in HOD/D2O. The latter comparisons imply that resonance coupling, which significantly influences the OH stretch band of pure H2O, is significantly suppressed in water molecules that are strongly coupled to anions. Our results confirm previous experimental and theoretical findings that the hydrogen bonds between halide anions and water are weaker than those in bulk water (on the average). The intensity of the ion-correlated OH band increases with increased anion size both because of the larger number of water molecules and because of the increase in the Raman cross section of water with increasing anion size. Moreover, both the strength of the ion-water hydrogen bonds and the width of the corresponding OH bands decrease with increasing anion size. Our polarized Raman results reveal that water molecules around F- produce an OH stretch Raman band that is highly polarized, while water around larger anions has a depolarization ratio that is similar to that of bulk water. The present results may be viewed as following and largely confirming the extensive experimental and theoretical analysis recently published by Smith et al.12 However, our ion-correlated

Letters OH spectral band areas are consistently smaller (by about a factor of 2) than those obtained by Smith et al. (using manual spectral subtraction procedures). Moreover, our experimentally derived Raman cross section estimates are also smaller than those obtained Smith et al. (using DFT calculations of ion-water clusters). These results, as well as the unrealistically small water coordination numbers inferred by Smith et al., all suggest that the DFT Raman cross sections obtained from ion-water clusters overestimate the corresponding Raman cross sections of water around large halide anions. References and Notes (1) Bergstrom, P. A.; Lindgren, J.; Kristiansson, O. J. Phys. Chem. 1991, 95, 8575. (2) Chen, Y.; Zhang, Y. H.; Zhao, L. J. Phys. Chem. Chem. Phys. 2004, 6, 537. (3) Dubessy, J.; Lhomme, T.; Boiron, M. C.; Rull, F. Appl. Spectrosc. 2002, 56, 99. (4) Li, R. H.; Jiang, Z. P.; Chen, F. G.; Yang, H. W.; Guan, Y. T. J. Mol. Struct. 2004, 707, 83. (5) Rull, F. Pure Appl. Chem. 2002, 74, 1859. (6) Tauler, R.; Barcelo, D. TrAC, Trends Anal. Chem. 1993, 12, 319. (7) Saurina, J.; Hernandezcassou, S.; Tauler, R. Anal. Chem. 1995, 67, 3722. (8) Perera, P.; Wyche, M.; Loethen, Y.; Ben-Amotz, D. J. Am. Chem. Soc. 2008, 130, 4576. (9) Bakker, H. J.; Kropman, M. F.; Omta, A. W. J. Phys.: Condens. Matter 2005, 17, S3215. (10) Soper, A. K.; Weckstrom, K. Biophys. Chem. 2006, 124, 180. (11) Nickolov, Z. S.; Miller, J. D. J. Colloid Interface Sci. 2005, 287, 572.

J. Phys. Chem. B, Vol. 113, No. 7, 2009 1809 (12) Smith, J. D.; Saykally, R. J.; Geissler, P. L. J. Am. Chem. Soc. 2007, 129, 13847. (13) Schultz, J. W.; Hornig, D. F. J. Phys. Chem. 1961, 65, 2131. (14) Yoshimura, Y.; Kanno, H. J. Raman Spectrosc. 1996, 27, 671. (15) Walrafen, G. E. J. Chem. Phys. 1970, 52, 4176. (16) Terpstra, P.; Combes, D.; Zwick, A. J. Chem. Phys. 1990, 92, 65. (17) Pejov, L.; Spangberg, D.; Hermansson, K. J. Phys. Chem. A 2005, 109, 5144. (18) Bakker, H. J. Chem. ReV. 2008, 108, 1456. (19) Kropman, M. F.; Bakker, H. J. J. Am. Chem. Soc. 2004, 126, 9135. (20) Kropman, M. F.; Nienhuys, H. K.; Bakker, H. J. Phys. ReV. Lett. 2002, 88. (21) Scherer, J. R.; Kint, S.; Bailey, G. F. J. Mol. Spectrosc. 1971, 39, 146. (22) Zhang, D. M.; Jallad, K. N.; Ben-Amotz, D. Appl. Spectrosc. 2001, 55, 1523. (23) Schultz, J. W.; Hornig, D. F. J. Phys. Chem. 1961, 65, 2131. (24) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Science 2003, 301, 347. (25) Torii, H. J. Phys. Chem. A 2006, 110, 9469. (26) Torii, H. J. Mol. Liq. 2007, 136, 274. (27) Sovago, M.; Campen, R. K.; Wurpel, G. W. H.; Muller, M.; Bakker, H. J.; Bonn, M. Phys. ReV. Lett. 2008, 100. (28) Sokolowska, A. J. Raman Spectrosc. 1996, 27, 621. (29) Aoki, K.; Yamawaki, H.; Sakashita, M. Science 1995, 268, 1322. (30) Heuft, J. M.; Meijer, E. J. J. Chem. Phys. 2003, 119, 11788. (31) Heuft, J. M.; Meijer, E. J. J. Chem. Phys. 2005, 123, 094506:1. (32) Heuft, J. M.; Meijer, E. J. J. Chem. Phys. 2005, 122, 094501:1. (33) Schultz, Z. D.; Shaw, S. K.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 15916. (34) Wall, T. T.; Hornig, D. F. J. Chem. Phys. 1966, 45, 3424. (35) Ohtaki, H.; Radnai, T. Chem. ReV. 1993, 93, 1157.

JP808732S