34
J. Phys. Chem. B 2002, 106, 34-40
Solvation Structure of Bromide Ion in Anion-Exchange Resins Makoto Harada and Tetsuo Okada* Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
Iwao Watanabe Department of Chemistry, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed: April 12, 2001; In Final Form: October 19, 2001
The local structures of Br- in ion-exchange resins are studied with XAFS at the Br-K edge. Br--form anionexchange resins having different ion-exchange groups, i.e., tertiary ammonium (R-3) and quaternary ammonium ion (R-4), are examined after being soaked in water, methanol (MeOH), and some polar aprotic solvents. The resins soaked in aprotic solvents give almost the same spectral features as dry resins, indicating that Br- is tightly bound by the ion-exchange sites. Br- is entrapped inside the tripod composed of three equivalent methyl groups of the ion-exchange group in the R-4 resin, while it strongly interacts with the ammonium hydrogen of the ion-exchange group of the R-3 resin. In the latter case, the scattering from methyl and methylene carbon atoms in the ion-exchange group also contributes to XAFS spectra. In contrast, characteristic features are confirmed when the resins are soaked in water and MeOH. Detailed analyses of XAFS spectra imply that two species, i.e., solvated Br- and that bound on the ion-exchange sites, coexist in the resins soaked in protic solvents. The XAFS spectra for the individual species can be extracted by assuming linear combinations of two extreme spectra.
Introduction Ion-exchange materials have been successfully utilized to advance various techniques and methodology in fundamental and practical branches of chemistry.1 Ionic groups chemically bonded to organic or inorganic matrixes act as active ionexchange sites in most of synthetic ion-exchangers, such as resins and membranes; sulfonate and ammonium groups are typical cation- and anion-exchange groups, respectively. The presence of ionic groups in solid matrixes has led to a widely accepted electrostatic separation mechanism; i.e., ion-exchange separation occurs on the basis of electrostatic interactions between the active groups and counterions. This intuition is supported by the fact that multivalent counterions are generally better partitioned to an ion-exchanger than monovalent counterparts. However, simple electrostatics does not explain that ions with identical charges can be separated by ion-exchangers. Although the electrostatic interaction must be a primary (and essential) mechanism of ion-exchange, other mechanisms should also contribute to the determination of separation selectivity; in particular, their contribution must be important to discuss the selectivity between identically charged ions. It is known that ion-exchange selectivity depends on the sizes of counterions, the structures and density of active sites, types of matrixes, solvents, etc. These factors affecting ion-exchange selectivity have been well investigated not only by batch partition experiments2 but also by chromatography,3 which have enabled simple determination of selectivity coefficients for a variety of combinations of ions, ion-exchange resins, and solvent systems. Compiled knowledge has suggested the important involvements of ion solvation in the ion-exchange mechanism.3a,4,5 However, the discussions of a solvation effect based on * Corresponding author. Phone and Fax: +81-3-5734-2612. E-mail:
[email protected] thermodynamic data have been severely restricted for some reasons; (1) thermodynamic data include the contributions both from a counterion and from an ion-exchange group, (2) ionexchange data intrinsically have relative nature because partition selectivity is determined by a difference in affinity between two counterions, and (3) exchanging counterions should cause a change in the energetic state of the ion-exchange site. Thus, the origin in ion-exchange selectivity has not been completely elucidated despite an enormous number of partition data. Elucidation of structural aspects is essential to understanding the ion-exchange mechanisms, and novel methods can (should) thereby be advanced. However, since only very few approaches are available for structural studies of ion-exchange systems, the structures of ions in ion-exchanger have been scarcely understood. Tromp and Neilson6 studied the hydration of Ni(II) and Li+ in cation-exchange resins by neutron diffraction (ND) and pointed out the penetration of sulfonate groups of the cationexchanger into the primary hydration shell of these cations. Although this method must be effective in some applications, the distinct local structures of counterions in ion-exchangers have not been provided. Takahashi et al.7 studied the hydration structure of Eu(III) in cation-exchange resins with laser-induced fluorescence spectroscopy and elucidated the pH dependence of the hydration number of this cation in the resins. The applicability of their approach is extremely limited because only fluorescent solutes can be investigated. Very few researches have thus been attempted to study the structures of ions in ionexchange materials. X-ray absorption fine structure (XAFS) spectroscopy is one of the most powerful tools for the local structural analyses of solvated ions or molecules because of its very high atomic selectivity (if appropriately high energy is selected, the presence of solvents and matrixes does not interfere with XAFS measure-
10.1021/jp011390u CCC: $22.00 © 2002 American Chemical Society Published on Web 11/29/2001
Bromide Ion in Anion-Exchange Resins
J. Phys. Chem. B, Vol. 106, No. 1, 2002 35
ments of targeted atoms). This method thus has much higher applicability than other approaches and has been applied to some ion-exchange systems; e.g., the solid-state structures of metal complexes in an ion-exchange membrane8 and metal ions in zeolite9 have been studied. Solvated structures rather than solidstate ones should provide more efficient clues to solve puzzles involved in ion-exchange phenomena, and XAFS should thus be a suitable choice for this purpose. We can vary the structures of anion-exchange groups (i.e., changing the numbers and chain lengths of alkyl groups is possible), whereas there are only two usual choices for cationexchange groups, i.e., sulfonic and carboxylic groups. For this reason, in the present paper, we focus our attention on the structures of Br- in anion-exchange resins soaked in various solvents. The hydrated structure of Br- has been well investigated using a variety of techniques, such as X-ray diffraction (XRD),10 ND,11 IR spectroscopy,12 and XAFS.13 In addition, the solvation structures of Br- in various nonaqueous solutions have been studied with XAFS.13a The understanding of ionexchange mechanisms should be facilitated by examining differences in the solvated structures of Br- between in bulk solutions and in the resins. Does Br- form an ion-associate with an anion-exchange site? Is it still solvated in a resin? How does the structure around Br- change with solvent natures and the structures of ion-exchange groups? The aim of this paper is to elucidate these aspects through studying the structures of Brin the resins soaked in solvents. Experimental Section Br--form
Resins and Solvents. anion-exchange resins were prepared from Amberlyst A-21 and A-26 (Rohm and Hass Co.); Amberlyst A-21 is a weak-base anion-exchange resin having -N(CH3)2 groups as anion-exchange sites, whereas Amberlyst A-26 is a strong-base anion-exchange resin having -N+(CH3)3 groups as anion-exchange sites. Amberlyst A-21 was treated with aqueous hydrochloric acid (ca. 0.1M) to allow the complete protonation of the tertiary amino groups. After protonation, the resin was treated with aqueous sodium bromide solutions (ca 0.2M) several times to allow full replacements of Cl- by Br-. Amberlyst A-26 was simply treated with aqueous sodium bromide solutions (ca. 0.2M) several times. The resins having tertiary ammonium groups and having quaternary ammonium groups are hereinafter referred to as R-3 and R-4, respectively. The resins were repeatedly rinsed with water until the conductivity of water became lower than 10 µs cm-1, powdered (particle sizes smaller than 40 µm, typically 5-20 µm), and dried in a vacuum. The ion-exchange capacities of the R-3 and R-4 resins were 3.57 mmol g-1 and 3.25 mmol g-1, respectively. The resins were soaked in water, methanol (MeOH), acetonitrile (AN), acetone, dimethyl sulfoxide (DMSO), or N,N-dimethylformamide (DMF) and then sealed in polyethylene pouches. Organic solvents of analytical grade (Wako Chemicals, Osaka) were dried and distilled in usual ways. Water was purified with a Milli-Q system. XAFS Measurements. All of XAFS spectra at Br K-edge were obtained at BL-10B of Photon Factory, High Energy Accelerator Research Organization (KEK-PF) in Tsukuba, Japan.14 The monochromator was equipped with a Si(311) channel-cut crystal. The intensity of incident X-ray was measured with an ionization chamber of 17 cm long filled with 15%/85% Ar/N2 and that of transmitted X-ray with an ionization chamber of 31 cm long filled with 50%/50% Ar/N2. The samples sealed in polyethylene pouches were sandwiched by plastic holders. Sample thickness (1.5-3.0 mm) was adjusted by
Figure 1. XAFS χ(k)k3 spectrum for R-4 in water (a) and after background correction. (a) Solid curve, χ(k)k3 spectrum; dashed curves, µmax(k) and µmin(k); dot-dashed curve, µ0(k). (b) Solid curve, χmod(k)k3 spectrum; dashed curves, µmax(k)′ and µmin(k)′; dotted-dashed curve, µ0(k)′.
inserting spacers between the plastic holders so that an appropriate signal jump could be obtained at Br K-edge. All of XAFS measurements were carried out at room temperature (ca. 297 K). Data Analysis. The normalized XAFS interference function, χ(k), is defined as
χ(k) )
µ(k) - µb(k) - µ0(k)
k)
µ0(k)
x
2m (E - E0) p2
(1)
(2)
where m is the electron mass, E is the incident X-ray energy, E0 is the threshold energy, and µ(k), µ0(k), and µb(k) are the total absorption, the absorption due only to the K shell excitation of a priori isolated bromide ion, and the background absorption depending on the circumstances of the absorbing atom, such as absorption from the other shells and long-range solvation effects, respectively. The Victreen’s parameters, a, b and c, were determined by fitting µb(k) to Victreen’s formula, aE-3 - bE-4 - c. Figure 1a shows a χ(k) k3 XAFS spectrum for Br- in the R-4 resin soaked in water. The extraction of µ0(k) is one of the most important issues under debate in XAFS analyses. The XAFS spectra at Br K-edge are strongly influenced by the multielectron excitaions(MEE).13 MEE effects on the spectra of other atoms are usually marginal and can be simply eliminated by a common background correction,15 whereas MEE
36 J. Phys. Chem. B, Vol. 106, No. 1, 2002
Harada et al.
gives specific structures at Br K-edge that cannot be corrected by usual procedures.13 In the present study, we used the following procedure to remove MEE effects.16 The envelope curves µ max(k) and µ min(k) were estimated by connecting the maxima and the minima of the spectrum of the smoothed χ(k) k3 (dashed curves in Figure 1a). µ 0(k) is given by
1 µ0(k) ) µt(k) + [µmax(k) + µmin(k)] 2
(3)
where µt(k) is a temporary baseline. The solid curve in Figure 1b was obtained by subtracting µ0(k) from χ(k) k3. The essential XAFS spectra χ(k) was extracted by subtracting the calculated baseline (dot-dashed curve in Figure 1b) from the modified χ(k) k3 spectrum (χmod(k) k3); this χmod(k) k3 spectrum is free from ΜΕΕ because its effect, included in µ 0(k) calculated by eq 3, is effectively eliminated by the baseline correction. When the envelope curves were again estimated for χmod(k) k3, a new baseline, µ0(k)′, was obtained:
1 µ0(k)' ) [µmax(k)' + µmin(k)'] 2
Figure 2. XANES spectra at Br K-edge with (a) R-3 and (b) R-4 soaked in various solvents.
(4)
The new baseline µ 0(k)′ has a specific periodic oscillation structure, as shown in Figure 1b. This oscillation disagrees with the k values coming from MEE (k ) 1.9, 2.7, 5.0, and 7.6 Å-1), and can be considered as an atomic XAFS of Br-.16,17 χmod(k) is calculated as
µ(k) - µb(k) - µ0(k) - µ0(k)'
χmod(k) )
µ0(k)
(5)
The XAFS spectra in k space, χ(k), were analyzed by curvefitting with the eqs 6 and 7:
χ(k) )
∑j
SjNjFj(kj) 2
exp(-2σj2kj2) sin[2kjrj + φj(kj)] (6)
krj
kj )
x
k2 -
2m ∆E0j p2
(7)
where j is the coordination shell number, rj is the distance between Br- and a scattering atom, SjNj is the amplitude factor (where Sj is the amplitude reduction factor), σj is the DebyeWaller factor, E0j is the absorption edge shift, and Fj(kj) is the backscattering amplitude. The factors, φj(kj) and Fj(kj), were calculated with FEFF ver. 7.02. The k3χ(k) spectra were analyzed in the ranges of k )1.9-4.8 Å-1 for water and MeOH and k ) 2.3-5.8 Å-1 for other solvents. Results and Discussion XAFS Spectra and Parameters. Figure 2 shows the X-ray absorption near edge structure (XANES) spectra at the Br-K edge obtained with the R-3 and R-4 resins soaked in various solvents. The resins soaked in aprotic solvents, i.e., AN, acetone, DMSO, and DMF, give similar XANES features, implying similar solvated and electronic structures of Br- under these conditions. In contrast, specific XANES spectral features can be seen for the resins soaked in protic solvents. Figure 3 shows χ(k)k3 spectra for resins samples and for Brin bulk of some solvents. In all cases, coordinating atoms should be hydrogen. The contributions of hydrogen atoms to overall XAFS oscillations are very small and can be neglected. X-ray
Figure 3. XAFS χ(k)k3 spectra at Br K-edge with (a) R-3 and R-4 soaked in various solvents and (b) tetraethylammonium bromide solutions.
scattering thus occurs at the atoms bonded to the hydrogen atom directly interacting with Br-, i.e., in the present instances, oxygen, nitrogen, and/or carbon atoms. This coordination nature of Br- results in rather long interaction distances (>3 Å) and
Bromide Ion in Anion-Exchange Resins
J. Phys. Chem. B, Vol. 106, No. 1, 2002 37 TABLE 1: XAFS Parameters for Br- in the R-3 and R-4 Resins R-3 solvents
r/Å
Na
σ/Å
R
water MeOH AN first second acetone first second DMSO first second DMF first second
3.24 3.12
3.45 1.68
0.119 0.101
0.140 0.182 0.221
3.46 4.16
2.92 3.49
0.186 0.104
3.44 4.16
3.25 3.88
0.201 0.107
3.61 4.28
3.04 3.68
0.169 0.139
3.47 4.14
2.38 3.67
0.184 0.104
0.213 0.245 0.140
R-4 solvents
r/Å
Na
σ/Å
R
water MeOH AN acetone DMSO DMF
3.22 3.02 3.50 3.55 3.61 3.64
3.44 1.74 4.45 3.79 4.57 4.10
0.121 0.101 0.174 0.185 0.169 0.169
0.116 0.124 0.234 0.269 0.162 0.212
Figure 4. Fourier Transforms of XAFS spectra at Br K-edge with R-3 and R-4 soaked in various solvents.
Br- in bulk solnb
generally makes the XAFS oscillation amplitude weak. This is a common problem to XAFS studies for halides. XAFS oscillation should thus reflect Br-‚‚‚H-O or Br-‚‚‚H-C interactions when it is solvated, whereas the oscillation comes from Br-‚‚‚H-C and Br-‚‚‚H-N interactions when it is bound by an anion-exchange group. Some characteristic features are seen in the χ(k)k3 spectra shown in Figure 3. χ(k)k3 amplitudes simply decrease at k > 2 Å -1 in most cases but show maxima at k ) ca. 5 Å -1 for the R-3 resin soaked in aprotic solvents, implying that Br- has scattering atoms at two different distances. Also, band broadenings at k ) 2-3 Å -1 are seen in the spectra when the resins are soaked in water and MeOH. These strongly suggest that the local structures of Br- in a resin depend on the types of solvents and the structures of ion-exchange groups. These specific spectral features cannot be seen for the XAFS spectra of Br- in bulk solutions (Figure 3b). Figure 4 shows the Fourier transforms of XAFS spectra. Fourier transformations were carried out over the range of k ) 1-6 Å -1 with the parameters calculated by assuming a single scattering atom. Table 1 summarizes the results of curve-fittings, together with the literature values obtained for solvated Br- in various solutions.13a The numbers of the closest scattering atoms were estimated by assuming N ) 6 for Br- in bulk water; the amplitude reduction factor was S ) 0.339. Although single and double shell calculations were applied to all cases, the former gave more appropriate results except for the R-3 resin soaked in aprotic solvents. Although the parameters listed in Table 1 provide some significant insights into the structure of Br- in the resins, unambiguous interpretations based only on these parameters are very limited. Details for individual systems are discussed below. Structures of Br- in the Resins Soaked in Aprotic Solvents. As discussed above, it is reasonably concluded that Br- in the R-3 resin has scattering atoms at two different distances, which were determined as ca. 3.5 and 4.2 Å in any aprotic solvents tested. The former distance, which agrees with that between Br- and the closest carbon atom of an aprotic solvent in bulk (see Table 1), is possibly characteristic of the
solvents
r/Å
Na
σ/Å
water MeOH AN acetone DMSO DMF
3.19 3.15 3.48 3.57 3.53 3.61
6.0 3.7 4.7 5.8 5.6 6.3
0.126 0.089 0.162 0.203 0.177 0.192
a
N for Br- in water was assumed to be 6. b Taken from ref 13a.
weak interactions of Br- with its surroundings. It should be noted that double shell structures emerge only for the resin samples and are not seen for Br- in bulk solutions. There are two possible explanations for scattering atoms at ca. 3.5 Å: (1) the nitrogen or carbon atom of the ion-exchange group and (2) carbon atoms of solvents. It has been reported that tertiary ammonium ions form stable ion-pairs with Br- even in solution, suggesting the direct interaction of Br- with hydrogen atoms in the -N+H(CH3)2 group.18 The charge distributions and structure of the ion-exchange site binding Br- in a vacuum were calculated by a semiempirical MO calculation program, MOPAC ver. 6.01, with a PM3 Hamiltonian.19b Because the R-3 resin is derived from polystyrene-type polymers, CH3-C6H4-CH2N+H(CH3)2 Br- can model the active site of this resin. Figure 5 shows the results of calculation. The local charge of the ammonium hydrogen atom (CH3-C6H4-CH2-N+H(CH3)2) is more positive (+0.21) than that of methyl hydrogen atoms (ca +0.09). This indicates the direct interaction of the ammonium hydrogen with Br-. Thus, the nitrogen atom of the -N+H(CH3)2 group is a scattering atom involved in the primary shell. The scattering atoms in the secondary shell are also reasonably interpreted by considering the tight ion-associate formation between Br- and the ammonium hydrogen. In this configuration, the secondary shell (r ) ca. 4.2 Å) is composed of the methylene carbon atom (-CH2-N+H(CH3)2) as well as two methyl carbon atoms (-N+H(CH3)2). If three atoms (Br-‚‚‚H -N) are aligned and the distance between Br- and the nitrogen is 3.5 Å, the methyl and methylene carbon atoms should be ca. 4.2 Å apart from Br-; this distance is consistent with the XAFS parameters
38 J. Phys. Chem. B, Vol. 106, No. 1, 2002
Harada et al.
Figure 6. Comparison of the XAFS χ(k)k3 spectrum for the R-4 resin soaked in water (solid curves) with XAFS χ(k)k3 spectra calculated by FEFF for Br-O 3.2 Å apart and Br-C 3.5 Å apart.
Figure 5. Schematic representations of MO calculations for the active sites of the R-3 and R-4 resins in a vacuum.
calculated based on the double shell model. However, our analyses implied that more than two different molecules should be involved in the primary shell. As listed in Table 1, the analyses give r ) ca. 3.5 Å and N ) 2-3 for the primary shell and r ) ca. 4.1 Å and N )3-4 for the secondary shell. We therefore conclude that one or two solvent molecules also coordinate Br-. This model explains the larger Debye-Waller factor for the primary shell than for the secondary shell because the atoms involved in the secondary shell are more rigidly fixed than those in the primary shell, which involves the atoms of solvent molecules. XAFS spectra suggest a single shell structure of Br- when the R-4 resin is soaked in an aprotic solvent. The N values () 4-5) are smaller than the corresponding values determined for Br- in bulk, while the interaction distances in the resin are almost the same as those in bulk solutions (r ) 3.5-3.6 Å). Unfortunately, we cannot clearly distinguish scattering atoms in solvent molecules from those in ion-exchange groups based solely on XAFS spectra because of similar backscattering amplitudes and phase shifts of carbon and nitrogen atoms. The XAFS spectra shown in Figure 4 indicate that the nature of an aprotic solvent does not affect the local structure of Br- in the R-4 resin. This suggests a minor contribution from the atoms in solvent molecules to XAFS oscillation. It is another important feature that N values obtained with the resin samples are substantially smaller than the corresponding values obtained for Br- in bulk solutions. The smaller N values for the resin samples possibly imply that a rather large trimethylammonium group occupies the major part of the surface of the primary shell around Br-. Figure 5 illustrates the result of the MO calculation for CH3-C6H4-CH2-N+(CH3)3 Br-, which is a model compound for the active site of R-4. This calculation shows that Br- is entrapped inside the tripod composed of three methyl groups. The electrical charge on the methyl hydrogen atoms of
the ion-exchange group was +0.19, whereas that on the methyl hydrogen atoms of AN was calculated as +0.06. This result supports the direct interaction of Br- with the ion-exchange group rather than solvent molecules. If it is assumed that the distance between Br- and methyl carbon atoms is 3.5 Å, the Br--N distance is ca. 3.7 Å. This difference 0.2 Å is not large enough to be clearly distinguished by XAFS because of similar backscattering nature of C and N atoms and long interaction distances. Thus, both the methyl carbon atoms and ammonium nitrogen atom contribute to the XAFS spectra obtained with the R-4 resin in aprotic solvents. If Br- is tightly bound by the ion-exchange groups, the Debye-Waller factor should be smaller than listed in Table 1; it ranges from 0.169 to 0.185 Å and almost equals to that obtained in bulk aprotic solutions. This must come from the contribution of solvent molecules weakly coordinating Br- from the opposite side of the ammonium ion. Hence, it can be concluded that Br- is tightly bound by the ion-exchange group of R-4 and is simultaneously coordinated by a few aprotic solvent molecules. Structures of Br- in the Resin Soaked in Protic Solvents. As summarized in Table 1, the distance between Br- and the oxygen atom of water has been reported to be 3.19 Å in bulk solutions;13a our analysis have provided almost the same distance (3.20 Å) for hydrated Br-, which is consistent with distances determined by other approaches, i.e., XRD (3.12-3.43 Å),10 ND (3.21 Å),11 and IR (3.43 Å).12 The Br‚‚‚O distance determined for Br- in bulk water is almost the same as that determined for Br- in the resins soaked in water; 3.24 Å for R-3 and 3.22 Å for R-4, indicating that the hydrated structure in bulk is maintained even in the resins to a large extent. In contrast, the coordination number is significantly different; N ) 3.45 for R-3 and 3.44 for R-4 but, for Br- in bulk, N ) 6,10a-c 8-9.410d,e (determined by XRD), and 611 (by ND). Another important feature of the XAFS spectra obtained with the resins soaked in water is the broadening and phase change seen at k ) 2-3 Å-1; this is characteristic feature found for the resin samples (see Figure 3). Dashed and dotted curves in Figure 6 show the χcalc(k)k3 spectra calculated for Br‚‚‚C (3.5 Å apart) and Br‚‚‚O (3.2 Å apart) pairs, respectively. The spectrum obtained with the R-4 resin soaked in water (shown by solid curves in this figure) agrees with the dashed curve in the range k ) 1-2.5 Å-1, but with dotted curve in the range k ) 3-6.5 Å-1. Thus, Br- in the R-4 resin soaked in water has two types of scattering atoms. It should be noted that a doubleshell model gave no relevant results, indicating that the XAFS spectra suggest the coexistence of more than two species.
Bromide Ion in Anion-Exchange Resins
J. Phys. Chem. B, Vol. 106, No. 1, 2002 39
-N+R3‚‚‚Br-(solv)n a Br-(solv)m We can assume that observed spectra (χoriginal(k)k3) are represented by the linear combination of the spectrum of a bound form (χresin(k)k3) and that of a solvated form (χsolution(k)k3). The latter was considered identical with the χ(k)k3 spectrum obtained in a bulk solution. The χresin(k)k3 spectra were estimated by subtracting the χsolution(k)k3 spectra from χoriginal(k)k3: χ
resin(k)k
Figure 7. χresin(k)k3 spectra (solid curves) extracted from the χ(k)k3 spectra obtained with the R-3 and R-4 resins soaked in water and MeOH. Dashed curves represent the χ(k)k3 spectra obtained with the resins soaked in AN.
In MeOH, both the interaction distance and Debye-Waller factor are considerably smaller than those obtained in other solvents, indicating that the solvent molecules strongly coordinate Br-. The strong coordination of MeOH is not characteristic of Br- in the resins and can be seen for Br- in solutions as well (see Table 1). The shorter interaction distance in MeOH than in water is intuitively felt strange because Br- is more preferably solvated in water than in MeOH; the solvation energy difference is ca. 11 kJ mol-1.20 This inconsistency may be explained by (1) the strong intermolecular forces of water and (2) water molecules coordinating Br- at more than two interaction distances. Although the latter explanation may be supported by a larger Debye-Waller factor in water, we do not have obvious experimental evidence. These XAFS parameters indicate that the solvated structure of Br- in a MeOH solution is substantially maintained in the resins. However, the shoulders in the χ(k)k3 spectra at k ) 2-3 Å-1 (Figure 3a) suggest that the structures in the resins be different from those in the solution (Figure 3b). The spectral broadening, which is seen in the same k range for the resins soaked in water as well, implies the similar structural feature of Br- in the resins soaked in these protic solvents. Solvated and bound Br- should coexist in the resins:
3
N ) χoriginal(k)k3 - χsolution(k)k3 m
(8)
where N () 3.45 for R-3 and 3.44 for R-4 in water and 1.68 for R-3 and 1.74 for R-4 in MeOH) and the solvation numbers in bulk solutions (m) are included to represent the contributions from solvated Br-. Equation 8 was derived by assuming that Br--solvent interaction predominantly determines the N values in comparison with Br--resin interaction when the resins are soaked in water or MeOH. This assumption can be justified by the shorter Br-‚‚‚O distances than the Br-‚‚‚N or C (resin) distance as well as larger backscattering amplitude of oxygen than those of nitrogen and carbon atoms. Figure 7 shows χresin(k)k3 spectra calculated by eq 8 (solid curves). The χ(k)k3 spectra obtained with the resins soaked in AN are also shown in Figure 7 for comparison (dashed curves); the amplitudes of the spectra are reduced by a factor of 0.45, 0.45, 0.55, and 0.6 for R-3 in water, R-4 in water, R-3 in MeOH, and R-4 in MeOH, respectively. As mentioned above, in AN, Br- forms a tight ion-associate with an ion-exchange group. The agreement of χresin(k)k3 with χ (k)k3 in AN thus shows that χresin(k)k3 spectra comes from the structure of Br- tightly bound by ion-exchange groups in a protic solvent. As listed in Table 2, the XAFS parameters calculated from the χresin(k)k3 spectra are fairly consistent with those obtained in an aprotic solvent. For the R-4 resin, the interaction distances are 3.43 and 3.67 Å in water and MeOH, which are comparable to those obtained in aprotic solvents, e.g., AN (r ) ca. 3.5 Å). In contrast, the interaction distances for the primary shell of the R-3 resin are slightly shorter than those obtained in aprotic solvents. All of the contributions cannot be separated from experimental spectra by the calculations based on eq 8, and thus, effects of solvation still remain in the χresin(k)k3 spectra and results in shorter interaction distance. Even in this case, the distance to the secondary shell should be independent of types of solvents, and almost the same as determined in an aprotic solvent.
Figure 8. Schematic illustration of the structures of Br- in (a) the R-3 and R-4 resins and (b) R-4 and R-3 in AN; (c and d) Br- bound by the active sites of the R-4 and R-3 resins soaked in water.
40 J. Phys. Chem. B, Vol. 106, No. 1, 2002
Harada et al.
TABLE 2: XAFS Parameters Determined for Br- Bound on the Resin in Water and MeOH R-3 solvents water first second MeOH first second
Na
r/Å
σ/Å
References and Notes 3.26 4.21
2.68 2.85
0.270 0.141
3.27 4.16
3.00 2.88
0.260 0.107
R-4
a
Education, Culture, Sports, Science, and Technology, Japan. This work has been carried out under the approval of the Photon Factory Program Advisory Committee (Proposal No. 98G311).
solvents
r/Å
Na
σ/Å
water MeOH
3.43 3.67
2.70 1.49
0.174 0.113
N value was determined by assuming N ) 6 for Br- in water.
A ratio of solvated Br- to that bound by the resin site can be roughly estimated. As stated above, the structure of Br- bound on the resin is conservative, and little affected by the nature of solvents. In AN, Br- forms a strong ion-associate with an ionexchange site. Taking R-4 resin in water as an example, N was determined as 4.45 in AN. Since the structure of Br- bound on the resin in water is identical with that seen in AN, the contribution from this structure to N can be estimated by the ratio of the amplitude of χresin(k)k3 to that of χ (k)k3 in AN () 0.45, see Figure 7). Thus, the molar ratio of Br- bound by the ion-exchange groups to hydrated one can be calculated as 0.58() 0.45 × 4.45/3.45):0.42. For the R-3 resin, the corresponding ratio (resin-bound:solvated) is 0.45:0.55 in water and 0.55:0.45 in MeOH. Thus, ca. 50% of total Br- are dissociated from ionexchange groups. In conclusion, we have thus elucidated some structural features of Br- in the anion-exchange resins by XAFS. Figure 8 illustrates the selected local structures of Br- in the resin. One of advantages of the structural approach to ion-exchange is that the absolute interactions of a single counterion can be discussed, while thermodynamic approaches provide relative information that is evaluated from the ion-exchange experiments of two components. We believe that both approaches should be necessary for the further understanding of ion-exchange mechanisms, and XAFS will make significant contributions to structural elucidation. This approach should be extensively applied to other systems to clarify its efficiency. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of
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