Article pubs.acs.org/JPCA
Identification of Multiple Water−Iodide Species in Concentrated NaI Solutions Based on the Raman Bending Vibration of Water Matthieu Besemer,†,‡,⊥ Rob Bloemenkamp,‡ Freek Ariese,† and Henk-Jan van Manen*,‡ †
LaserLaB VU University, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands Supply Chain, Research & Development, Strategic Research Group Measurement & Analytical Science, AkzoNobel, Zutphenseweg 10, 7418 AJ Deventer, The Netherlands ⊥ TI-COAST, Science Park 904, 1098 XH Amsterdam, The Netherlands ‡
ABSTRACT: The influence of aqueous electrolytes on the water bending vibration was studied with Raman spectroscopy. For all salts investigated (NaI, NaBr, NaCl, and NaSCN), we observed a nonlinear intensity increase of the water bending vibration with increasing concentration. Different lasers and a tunable frequency-doubled optical parametric oscillator system were used to achieve excitation wavelengths between 785 and 374 nm. Focusing on NaI solutions, the relative enhancement of the water bending vibration was found to increase strongly with excitation photon energy, in line with a preresonance effect from the iodide−water charge-transfer transition. We used multivariate curve resolution (MCR) to decompose the measured Raman spectra of NaI solutions into three interconverting spectral components assigned to bulk water and water molecules interacting with one (X···H−O−H···O) and two (X···H− O−H···X) iodide ions (X = I−). The Raman spectrum of solid sodium iodide dihydrate supports the assignment of the latter. Using the MCR results, relative Raman scattering cross sections of 4.0 ± 0.6 and 14.0 ± 0.1 were calculated for the mono- and di-iodide species, respectively (compared to that of bulk water set to unity). In addition, it was found that at relatively low concentrations each iodide ion affects the Raman spectrum of roughly 22 surrounding water molecules, indicating that the influence of iodide extends beyond the first solvation shell. Our results demonstrate that the Raman bending vibration of water is a sensitive probe, providing new insights into anion solvation in aqueous environments.
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INTRODUCTION Understanding the effects of ions on the molecular structure and dynamics of water in aqueous solutions is of crucial importance in fields as diverse as biophysics and biology, physical chemistry, geology, and industrial chemistry.1,2 In recent years, vibrational spectroscopic studies have challenged the long-held empirical notion that ions can be classified as “structure makers” or “structure breakers” based on their capacity to increase or decrease the strength or number of hydrogen bonds between water molecules in their surroundings.2−4 For example, based on femtosecond pump−probe infrared (IR) spectroscopy and Raman spectroscopy combined with computer simulations, the groups of Bakker5 and Geissler6 conclude that the effects of ions on the hydrogen-bond network in water is negligible beyond the first solvation shell. Supporting evidence for this conclusion has come from X-ray absorption studies7 and dielectric relaxation (DR) spectroscopy.8 However, a combined DR and femtosecond IR study has revealed that in certain cases the coupled effect of cations and anions on the dynamics of water molecules extends well beyond the first solvation shell.9 Also a recent publication by Okazaki et al. suggests that a halide ion affects the spectroscopic properties of 25−27 water molecules, that is, well beyond the first solvation shell.10 © XXXX American Chemical Society
The majority of Raman spectroscopy work on aqueous solutions of electrolytes has dealt with O−H stretching vibrations at ∼3200 cm−1 (or O−D stretching at lower frequencies), whereas the bending vibration of H2O at ∼1640 cm−1 has received comparatively little attention. The bending vibration of water in aqueous electrolytes has traditionally been regarded as a single band that increases linearly in intensity with increasing concentration of dissolved anions.11 Other studies have shown that the water bending vibration undergoes spectral changes, such as signal enhancement, red shift, and peak asymmetry, when interacting with anions.3,4,12,13 The wavelength-dependent signal enhancement of the Raman water bending vibration induced by halides can be explained by a charge transfer (CT) transition.3,12,14 In the CT state an electron is donated to a neighboring water molecule, which affects the bond angle and polarizability and thus the Raman cross-section. Abe and Ito used 514.5 and 337.1 nm excitation sources and reported that at a concentration of ∼7 M LiI the relative enhancements of the water bending vibration were 8.35 and 27.7, respectively.12 Piatkowski and Bakker also studied the Received: October 15, 2015 Revised: January 16, 2016
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DOI: 10.1021/acs.jpca.5b10102 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
measurement times were six accumulations of 10 s. All measurements were performed at least in triplicate, and concentration series were measured in random order. For this work the SHG-OPO setup was used to produce a series of excitation wavelengths over the range of 374−446 nm. Additional measurements at longer wavelengths were performed using a Renishaw InVia system (532 and 632.8 nm) and a Kaiser Optical Systems RXN-4 system (785 nm) coupled with fiber optics to an immersion probe with a short focal length.16 Data Processing. For quantitation of the enhancement factors, the spectra were baseline-corrected with a second-order polynomial, and the integration boundaries were set at 1500− 1800 cm−1. Next, the peak areas were intensity-corrected using the intensity of the thiocyanate peak as internal standard. The resulting peak areas were corrected for the actual concentration of water. Finally, relative enhancements were calculated by normalizing to the spectral intensity (peak area) of bulk water. The data preprocessing was executed with the PLS_Toolbox version 6.0.1 (eigenvector Research, Inc.) operating in MATLAB R2007B (The MathWorks, Inc.) and open-source peak fitting software Peakfit.m from the University of Maryland.17 For the MCR routine we used the data recorded at 398 nm excitation. The contribution of oxygen in the Raman spectra at a wavenumber of 1555 cm−1 was removed in the following way: the experimental water bulk spectrum (0 M) was fitted with a Gaussian distribution, and this broad band was subtracted from the measured water spectrum. The difference spectrum, consisting only of the sharp oxygen contribution, was subsequently subtracted from the spectra of the various NaI solutions. The spectra were then smoothed with the Savitzky− Golay algorithm (window size 13 point, second-order polynomial), baseline corrected with a second-order polynomial, and normalized to unit area.18 As MCR constraints we set the contribution of bulk water to 100% for 0 M NaI and to 0% above 3 M NaI. The latter constraint is based on the notion that above 3 M NaI all water molecules are found in hydration layers of iodide and therefore do not resemble bulk water, as was demonstrated by Marcus.15 This is supported by our observation that at 2.5 M NaI only a minimal fraction of bulk water exists (see below).
water bending vibration by means of pump−probe IR spectroscopy. They observed peak asymmetry and a red shift in the vibrational spectrum at high iodide levels, which was interpreted on the basis of an interconversion of two Gaussian distributions.4 They observed that in addition to the bulk water bending vibration a second water bending vibration becomes visible, originating from water−iodide interactions. Their peak fitting results predicted that at a concentration of ∼12.0 M LiI still ∼35% of the water molecules behaved bulk-like, and ∼60% of the water molecules were interacting with iodide via a single hydrogen bond. Only a minority of the water molecules were found to be doubly hydrogen bonded with their protons to iodide. These results are in contrast to geometrical calculations by Marcus, who argued that it is not possible to have bulk water at concentrations of ∼6 M electrolyte, assuming that ion pairing does not occur.15 In the present study we aim to shed light on these issues by studying the Raman H−O−H bending mode (e.g., Raman shift, band shape, and intensity) in concentration series of aqueous halide solutions, in particular, those of iodide. With the use of multivariate curve resolution (MCR) as data analysis tool, we demonstrate the interconversion of three spectral components and calculate the different relative Raman scattering cross sections attributed to each associated species, as well as the number of water molecules affected by an iodide ion. In addition, we applied multiple wavelengths ranging from 785 to 374 nm using various lasers and a frequency doubled optical parametric oscillator (OPO) system to determine the wavelength-dependent enhancement of the water bending vibration in more detail.
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EXPERIMENTAL SECTION Materials. NaCl, NaBr, NaSCN, and NaI were purchased from Sigma-Aldrich and used without further purification. Concentration series of the salts were prepared with demineralized water (Milli-Q) in volumetric flasks. To correct for laser power fluctuations and absorption inner filter effects at shorter wavelengths due to traces of iodine formed, an internal standard of 0.2 M NaSCN was added to every NaI solution. At that concentration NaSCN did not enhance the water bending vibration significantly; the stretch vibration at 2070 cm−1 was used to normalize the spectra. Equipment. The Raman setup is based on a frequencydoubled OPO Levante Emerald system from APE (Berlin, Germany) pumped by a 20 W, 10 ps 532 nm laser (Coherent Palladin, Santa Clara, CA, USA). The signal output of the OPO is tunable from 690 to 980 nm and is frequency doubled by a second harmonic generator (SHG, APE Harmonixx). The beam from the SHG unit is aimed at a Pellin Broca prism and a diaphragm to block any remaining unwanted wavelengths. After the diaphragm the light beam hits a beam splitter, which directs 50% of the light toward a 20× quartz objective and eventually reaches the sample inside a 1 × 1 cm quartz cuvette. Backscattered light is collected by the same objective, passes through the 50% beam splitter, through an appropriate edge filter (Semrock AELP series, angle-tuned) for the removal of the Rayleigh scatter and reflected laser light, and through a focusing lens to enter the spectrograph (Andor Shamrock 30 cm). Finally the Raman scattered photons are diffracted by a 1200 lines/millimeter grating and detected by a CCD camera (Andor Newton, cooled to −60 °C). The spectrograph was recalibrated for every excitation wavelength using cyclohexane and quadratic fitting for wavenumber calibration. Typical
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RESULTS AND DISCUSSION Enhancement of the Water Bending Vibration by Different Anions. To study the effects of different anions on the water bending vibration, concentration series of NaCl, NaBr, NaI, and NaSCN were prepared to nearly the solubility limit. Raman spectra were recorded for all these solutions, focusing on the relatively weak water bending vibration. Peak areas were determined and normalized to that of blank demineralized water. The relative enhancement of the water bending vibration by different salts, measured at 785 nm, is shown in Figure 1. For every type of anion tested the normalized peak area of the water bending vibration increases nonlinearly with the salt concentration. This is not due to optical artifacts, since the Raman intensity of the CN stretch vibration of the thiocyanate anion was found to increase linearly with the NaSCN concentration (R2 = 0.9996; not shown). As illustrated in Figure 1, the intensity data were fitted with second-degree polynomials; statistical significance tests showed that linear fits could not properly fit the observed enhancements. Throughout the halide series the observed relative enhancement increases in B
DOI: 10.1021/acs.jpca.5b10102 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Figure 1. Relative enhancement of the Raman water bending vibration in the presence of four different salts at different concentrations, measured at 785 nm excitation in triplicate. The relative standard deviations are below ∼1.4% (error bars would not be visible at this scale). Peak areas were normalized to the area of the water blank and corrected for differences in water concentration.
Figure 2. Wavelength-dependent enhancement of the water bending vibration at different NaI concentrations of 1.2 (bottom), 4.5 (middle), and 6.3 M (top). All measurements were done at least in triplicate (standard deviations were less than 1%), and peak areas were normalized to the blank water signal at that wavelength and corrected for the actual water concentration. NaSCN was added as an internal standard to correct for absorption inner filter effects (due to formation of iodine) and laser power fluctuations.
the order chloride < bromide < iodide, presumably due to an increase in polarizability of water−halide species in that order.19,20 It is clear from Figure 1 that among the four anions tested the water−iodide interaction causes the greatest enhancement. Therefore, we will focus in this paper on the water−iodide interactions. Wavelength Dependence of Enhancement Factors. Using commercial Raman instruments and a tunable frequencydoubled OPO system, Raman spectra of NaI solutions were recorded at different wavelengths ranging from 374 to 785 nm. In all cases the band intensities were normalized to that of the water blank measured at the same wavelength and corrected for the actual water concentration in the solutions. At all wavelengths a concentration series was measured, and in all cases a nonlinear curve was obtained (not shown), similar in shape to that of Figure 1 (top). However, at each iodide concentration level the enhancement factor was found to depend on the excitation wavelength, with shorter wavelengths causing stronger enhancements. The enhancement factors observed for 1.2, 4.5, and 6.3 M iodide solutions at various excitation wavelengths are shown in Figure 2. According to the mechanism suggested by Xiong & Asher14 and Abe & Ito,12 the enhancement at shorter wavelengths is caused by a CT transition from the anion to the water. The change in bond angle related to this transition would cause preresonance enhancement of the Raman signal of the water bending vibration. This preresonance effect is expected to increase at shorter wavelength (because the energy gap between the virtual excited state and the CT-excited state decreases) and at higher iodide concentrations (which not only increases the number of hydration water molecules involved in CT from iodide but also determines the type of water−iodide interaction), as is indeed observed in Figure 2. Note that even at 374 nm the excitation wavelength is still far (ca. 17 000 cm−1) from true resonance at the 227 nm CT absorption band of iodide,12 but preresonance enhancement at such energy differences is in line with the Raman excitation profile reported by Xiong & Asher for sodium chloride solution. That study showed enhancement at similar distances below the estimated CT level of chloride of ∼55 000 cm−1 and strong enhancement at shorter wavelengths; the data could be fitted successfully to an Albrecht A-term expression.14
Concentration Dependence of Water Bending Vibration Spectral Shapes. Mizuno and Tahara argued that an anion with water molecules in the first solvation shell can be considered as a quasi-molecule, with its own vibrational frequency and increased Raman cross-section relative to bulk water.3 Figure 3 shows the Raman spectra of NaI solutions at
Figure 3. Water bending Raman peaks of NaI solutions at 398 nm excitation, illustrating differences in intensity, peak maximum, and spectral shape at NaI concentrations increasing from 0 to 6.3 M. Note the spectral asymmetry at intermediate iodide levels. The vertical line indicates the peak maximum for bulk water at 1638 cm−1.
different concentrations. Apart from an increase in intensity, also changes in the position, shape, and width of the water bending vibration band are observed. The maximum of the symmetric band of bulk water (bottom spectrum) was found at 1638 cm−1, and the maximum of the 6.3 M NaI solution (top) showed at 1628 cm−1. A similar red shift was also observed in the IR spectra of concentrated LiI solutions by Piatkowski and Bakker4 and in recent Raman spectra of concentrated NaI solutions (reported by the authors as Supporting Information) by Ahmed et al.13 C
DOI: 10.1021/acs.jpca.5b10102 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A However, at an intermediate concentration of 1.2 M iodide the maximum of the Raman band (highlighted in Figure 3) is blue-shifted by 7 cm−1 relative to bulk water. As the concentration is increased further, the peak becomes more symmetric again, and an overall red shift to 1628 cm−1 is observed. This indicates that a third species is being formed at higher anion concentrations. Abe and Ito also observed a highly symmetric peak at a concentration of 7 M iodide.12 Piatkowski and Bakker reported4 that the spectral changes observed in IR experiments over a comparable concentration range in H2O/ D2O can be explained by the interconversion of only two Gaussian distributions with fixed frequencies and widths for bulk-like and anion-bound water. However, an initial peak analysis of our Raman data in H2O showed that it is impossible to deconvolute the bands into only two Gaussian distributions. For a more in-depth analysis and quantitative determination of the different water species, we analyzed the spectra using MCR. Multivariate Curve Resolution on Raman Spectra of NaI Solutions. MCR is a multivariate analysis technique that decomposes the experimental data into a linear combination of pure spectral components and their corresponding contributions.21 In recent years, MCR has been applied to Raman spectra of solutions (e.g., solvent mixtures, aqueous electrolytes) to obtain specific spectroscopic details about solvation shell molecules.13,18,22−25 In our case, we used MCR to describe the spectral changes such as Raman signal enhancement and peak shifts of the water bending vibration that occur in NaI solutions at 12 different concentrations. As input we used the preprocessed water Raman spectra, as described in the Experimental Section. Our MCR analysis showed that these 12 spectra could be adequately described with three interconverting spectral components, of which one corresponds with the Raman bulk water bending vibration at 1638 cm−1 with a full width at halfmaximum (fwhm) of 104 cm−1. The other two spectral components represent a blue-shifted band at 1645 cm−1 (fwhm 79 cm−1) and a red-shifted band at 1628 cm−1 (fwhm 50 cm−1). Figure 4a shows these component spectra; each component can be fitted with a Gaussian distribution within an error margin of
