Anal. Chem. 2001, 73, 366-372
Water Structures in Ion-Exchange Resin Particles: Solvation Dynamics of Nile Blue A Satoshi Habuchi, Haeng-Boo Kim, and Noboru Kitamura*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
The structures of water, partitioned in cation-exchange resin particles, were studied on the basis of fluorescence dynamics of Nile Blue A (NB). The fluorescence lifetime of NB in the resin was longer than that in water and increased with increasing cross-linking density of the resin (G). The results demonstrated that the water structures in the resin were significantly different from those in water and dependent on G. A study on solvation dynamics of NB in the resin, reflecting structured water around the ionexchange group, revealed the roles of “bound water” molecules in the water structures, since the solvent relaxation time (τS) in the resin was much longer than that in water and depended on G; τS increased from 34 to 55 ps with increasing G from 2 to 8%. The origin of the G dependence of τS was discussed in terms of the separation distance between the ion-exchange groups, and the effects of the counterion of the ion-exchange group on the solvation processes were also discussed. Ion-exchange resin is widely used in both fundamental and applied research fields,1,2 and the ion-exchange mechanisms in the resin have been extensively investigated by various techniques. Studies so far made indicate that ion exchange in the resin proceeds via three steps: diffusion of an ion in an adherent film around the resin, diffusion of an ion inside the resin, and an ionexchange reaction at a fixed ionic group.1 Among these steps, the rate-determining step of ion exchange has been reported to be intraparticle diffusion of an ion in the resin. Therefore, a study on the diffusion processes of an ion in the resin particles is of primary importance for understanding the whole of the ionexchange processes. In practice, intraparticle diffusion of an ion in the resin has been investigated by a tracer method using a radioactive isotope,3 flame photometry,4 and so on. Recently, diffusion of an ion and water in the resin has been directly (1) (a) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962. (b) Samuelson, O. Ion Exchange Separations in Analytical Chemistry; John Wiley: New York, 1963. (2) (a) Bunzl, K. J. Phys. Chem. 1991, 95, 1007-1012. (b) Juang, R. S.; Lin, H. C. J. Chem. Technol. Biotechnol. 1995, 62, 132-140. (c) Lucas, A.; Can ˜izares, P.; Rodrı´guez, J.; Gracia, I. Chem. Eng. J. 1997, 66, 137-147. (d) Linnekoski, J. A.; Krause, A. O.; Struckmann, L. K. Appl. Catal. A 1998, 170, 117-126. (e) Rhee, I. H.; Dzombak, D. A. Langmuir 1999, 15, 6875-6883. (3) (a) Boyd, G. E.; Soldano, B. A. J. Am. Chem. Soc. 1954, 75, 6091-6099. (b) Ferna´ndez-Prini, R.; Philipp, M. J. Phys. Chem. 1976, 80, 2041-2046. (c) Yeager, H. L.; Kipling, B. J. Phys. Chem. 1979, 83, 1836-1839. (4) (a) El-Naggar, I. M.; Zakaria, E. S.; El-Wahab, M. A.; Belacy, N.; Aly, H. F. Solid State Ionics 1996, 92, 309-315. (b) Varshney, K. G.; Pandith, A. H. Langmuir 1999, 15, 7422-7425.
366 Analytical Chemistry, Vol. 73, No. 2, January 15, 2001
measured by a pulsed-field-gradient spin-echo NMR technique as well.5 However, a direct method capable of interrogating ionexchange processes in the resin particles is still limited. Clearly, further detailed studies on ion exchange in the resin particles are needed for development of the relevant research fields. Previously, we reported direct measurements of diffusion processes of a cationic fluorescence dye in single ion-exchange resin particles on the basis of laser trapping microspectroscopy and confocal fluorescence microspectroscopy techniques.6 A direct excitation energy-transfer method has been also shown to be a powerful means to study ion diffusion processes in the resin particles.7 These studies revealed that the spatial distribution of the ion-exchange group was dependent on the cross-linking density of the resin.8 Since fluorescence dynamic anisotropy measurements demonstrated that an ion was tightly bound to the ion-exchange group,8 the intraparticle diffusion processes of an ion are governed strongly by the three-dimensional network structures in the resin. In practice, we showed previously that the diffusion coefficient of a fluorescent cationic dye in a single ion-exchange resin particle was as slow as 10-13 cm2 s-1,6 reflecting characteristic microstructures in the resin particles. To further elucidate the ion diffusion processes, one must know microenvironments in the network structures in the resin, especially the water structures. An ion-exchange reaction is based essentially on hopping of an ion between exchange sites and this might accompany a change in hydration structures around an exchangeable ion and/or ion-exchange group. Furthermore, water molecules are confined in three-dimensional network structures in the resin. Therefore, the water structures in the resin could be different from those in bulk water, and such characteristic water structures and motions would determine the ion-exchange processes in the resin. One method for investigating solvent motions in a restricted environment is to measure solvation dynamics of a probe molecule by using time-resolved fluorescence spectroscopy. Solvation dynamics is the measure of the polar solvent response to an (5) (a) Busch, M.; Goldammer, E. V. J. Solution Chem. 1982, 11, 777-791. (b) Ohuchi, M.; Horiuchi, H.; Sakai, Y. Kobunshi Ronbunshu 1996, 53, 322329 (in Japanese). (c) D’Archivio, A. A.; Galantini, L.; Panatta, A.; Tettamanti, E.; Corain, B. J. Phys. Chem. B 1998, 102, 6774-6779. (d) Matsukawa, S.; Yasunaga, H.; Zhao, C.; Kuroki, S.; Kurosu, H.; Ando, I. Prog. Polym. Sci. 1999, 24, 995-1044. (6) Kim, H.-B.; Hayashi, M.; Nakatani, K.; Kitamura, N.; Sasaki, K.; Hotta, J.; Masuhara, H. Anal. Chem. 1996, 68, 409-414 (correction: 1996, 68, 1987). (7) Kim, H.-B.; Habuchi, S.; Hayashi, M.; Kitamura, N. Anal. Chem. 1998, 70, 105-110. (8) Kim, H.-B.; Habuchi, S.; Kitamura, N. Anal. Chem. 1999, 71, 842-848. 10.1021/ac0007276 CCC: $20.00
© 2001 American Chemical Society Published on Web 12/08/2000
instantaneous change of the charge distribution in a probe molecule upon excitation by recording the changes in the relevant fluorescence characteristics: fluorescence dynamic Stokes shift. A fluorescence dynamic Stokes shift reflects directly the rate of solvent reorganization around a probe molecule and has been studied for a variety of solvents.9 Solvation dynamics measurements have also been performed in various heterogeneous systems such as reverse micelles,10 cyclodextrin,11 vesicles,12 zeolites,13 liquid crystals,14 solid/liquid interfaces,15 liquid/liquid interfaces,16 DNA,17 and so on. These studies revealed that the solvent motions in restricted spaces were significantly different from those in bulk phases. Therefore, we expected that solvation dynamics of a probe molecule would provide invaluable information about water structures in an ion-exchange resin. In the present study, we applied solvation dynamics measurements to an ion-exchange resin for the first time by using Nile Blue A (NB) as a fluorescence probe. NB is a solvatochromic dye and is known to show a fluorescence dynamic Stokes shift in a bulk solvent.18 The fluorescence lifetime of NB also decreases with increasing a solvent polarity.19 Furthermore, since NB is a cationic dye, it can bind to a cation-exchange group in the resin. NB is thus very suitable for the present purpose of the experiments, and water structures around the ion-exchange group could be discussed in detail by observing the fluorescence dynamic Stokes shift of NB in a cation-exchange resin. In the following, we report the water structures in a cation-exchange resin on the basis of the picosecond fluorescence dynamics of NB. The water structures in the resin are then discussed in terms of both the cross-linking density of the resin and the nature of the counterion of the ionexchange group. EXPERIMENTAL SECTION Chemicals and Sample Preparation. Water was purified by using a Toraypure LV-08 system (Toray, conductivity >17 MΩ‚ cm) prior to use. Lithium chloride (Kanto Chemical Co., Inc.), (9) (a) Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1987, 86, 6221-6239. (b) Kahlow, M. A.; Jarzeˆba, W.; Kang, T. J.; Barbara, P. F. J. Chem. Phys. 1989, 90, 151-158. (c) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471-473. (d) Laitinen, E.; Salonen, K.; Harju, T. J. Chem. Phys. 1996, 105, 9771-9780. (e) Chapman, C. F.; Fee, R. S.; Maroncelli, M. J. Phys. Chem. 1995, 99, 4811-4819. (f) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys. Chem. 1995, 99, 9, 17311-17337. (10) (a) Riter, R. E.; Undiks, E. P.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 6062-6067. (b) Riter, R. E.; Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 2705-2714. (c) Willard, D. M.; Riter, R. E.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 4151-4160. (d) Zhang, J.; Bright, F. V. J. Phys. Chem. 1991, 95, 7900-7907. (e) Sarkar, N.; Das, K.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem. 1996, 100, 15023-10527. (11) (a) Vajda, Sˇ .; Jimenez, R.; Rosenthal, S. J.; Fidler, V.; Fleming, G. R.; Castner, E. W. J. Chem. Soc., Faraday Trans. 1995, 91, 867-873. (b) Nandi, N.; Bagchi, B. J. Phys. Chem. B 1996, 100, 13914-13919. (12) Datta, A.; Pal, S. K.; Mandal, D.; Bhattacharyya, K. J. Phys. Chem. B 1998, 102, 6114-6117. (13) Das, K.; Sarkar, N.; Das, S.; Datta, A.; Bhattacharyya, K. Chem. Phys. Lett. 1996, 249, 323-328. (14) (a) Rau, J.; Ferrante, C.; Deeg, F. W.; Bra¨uchle, C. J. Phys. Chem. B 1999, 103, 931-937. (b) Saielli, G.; Polimeno, A.; Nordio, P. L.; Bartolini, P.; Ricci, M. J. Chem. Soc., Faraday Trans. 1998, 94, 121-128. (15) Yanagimachi, M.; Tamai, N.; Masuhara, H. Chem. Phys. Lett. 1992, 200, 469-474. (16) Bessho, K.; Uchida, T.; Yamauchi, A.; Shioya, T.; Teramae, N. Chem. Phys. Lett. 1997, 264, 381-386. (17) Brauns, E. B.; Madaras, M. L.; Coleman, R. S.; Murphy, C. J.; Berg, M. A. J. Am. Chem. Soc. 1999, 121, 11644-11649.
Table 1. Properties of the Cation-Exchange Resin Particles Used in This Study sample Fa/% counterion 8
4 2
Li+ Na+ Cs+ TEA+ Na+ Li+ Na+ Cs+ TEA+
diameterb/ µm
ion-exchange capacityc /mequiv/mL
[NB]Rd/ 10-4 M
SDe/Å
11.8 ( 1 11.6 ( 1 11.8 ( 1 12.4 ( 1 12.2 ( 1 21.7 ( 1 21.6 ( 1 21.0 ( 1 22.7 ( 1
1.92 2.02 1.92 1.65 1.24 0.69 0.70 0.76 0.60
6.4 6.7 6.4 5.5 3.0 1.8 1.8 2.0 1.6
11.8 11.6 11.8 12.4 13.7 16.6 16.5 16.1 17.3
a Cross-linking density. b Determined under an optical microscope in this study. c The data of the Na+ form were taken from the data sheet supplied from Mitsubishi Chemicals Co. Ltd., and the values for other resins with different counterions were estimated by the diameter of each resin. d Absorbance of NB in the particles was determined by a laser trapping-absorption microspectroscopy system reported earlier.21 The NB concentration in the particle was calculated on the basis of the molar absorptivity at the maximum wavelength (657 nm; 4.7 × 104 M-1 cm-1) and the diameter of the particle being assumed to be the optical path length. e The separation distance between the ionexchange groups, calculated by the ion-exchange capacity and the size of the resin.
cesium chloride (Wako Pure Chemical Industries, Ltd.), and tetraethylammonium chloride (Wako Pure Chemical Industries, Ltd.) were used without further purification. The properties of the cation-exchange resins used in this study (Mitsubishi Chemicals; MCI-GEL CK08S, CK04S, and CK02A) are summarized in Table 1. The resins are made of a styrenedivinylbenzene copolymer having sulfonate as an ion-exchange group. The cross-linking density of the resin (F in percent; mole fraction of divinylbenzene) was 8, 4, or 2% for CK08S, CK04S, or CK02A, respectively. The resins (supplied as a Na+ form) were washed thoroughly with water and dried in the same manner as reported previously.7,8 The countercation of the ion-exchange group was replaced by Li+, Cs+, or tetraethylammonium cation (TEA+) by soaking 1 g of the resin in 100 mL of 2 M LiCl, CsCl, or TEACl solution for 1 h, respectively. The procedures were repeated three times to attain complete exchange of the countercation. These resins were washed thoroughly with water and then dried in air at room temperature. Nile Blue A perchlorate (NB, Acros Organics; laser grade) was used as supplied. The purity of NB was checked by comparing the fluorescence lifetime in a dilute aqueous solution with the reported value.19 NB-adsorbed resins were prepared by soaking each resin (10 mg, dry weight) in an aqueous NB solution (4 mL, 2 × 10-6 M) for 1 day. Under such conditions, the concentration of NB in the resin was 6.7 × 10-4, 3.0 × 10-4, and 1.8 × 10-4 M for the resin with F ) 8, 4, or 2%, respectively, as determined by laser trapping-absorption microspectroscopy.20 After filtration, the NB-adsorbed resin was then dispersed in water as a sample. Spectroscopic Measurements. Fluorescence decay profiles of NB in the resin were measured by using a time-correlated (18) Mokhtari, A.; Chesnoy, J.; Laubereau, A. Chem. Phys. Lett. 1989, 155, 593598. (19) (a) Dutt, G. B.; Doraiswamy, S.; Periasamy, N.; Venkataraman, B. J. Chem. Phys. 1990, 93, 8498-8513. (b) Grofcsik, A.; Kubinyi, M.; Jones, W. J. J. Mol. Struct. 1995, 348, 197-200. (20) (a) Kim, H.-B.; Yoshida, S.; Kitamura, N. Anal. Chem. 1998, 70, 51-57. (b) Kitamura, N.; Hayashi, M.; Kim, H.-B.; Nakatani, K. Anal. Sci. 1996, 12, 49-54.
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367
Figure 1. Fluorescence (left) and fluorescence excitation (right) spectra of NB in water (solid line) and in the resin particles (F ) 8, Li+ form, dotted line).
single-photon-counting system reported earlier.8 Briefly, laser pulses (800 nm) from a mode-locked Ti:sapphire laser (Coherent, Mira model 900F) were amplified by a regenerative amplifier (RegA model 9000). Optical parametric amplification (model 9400) of the output gave 600-nm laser pulses (fwhm ) 150 fs, repetition rate 100 kHz) as an excitation light source. The polarized direction of the excitation laser pulse was set at a vertical direction. The fluorescence from a sample was passed through a polarizer set at a magic angle for fluorescence decay measurements, while the polarizer was set at a perpendicular or horizontal angle for fluorescence decay or dynamic anisotropy measurements. The fluorescence was then analyzed by a time-correlated single-photoncounting module, equipped with a microchannel-plate photomultiplier and a polychromator. Decay profiles were analyzed by using an iterative nonlinear least-squares deconvolution method. Time-resolved fluorescence spectra were generated from a set of decays taken at 5-nm intervals spanning the fluorescence spectra with the accumulation time of each datum being set 150 s. To deconvolve the instrument response from the decay data, each decay was fit to a sum of exponential functions. Three components were generally required to obtain a satisfactory fit to the data. The purpose of these fits is simply to reproduce the decay curve, and no physical meaning is ascribed to the derived exponential parameters. The spectrum at a given delay time was obtained from the fitted decay series by relative normalization of the photon counts at different wavelengths. Steady-state fluorescence and excitation spectra of NB in the resin were measured with a Hitachi F4500 fluorescence spectrophotometer. All the experiments were performed under aerated conditions in a temperature-controlled room (294 K). RESULTS AND DISCUSSION Steady-State Fluorescence and Excitation Spectra of NB in the Resin Particles. Figure 1 shows a typical example of steady-state fluorescence and excitation spectra of NB in the resin, together with those in water. We explored absorption spectroscopy of single NB-adsorbed resin beads by a laser trapping-microspec368
Analytical Chemistry, Vol. 73, No. 2, January 15, 2001
troscopy technique.21 The absorption spectrum of NB in the resin agreed well with the relevant excitation spectrum determined for an ensemble sample. Nonetheless, the spectral resolution was much better for the excitation spectrum compared to the absorption spectrum. Therefore, the following discussions are made on the basis of the excitation spectrum data instead of the absorption data. All of the spectroscopic data observed in this study are summarized in Table 2. The peak frequency of the excitation spectrum (νex) of NB in the resin (15 165-15 295 cm-1) was shifted to a lower frequency compared with that in water (νex ) 15 699 cm-1). Similarly, the fluorescence of NB in the resin showed a red-shifted spectrum compared with that in water: peak frequency: νem ) 14 815-14 930 cm-1 in the resin and 14 970 cm-1 in water. Analogous results have been obtained for another cationic dye (i.e., rhodamine B, RhB), and the spectral shift has been ascribed to binding of the dye to the ion-exchange group (sulfonate group) in the resin.8 Since NB is also a cationic dye, such a spectral shift of NB could be due to a strong interaction between NB and the ion-exchange group in the resin. The band shapes of both excitation and fluorescence spectra were independent of the cross-linking density of the resin and the nature of the counterion of the ion-exchange group. On the other hand, νex and νem were dependent on F and the counterion. Assuming νex is identical with the absorption maximum of NB, we discuss the present results on the basis of a Stokes shift (∆ν), as the results of ∆ν ) νex - νem are included in Table 2. In the present case, the Stokes shift reflects the polarity around the dye molecule. The dipole moments of NB in the ground and excited states have been reported to be 2.0 and 8.6 D,19a respectively, so that the excited state is more stabilized in energy compared to the ground state. Therefore, the Stokes shift of NB should be larger in a polar environment. Table 2 indicates that the Stokes shift of NB in the resin (253450 cm-1) is very small compared to that in water (729 cm-1), demonstrating that the microenvironment around NB in the resin is hydrophobic owing to the presence of the styrene-divinylbenzene copolymer matrix. The presence of the polymer matrix is also reflected on a F dependence of ∆ν; ∆ν increases with a decrease in F (286, 322, and 418 cm-1 for the resins (Na+ form) with F ) 8, 4, and 2%, respectively). The results indicate that the hydrophobicity in the resin varies with the cross-linking density. Nonetheless, the hydrophobicity of the polymer network itself would not depend on F. As discussed later, an increase in F brings about a decrease in the pore size of the three-dimensional polymer network in the resin. Therefore, the F dependence of the Stokes shift suggests that the structures of the water molecules presented in the resin are much different from that of a bulk water, and this effect would be more pronounced in the resin with a higher crosslinking density. The importance of the water structures in the pore is also suggested by a counterion dependence of ∆ν, by which ∆ν becomes larger for the ion with a smaller hydrodynamic radius: Li+ < Na+ < Cs+. Since the nature of the counterion itself would not determine the polarity in the pore, the hydration structures around the counterion are suggested to influence the water structures in the resin as discussed later again in detail. (21) Kitamura, N.; Nakatani, K.; Kim, H.-B. Pure Appl. Chem. 1995, 67, 79-86.
Table 2. Spectroscopic Properties of NB in the Resin Particles sample
C(t)
F/%
counterion
νexa/cm-1
νema/cm-1
∆νb/cm-1
τavc/ns
a1
τS1/ps
8
Li+ Na+ Cs+ TEA+ Na+ Li+ Na+ Cs+ TEA+ in water
15 184 15 211 15 216 15 165 15 225 15 267 15 281 15 295 15 216 15 699
14 930 14 925 14 894 14 912 14 903 14 863 14 863 14 846 14 815 14 970
254 286 322 253 322 404 418 450 401 729
2.32 2.34 2.26 2.36 2.06 1.73 1.72 1.66 1.62 0.38
1 1 1 0.48 1 1 1 1 0.42
53 55 62 72 47 29 34 27 37
4 2
a2
τS2/ps
0.52
264
0.58
116
a ν and ν b c ex em are the peak frequencies of the excitation and fluorescence spectra, respectively. ∆ν ) νex - νem. τav is the average fluorescence lifetime of NB calculated by the equation, τav ) Σiaiτi2/Σiaiτi.
Figure 2. Fluorescence anisotropy decay profiles of NB in (a) the resin particles (F ) 8, Li+ form) and (b) water. The solid curve shows the best fit by eq 1.
Fluorescence Dynamics of NB in the Resin Particle. To discuss further the microenvironments in the resin, adsorption states of NB in the matrix should be clarified since strong interactions between NB and the ion-exchange group have been suggested by steady-state spectroscopy. Therefore, we conducted dynamic fluorescence anisotropy of NB in the resin. Dynamic fluorescence anisotropy8 (r(t)) is defined as in eq 1,
r(t) )
I|(t) - I⊥(t) I|(t) + 2I⊥(t)
( )
) r0 exp -
t τr
(1)
where I|(t) and I⊥(t) are the parallel and perpendicular components of a fluorescence decay, respectively, and r0 is initial anisotropy at t ) 0. τr is the rotational correlation time of a probe molecule. Figure 2 shows r(t) of NB in the resin and water. Analysis of the r(t) curve determined in water gave τr ) 110 ps, which agreed very well with the reported value (τr ) 116 ps).19a On the other hand, r(t) in the resin did not decay over 10 ns after excitation. Analogous results were obtained for other resin samples, indicating that the rotational motions of NB were inhibited almost completely owing to tight binding of NB to the ion-exchange
Figure 3. Fluorescence decay profiles of NB in the resin particles (a) F ) 8, Li+ form, (b) F ) 4, Li+ form, (c) F ) 2, Li+ form, and (d) water. The solid curve shows the best fit by single- or doubleexponential functions.
group. It is noteworthy that NB is considered to be located in the close vicinity of the ion-exchange group irrespective of F and the counterion. Therefore, such results are very important for discussing the water structures in the resin. Figure 3 shows a typical example of fluorescence decay curves of NB in the resin particles (F ) 2, 4, and 8%), together with that in water. The fluorescence decay curve in water was fitted satisfactorily by a single-exponential function. On the other hand, the time response of the fluorescence in the resin showed a nonsingle-exponential decay: 2.7 and 0.73 ns as an example. The fluorescence decay of NB has been known to depend highly on a medium polarity. For instance, NB shows the fluorescence lifetime of 0.38 or 2.7 ns in water or 1-decanol, respectively. Therefore, the non-single-exponential decay of the NB fluorescence in the resin is probably due to the presence of mutually different microenvironments around NB. Previously, we reported that the fluorescence lifetime of RhB in resin particles showed a singleexponential decay irrespective of F,8 in marked contrast to the present results. Since both experiments have been performed with the same MCI-GEL samples, such a difference in the fluorescence dynamics is considered to be caused by the fact that the Analytical Chemistry, Vol. 73, No. 2, January 15, 2001
369
fluorescence dynamics of NB is more sensitive to the surrounding environments compared to that of RhB. Figure 3 and Table 2 indicate that the average lifetime of NB (τav) in the resin (1.62.4 ns) is much longer than that in water (0.38 ns), and the fluorescence decay becomes slower with increasing F. These results demonstrate clearly that the micropolarity around the ionexchange group in the resin is very low compared to that in water and becomes less polar with increasing F. Such the results agree quite well with those obtained by the steady-state spectroscopic measurements. Fluorescence Dynamic Stokes Shift of NB in the Resin Particles. The steady-state spectra and fluorescence dynamics of NB in the resin indicated that the microenvironments in the resin differ significantly from those in water. To obtain direct information about the microenvironments, therefore, we studied a fluorescence dynamic Stokes shift of NB in the resin. Figure 4 shows fluorescence decays of NB in the resin particles monitored at two different wavelengths. As seen in the figure, the time response of the fluorescence monitored at 640 nm showed a fast decay, while that determined at 750 nm showed both rise and decay components. Such a behavior is indicative of a continuous time-dependent shift of the spectrum rather than kinetics involving a few discrete excited states. Furthermore, NB is a relatively rigid and asymmetric dye, which ensures that the observed timedependent spectral shift can be attributed to solvation without contributions from the internal degree of freedom of the probe. A typical example of time-resolved fluorescence spectra of NB in the resin (F ) 8%), determined by the procedures described in the Experimental Section, is shown in Figure 5. A time-dependent spectral shift was observed in the range of several hundreds of picoseconds, demonstrating reorganization of water molecules in the resin. For further discussion, the fluorescence maximum at a given time after excitation (ν(t)) should be determined. Since the spectral shift with t is rather small, we determined ν(t) by fitting each spectrum by a log-normal line shape function,9a,22
[
g(ν) ) g0 exp - ln(2)
(
)]
ln[1 + 2b(ν - νp)/∆] b
2
(2)
where g0, νp, b, and ∆ are peak height, peak frequency, asymmetric parameter, and width parameter, respectively. The observed ν(t) data were then analyzed by the following correlation function,
C(t) )
ν(t) - ν(∞) ν(0) - ν(∞)
(3)
where ν(0) and ν(∞) are the optical frequencies of the fluorescence maximum at t ) 0 and an infinite time, respectively. In the case of solvation dynamics, C(t) is expressed as a sum of exponentials, n
C(t) )
∑A exp(-t/τ i
Figure 4. Fluorescence decay profiles of NB in the resin particles (F ) 8, Li+ form) at (a) 640 and (b) 750 nm. The solid curve shows the best fit by triple-exponential functions.
Si)
(4)
i)1
where τSi is a solvent relaxation time and Ai is the relevant amplitude. (22) Siano, D. B.; Metzler, D. E. J. Chem. Phys. 1969, 51, 1856-1861.
370 Analytical Chemistry, Vol. 73, No. 2, January 15, 2001
Figure 5. Time-resolved fluorescence spectra of NB in the resin particles (F ) 8, Li+ form) at time delays of 20 (b), 50 (O), 100 (2), and 200 ps (4). The solid curve shows the best fit by eq 2.
Figure 6 shows solvation dynamics of NB in the resin particles with different F (a, Na+) and counterions (b, F ) 8%). The time response of C(t) was well reproduced by eq 4 with one or two exponentials as the fittings were shown by solid lines. Analogous results were obtained for other samples, and the results of τSi were listed in Table 2. The important findings are as follows. First, τSi in the resins (34-55 ps with Na+ form) were 2 orders of magnitude slower than that in water (τS < 1 ps).9c Second, τS was longer for the resin with a higher F value: 55, 47, or 34 ps for F ) 8, 4, or 2% (Na+ form), respectively. Third, we observed two relaxation times in the resin with TEA+ as a counterion; a shorter relaxation time (37 or 72 ps) was comparable to that observed for the resin having an alkali metal ion (27-62 ps), while a longer relaxation time (116 or 264 ps) was ∼4 times that of the short component. In the present experiments, a decay component faster than 20 ps cannot be resolved owing to time resolution of our experimental setup. Indeed, a dynamic Stokes shift of NB in water
Figure 6. Solvation dynamics of NB in the resin particles with (a) F ) 8, Na+ form (b), F ) 4, Na+ form (O), and F ) 2, Na+ form (2) and (b) F ) 8, Li+ form (b), F ) 8, Na+ form (O), F ) 8, Cs+ form (2), and F ) 8, TEA+ form (4). The solid curve shows the best fit by eq 4.
could not be observed. It is worth noting, however, that observation of both F and counterion dependencies of τSi is very important to elucidate water structures in the resin as discussed in the following section in detail. Structural Characteristics of Water in the Resin Particle. An organic ion-exchange resin consists of three-dimensional crosslinked polymer networks, and ion-exchange groups attached to the polymer are exposed to water pools in the resin. The size of the water pool in the present resin has been estimated to be 60, 80, or 100 Å in diameter for the resin with F ) 8, 4, or 2%, respectively.8 Since NB could be located in the periphery of the water pool, its fluorescence dynamics will be influenced by the water pool size. For NB in the cation-exchange resins having an alkali metal ion as a counterion (i.e., Na+, Li+, or Cs+), C(t) was best fitted by a single-exponential function with τS being several tens of picoseconds. It is worth noting that τS is independent of the nature of the counterion, while that depends on F and, therefore, on the
water pool size in the resin. In the present case, dynamic fluorescence anisotropy of NB in the resin clearly demonstrated that NB was strongly bound to the ion-exchange group. NMR spectroscopic studies on an ion-exchange resin suggested that the first hydration sphere of a counterion involved three water molecules, and the counterion and the ion-exchange group interacted significantly with each other.23 It is noteworthy, furthermore, that a sulfonate group as an ion-exchange site is not hydrated since the electronic charge is delocalized over the sulfonate group.23a,24 In the present case, the total amount of NB adsorbed onto each particle is far below the ion-exchange capacity of the resin (0.033, 0.024, or 0.026% for the resin with F ) 8, 4, or 2%, respectively), so that NB interacting electrostatically with a sulfonate group is surrounded considerably by the countercations. It is worth noting that an NMR study on spin-lattice relaxation in an analogous system (F ) 1%, Na+ form) has demonstrated the rotational correlation time of water molecules in the first hydration shell of the ion-exchange groups to be 30 ps, “bound water”. The value is comparable to the solvent relaxation time obtained in this study.5a Therefore, the slow relaxation times observed in the resin particles could be attributed to bound water around the countercations whose mobility is much lower than that of free water. Although the τS value is certainly dependent on F, the water pool size in the resin of 60-100 Å is not small enough to explain directly observed τS through the structured water (i.e., bound water in the resin). An another factor characterizing the ionexchange resin might influence the water structures in the resin. A close inspection of the data in Table 1 indicates that the ionexchange capacity increases with increasing F. A variation of the ion-exchange capacity per unit volume brings about that of the average separation distance between the ion-exchange groups in the resin: 12, 14, or 16 Å for the resin with F ) 8, 4, or 2%, respectively. The ratio of the number of bound waters to that of free waters in the pool is considered to increase with decreasing separation distance, so that the density of the counterion influences the water structures in the periphery of the pool through hydration around the counterion as discussed above. The F dependence of τS could be thus explained by such distribution characteristics of the ion-exchange group in the resin. It is worth noting, however, that τS is almost independent of the nature of an alkali metal ion as the counterion. It has been reported that an alkali metal ion in the resin possesses hydration numbers (3) analogous to each other as determined by NMR spectroscopy.23 Furthermore, the diameter of the resin particle is almost constant irrespective of the counterion, indicating analogous swelling properties. Therefore, we suppose that the hydration structures in the resin do not vary with the nature of the alkali metal ion. The present results are therefore reasonably explained by bound water around the counterion. In the resin having TEA+ as a counterion, on the other hand, both short (37-72 ps) and very slow (116-264 ps) relaxation times were observed. The short relaxation time would be attributed to bound water around TEA+ analogous to those observed (23) (a) Creekmore, R. W.; Reilley, C. N. Anal. Chem. 1970, 42, 570-575. (b) Komoroski, R. A.; Mauritz, K. A. J. Am. Chem. Soc. 1978, 100, 7487-7489. (24) (a) Uedaira, H.; Uedaira, H. J. Phys. Chem. 1970, 74, 1931-1936. (b) Tamaki, K.; Ohara, Y.; Kurachi, H.; Akiyama, M.; Odaki, H. Bull. Chem. Soc. Jpn. 1974, 47, 384-388.
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for other resin particles having an alkali metal ion. Furthermore, very slow relaxation indicated that the microenvironments in the TEA+ resin are quite different from that in other resins. Relaxation times as slow as 116-264 ps would not be explained by the bound water alone. Since NB will be surrounded by the tetraalkyl chains of TEA+, we suspect that coupled motions of water and the alkyl chains are the possible reason for the very slow relaxation time rather than the hydration structure of the counterion itself. Both the cross-linking density and the nature of the counterion were shown to influence the water structures in the ion-exchange resin. CONCLUSIONS We demonstrated that time-resolved fluorescence spectroscopy gave valuable information about the microenvironments in the network structure of ion-exchange resin particles. The fluorescence lifetime of NB indicated that the micropolarity around the ion-exchange group in the resin was very low compared to that in water and became less polar with increasing F. The solvent relaxation time in the resin was much slower than that in water and was shown to increase with increasing cross-linking density of the resin. The slow relaxation could be attributed to the
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hydration of the countercations of the ion-exchange group, “bound water”. The F-dependent relaxation time would be explained by the density of the ion-exchange group. Although the nature of the alkali metal ion as the counterion of the ion-exchange group had almost no effect on the solvation dynamics in the resin, tetraethylammonium ions exhibited large effects on τS, probably through the coupled motions of water and the alkyl chains. Such characteristic microenvironments in the resin could play essential roles in intraparticle diffusion of an ion in the resin. ACKNOWLEDGMENT The authors acknowledge a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, Japan (11554032 to H.-B.K. and 10207201 to N.K.) for partial support of the research. S.H. acknowledges the JSPS for a fellowship.
Received for review June 23, 2000. Accepted October 19, 2000. AC0007276
