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Optical beam deflection approach for studying ion-exchange reactions occurring at a single ion-exchange resin particle
REACTIVE & FUNCTIONAL POLYMERS
Reactive & Functional Polymers 67 (2007) 113–119
www.elsevier.com/locate/react
Optical beam deflection approach for studying ion-exchange reactions occurring at a single ion-exchange resin particle Xing-Zheng Wu a
a,*
, Yumiko Tsuji a, Norio Teramae
b
Department of Materials Science and Engineering, Faculty of Engineering, Fukui University, Bunkyo 3-9-1, Fukui 910-8507, Japan b Department of Chemistry, Graduate School of Science, Tohoku University, Japan Received 13 December 2005; received in revised form 26 July 2006; accepted 19 October 2006 Available online 27 November 2006
Abstract The optical beam deflection method was applied for exploring chemical reactions occurring at a single particle. A probe beam from a diode laser was introduced into a microscope, and focused to the vicinity of a particle. When a chemical reaction occurs at the surface of the particle, concentration gradients exist in the vicinity of the particle due to the diffusion of chemical species involved in the reaction. Ion exchange reactions occurring at a single particle of ion exchangers were used as model systems. The results showed that the tendency of change in the OBD signal with time is opposite of that seen in the reverse ion exchange reactions, both theoretically and experimentally. Also, the experimental results indicated that the concentration gradients changed with time in the diffusion layer around the particles, implying that the actual reaction-rate determining step was not the particle diffusion but film diffusion. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Optical beam deflection; Single particle; Ion exchanger; Reaction; Microscope
1. Introduction Characterization of a single particle is becoming more and more important in many fields, such as in studies on aerosols [1–3] and polymerization kinetics [4,5]. Several methods, including single particle-mass spectroscopy [1–4], small-angle neutron scattering [6], and light scattering [7] techniques, *
Corresponding author. Tel.: +81 776 27 8610; fax: +81 776 27 8767. E-mail address: [email protected] (X.-Z. Wu).
have been used for characterization of a single particle. Confocal raman microscopy has been applied for monitoring chemical reactions on single optically trapped, solid-phase support particles [8]. Laser trapping microspectroscopy and confocal fluorescence microspectroscopy have been used for in situ measurements of ion-exchange processes in a single polymer particle [9–12]. The optical beam deflection method, which is based on probing a refractive index gradient (mirage effects), [13] has been used extensively in photothermal spectroscopy, where a temperature gradient is
1381-5148/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.10.002
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X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
generated by nonradiative relaxation of the photoenergy adsorbed from a powerful excitation light source [13,14]. On the other hand, the OBD method has also been applied for exploring a concentration gradient [15–19] in chemical and electrochemical systems. The OBD method also can be used for exploring a concentration gradient in flow systems [20–22]. Recently, we have explored chemical reaction heat-induced OBD and concentration gradient-induced OBD in chemical reactions in aqueous solution [23–30]. In principle, any chemical reaction occurring at a single particle in aqueous solution will generate concentration changes or gradients of chemical species, and a reaction heat-induced temperature gradient around the particle. Therefore, the OBD method should be applicable to the study of chemical reactions occurring at a single particle. Here, we explore the possibility of applying the chemical reaction-induced OBD for studying chemical reactions occurring at a single particle. Ionexchange reactions occurring at a single particle of ion exchangers were used as model systems.
a
2. Experimental As shown in Fig. 1a, a probe beam from a diode laser (3 mW, 670 nm) was introduced into a microscope. The probe beam was reflected inside the microscope, and then focused (spot size, 5 lm) to the vicinity of a particle of ion-exchange resin (Amberlite IR-120 B, IRA-400) or a glass bead by an objection lens (magnification 4). The focused beam passed just beside the particle surface (a very small portion of the focused beam directly stroked the surface of the particle). A part of the resin particle and of the glass bead was glued onto a slide glass with epoxy glue, and an O-ring (diameter 1 cm) was also fixed on the slide glass as a reaction cell (Fig. 1a). A position sensor (Hamamatsu) was used for detection of the deflection of the probe beam. Most of the O-ring was covered by a cover glass for preventing water evaporation during experiments. A capillary syringe was used for introducing or removing aqueous solution (the volume was about 100 ll) to/from the O-ring.
Microscope
Diode laser Capillary Syringe Cover glass
Resin part particle ~ 400 μm) m) Glass beads Slide glass
O-ring
θ Epoxy glue
Position sensor
c
Immersed in 1 M NaCl solution for one day
Introducing and withdrawing water twice Introducing and withdrawing sample soon twice Introducing samples Monitoring of OBD signal
R-B + A
R-A + B Concentration or Temperature
b
CB
T CA
x
Distance from the particle surface
Fig. 1. Illustrations of the experimental setup (a), the experimental procedure for OBD measurement of the ion-exchange reaction between R–Na and Ca2+, and the concentration distribution (c) of ions in an ion exchange reaction occurring at a single particle of an ion exchanger.
X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
Commercial particles of ion-exchange resin (Amberlite IR-120 B and IRA-400) were Na-type cation ion exchangers (R–Na, with a particle size of about 400 lm), and Cl-type anion ion exchanges (R 0 -Cl, with a particle size of about 200 lm), respectively. The experimental procedure used for the ionexchange reaction between Ca2+ and R–Na is illustrated in Fig. 1b. First, 1 mol/L NaCl solution was added into the O-ring for 1 day before the start of the experiments so that the cation ion exchangers could completely become Na-type. Then, the NaCl solution in the O-ring was withdrawn, and water was added and withdrawn twice to wash away any remaining film of NaCl solution from the surface of the particle. Third, a sample solution of 1 mol/L CaCl2 was added and quickly withdrawn to wash away any water film on the surface of the particles (the length of time for which the sample solution remained in the reaction cell was kept to less than about 2 s, since an ion-exchange reaction also occurred in this washing process). Fourth, the sample solution was added to the O-ring, and the OBD signal was monitored. In the experiments for other ion-exchange reactions, the experimental procedures were similar to those described above. In experiments with the IR-120 B particle, all cation solutions were prepared from salt of chloride. In experiments with the IRA-400 particle, all anion solutions were prepared from salt of sodium. All chemicals were of analytical grade and from Wako Chemicals (Tokyo, Japan). The water used in the experiments was distilled deionized water. 3. Results and discussion As illustrated in Fig. 1c, when a solution of ion B was added to the vicinity of a resin particle in R–A form (R and A represent an ion exchanger and exchangeable ion in the exchanger, respectively), an ion exchange reaction between R–A and –B occurs at the interface of the particle/liquid. The exchanged ion A from the particle diffuses toward bulk solution from the interface, while ion B exchanged to the resin particle will diffuse to the inside of the particle. Meanwhile, ion B in the bulk solution will diffuse toward the interface. The concentration distributions of A and B illustrated in Fig. 1c are expected in the vicinity of the particle. That is, concentration gradients of A and B exist in the vicinity of the particle. Also, the reaction heat of the ion-exchange reaction will cause a temperature gradient. Both the concentration gradients
115
and temperature gradient will induce refractive index gradients, which in turn will deflect a probe beam passing through the vicinity of the particle. Deflection angle h of the probe beam induced by the concentration and temperature gradients is expressed as follows:
dn dx dnA dC A dnB dC B dn dT ; þ þ ¼k dT dx dC A dx dC B dx ð1Þ
h¼k
where k is a constant determined by the optical pathlength and refractive index n of the reaction medium; dn/dx is the refractive index gradient; dnA/dCA and dnB/dCB are concentration coefficients of n; dn/dT is the temperature coefficient of n; and dCA/dx, dCB/dx, and dT/dx are concentration gradients of A, B, and the temperature gradient, respectively. Since dn/dc (on the level of 102 M1)[17] is much larger than dn/dT (on the level of 104 K1 in aqueous solution) [31], the third term in Eq. (1) can be ignored except for special cases with large temperature change due to the reaction heat. Therefore, the deflection angle can be expressed as dn h¼k dx dnA dC A dnB dC B ¼k þ : ð2Þ dC A dx dC B dx In a certain concentration range, dnA/dCA and dnB/dCB could be approximated as a constant. Therefore, the deflection signal depends on dCA/dx and dCB/dx. Fig. 2 shows typical OBD signals for ionexchange reactions occurring at a particle of cation rechangers (IR–B). In order to prevent the possibility of liquid film remaining on the particle surface when replacing solution in the reaction cell, the particle was washed with water and sample solution as illustrated in Fig. 1b before the OBD signal was monitored. Fig. 2a shows the change of the OBD signal with time for the ion-exchange reaction between R–Na and Ca2+. It can be seen that the OBD signal decreased with time. On the other hand, the OBD signal increased with time for the reverse ion-exchange reaction between R–Ca and Na+ (Fig. 2b). In comparing Fig. 2a with Fig. 2b, it is clear that the change tendency of the OBD signal with time was opposite for the reverse cation
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OBD signa l (A.U.)
CaCl2 -
0
a2+
3
-
3
+
a
-0.25 -0.5 -0.75 -1 100
0
200
400
300
Time (sec)
NaCl -
3Ca
Na+
-
3Na
Ca2+
OBD signa l (A.U.)
0.75 0.5 0.25 0 -0.25
0
100
200
300
400
Time (sec) Fig. 2. Experimental results for the ion-exchange reaction occurring at a particle of a cation ion exchanger. In (a), 1 mol/L CaCl2 solution was added to the vicinity of the particle of an R–Na type ion exchanger. In (b), 1 mol/L NaCl solution was added to the vicinity of the particle of an R–Ca type ion exchanger.
ion-exchange reaction. Fig. 2 also shows that the OBD signal changed quickly at the beginning of the ion exchange reaction, and more slowly thereafter. This means that the concentration gradients near the particle/water interface also changed quickly at the beginning and then more slowly. The change of OBD signal was caused by the change of the concentration gradients dCNa/dx and dCCa/dx, which were related with the ion exchange reactions. In principle, both of these concentration gradients can be obtained from their diffusion equations and initial and boundary conditions (here one-dimensional diffusion was considered for purpose of simplicity). oC Na =ot ¼ DNaðsolutionÞ ðo2 C Na =ox2 Þ; oC Ca =ot ¼ DCaðsolutionÞ ðo2 C Ca =ox2 Þ:
ð3Þ
Here, DNa(solution) and DCa(solution) are diffusion coefficients of Na+ and Ca2+ in solution, respectively. The initial conditions for experiments in Fig. 2A are as follows: C Na ðx; tÞ ¼ 0 C Ca ðx; tÞ ¼ C 0Ca
at
t¼0
ð4Þ
where C 0Ca is the concentration of Ca2+ in sample solution (the sample solution was assumed to be homogeneously distributed in the solution before the ion-exchange reaction occurred). The boundary conditions were as follows: C Ca ðx; tÞ ¼ f ðr; DCaðresinÞ ; DCaðsolutionÞ Þ C Na ðx; tÞ ¼ f 0 ðr; DNaðresinÞ ; DNaðsolutionÞ Þ x ¼ 0 DCaðsolutionÞ dC Ca ðx; tÞ=dt ¼ 2 DNaðsolutionÞ dC Na ðx; tÞ=dt;
ð5Þ
where r, DCaðresinÞ , and DNaðresinÞ are the reaction rate of the ion-exchange reaction, and the diffusion coefficients of Na+ and Ca2+ in resin, respectively. The first two expressions in Eq. (5) represent that the concentrations of both Na+ and Ca2+ at the surface of the particle depends on r, DCaðresinÞ , and DNaðresinÞ . The third expression in Eq. (5) indicates that the exchanged amount of Ca2+ was stoichiometrically equal to that of Na+ per unit time. As illustrated in Fig. 1B, the direction of dCNa/dx was opposite to that of dCCa/dx in the ion-exchange reactions. For experiments in Fig. 2A, the deflection signal is expressed as follows:
X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
dn h¼k dx dC Ca dC Na dnCa dnNa ¼k dx dC dx ; dC Ca Na
117
Fig. 3 shows a typical OBD signal when the probe beam passed through the vicinity of a particle of anion exchangers (IRA-400). In comparing Fig. 3A and B, it is clear that the tendency of change in the OBD signal with time was also reversed for reverse anion exchanger reactions. Also, the change of the OBD signal with time is different from that in Fig. 2. This means that the change in the concentration gradients with time in the vicinity of the resin particles was different for the cation and anion ion exchangers. This reflects the difference both in the reaction rate and diffusion coefficients between the anion and cation ion-exchange reactions. If the OBD signal could be analyzed quantitatively, information on the reaction rate and diffusion coefficients could be obtained. In the study of ion-exchange reaction kinetics, the ion-exchange reaction is usually considered to be fast. Thus two potential rate-determining steps exist. The first one is the particle diffusion, i.e., diffusion of ions within the ion exchanger. The second one is the film diffusion, i.e., diffusion of ions in the adherent film (or diffusion layer) around the particles [32]. In the first case, the concentration profiles
ð6Þ where jdCCa/dxj and jdCCa/dxj are magnitudes (absolute values) of dCCa/dx and dCNa/dx, respectively. On the other hand, the direction of both dCNa/dx and dCCa/dx in the experiments in Fig. 2b were the opposite of those in Fig. 2a. Therefore, the deflection signal in Fig. 2b was expressed as dn h¼k dx dC Ca dC Na dnCa dnNa ¼k dx þ dC dx : dC Ca Na ð7Þ Eqs. (6) and (7) theoretically show that the tendency of change in the deflection signal is the reverse for reverse ion-exchange reactions. This was experimentally confirmed as shown in Fig. 2a and b.
NaCl
OBD signa l (A.U.)
-
3 3
4
+
Cl-
-
3 3Cl
24
0.7 0.4 0.1 -0.2 0
300
200
100
400
Time (s)
b
Na2SO4
OBD signa l (A.U.)
0.2
-
3 3
4
2-
-
3 3
4
-
-0.1
-0.4
-0.7
0
100
200
300
400
Time (s) Fig. 3. Experimental results for the anion ion-exchange reaction occurring at a single particle of an anion ion exchanger. In (a), 1 mol/L NaCl solution was added to the vicinity of the particle of an R 0 –SO4 type ion exchanger. In (b), 1 mol/L Na2SO4 solution was added to the vicinity of the particle of an R 0 –Cl type ion exchanger.
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X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
or gradients of ions in the solution do not change with time, while the concentration profiles or gradients of ions inside the particles do change [32]. If the rate-determining step in the present experiments was the particle diffusion, concentration gradients of ions would be constant in the vicinity of the particle and thus no change in OBD signal could be detected. However, Figs. 2 and 3 show changes of the OBD signal with time. Therefore, the rate-determining step was not the particle diffusion. On the other hand, in the case of film diffusion the concentration gradients of ions in the diffusion layer change with time until the ion-exchange reaction reaches equilibrium. Since the thickness of the adherent film is on the order of 10–100 lm [32], the probe beam was passing through the adherent film or diffusion layer in these experiments. The results in Figs. 2 and 3 indicate that the concentration gradients of ions in the diffusion layer changed with time during the ion-exchange reaction. Therefore, the rate-determining step was the film diffusion for both the cation and anion ion-exchange reactions. Fig. 4 shows the OBD results for a glass bead when CaCl2 and NaCl solutions were added. No change in OBD signals was observed after addition of either CaCl2 or NaCl solution. Because no chemical reaction occurred on the surface of the glass
bead, no concentration gradients existed around the glass bead. Therefore, no change in OBD signals was observed. 4. Conclusion In conclusion, the OBD method was here shown to be applicable for the study of chemical reactions occurring at a single particle. This method can be used for non-invasive in-situ monitoring of the change in concentration gradient around a particle. Because it requires only a probe beam and detection of the concentration gradient, the method is very simple and can be applied to any chemical reaction, unlike other optical methods such as the absorbance and fluorescence methods where spectroscopic change is required. Information on the dependence of concentration gradients on the distance from the particle surface could be further obtained by adding a micro-stage to the microscope. By using the dependence of concentration gradients on the distance and a quantitative model to describe the film diffusion, it would be possible to obtain the spatial dynamic change of the concentration profile in the vicinity of the particle, as well as the fluxes of ions during a chemical reaction. This work is being done and will be reported later.
CaCl2 OBD signal (A.U.)
0.45 0.15 -0.15 -0.45
0
100
200
300
400
Time (sec)
OBD signal (A.U.)
NaCl 0.45 0.15 -0.15 -0.45 0
100
200 Time (sec)
300
400
Fig. 4. Experimental results for a glass bead. In (a) and (b), 1 mol/L CaCl2 or NaCl solution was added to the vicinity of a glass bead, respectively.
X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
References [1] D.S. Gross, M.E. Galli, P.J. Silva, K.A. Prather, Anal. Chem. 72 (2000) 416–422. [2] E. Woods III, G.D. Smith, Y. Dessiaterik, T. Baer, R.E. Miller, Anal. Chem. 73 (2001) 2317–2322. [3] G.D. Smith, E. Woods III, C.L. DeForest, T. Baer, R.E. Miller, J. Phys. Chem. A. 106 (2002) 8085–8095. [4] Y. Cai, W.-P. Peng, S.-J. Kuo, Y.T. Lee, H.-C. Chang, Anal. Chem. 74 (2002) 232–238. [5] T. Dobashi, T. Furukawa, T. Narita, S. Shimofure, K. Ichikawa, B. Chu, Langmuir 17 (2001) 4525–4528. [6] A. Elaissari, Y. Chevalier, F. Ganachaud, T. Delair, C. Pichot, Langmuir 16 (2000) 1261–1269. [7] T. Dobashi, T. Furukawa, T. Narita, S. Shimofure, K. Ichikawa, B. Chu, Langmuir 17 (2001) 4525–4528. [8] M.P. Houlne, C.M. Sjostrom, R.H. Uibel, J.A. Kleimeyer, J.M. Harris, Anal. Chem. 74 (2002) 4311–4319. [9] H.-B. Kim, M. Hayashi, K. Nakatani, N. Kitamura, Anal. Chem. 68 (1996) 409–414. [10] H.-B. Kim, S. Yoshida, N. Kitamura, Anal. Chem. 70 (1998) 51–57. [11] H.-B. Kim, S. Yoshida, M. Miura, N. Kitamura, Anal. Chem. 70 (1998) 105–110. [12] H.-B. Kim, S. Yoshida, M. Miura, N. Kitamura, Anal. Chem 70 (1998) 111–116. [13] A.C. Boccara, D. Fournier, J. Badoz, Appl. Phys. Lett. 36 (1980) 130–132. [14] J. Wu, T. Kitamori, T. Sawada, Appl. Phys. Lett. 57 (1990) 22–24. [15] J. Pawliszyn, M.F. Weber, M.J. Dignam, A. Mandelis, R.D. Venter, S.-M. Park, Anal. Chem. 58 (1986) 236–239.
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[16] J. Pawliszyn, M.F. Weber, M.J. Dignam, A. Mandelis, R.D. Venter, S.-M. Park, Anal. Chem. 58 (1986) 239–242. [17] J. Pawliszyn, Spectrochimica Acta Rev. 13 (1990) 311– 354. [18] R.E. Russo, F.R. McLarnon, J.D. Spear, E.J. Cairns, J. Electrochem. Soc. 134 (1987) 2783–2787. [19] C. Barbero, M.C. Miras, R. Kotz, Electrochim. Acta. 37 (1992) 429–437. [20] J. Pawlizyn, Anal. Chem. 60 (1986) 1751–1758. [21] D.O. Hancock, R.E. Synovec, Anal. Chem. 60 (1988) 2812– 2817. [22] C.D. Costin, R.E. Synovec, Anal. Chem. 74 (2002) 4558– 4565. [23] X.-Z. Wu, H. Shindoh, M. Yamada, T. Hobo, Anal. Chem. 65 (1993) 834–836. [24] X.-Z. Wu, H. Shindoh, M. Yamada, E. Kobayashi, T. Hobo, Anal. Sci. 10 (1994) 203–205. [25] X.-Z. Wu, H. Shindoh, T. Hobo, Microchem. J. 49 (1994) 213–219. [26] X.-Z. Wu, H. Shindoh, T. Hobo, Anal. Chim. Acta. 299 (1995) 333–336. [27] X.-Z. Wu, H. Shindoh, T. Hobo, Anal. Chim. Acta. 316 (1995) 111–115. [28] X.-Z. Wu, Bunseki Kagaku. 45 (1996) 55–64. [29] X.-Z. Wu, M. Yamaguchi, Y. Nagaosa, Bunseki Kagaku. 47 (1998) 381–385. [30] X.-Z. Wu, T. Morikawa, K. Uchiyama, T. Hobo, J. Phys. Chem. B. 101 (1997) 1520–1523. [31] L. Lorrain, M. Havaux, A. Tessier, R. Leblanc, Photochemi. Photobiol. 51 (1990) 491–495. [32] F. Helfferich (Ed.), Ion Exchange, McGraw-Hill Book Company, Inc., New York, 1962.
Reactive & Functional Polymers 67 (2007) 113–119
www.elsevier.com/locate/react
Optical beam deflection approach for studying ion-exchange reactions occurring at a single ion-exchange resin particle Xing-Zheng Wu a
a,*
, Yumiko Tsuji a, Norio Teramae
b
Department of Materials Science and Engineering, Faculty of Engineering, Fukui University, Bunkyo 3-9-1, Fukui 910-8507, Japan b Department of Chemistry, Graduate School of Science, Tohoku University, Japan Received 13 December 2005; received in revised form 26 July 2006; accepted 19 October 2006 Available online 27 November 2006
Abstract The optical beam deflection method was applied for exploring chemical reactions occurring at a single particle. A probe beam from a diode laser was introduced into a microscope, and focused to the vicinity of a particle. When a chemical reaction occurs at the surface of the particle, concentration gradients exist in the vicinity of the particle due to the diffusion of chemical species involved in the reaction. Ion exchange reactions occurring at a single particle of ion exchangers were used as model systems. The results showed that the tendency of change in the OBD signal with time is opposite of that seen in the reverse ion exchange reactions, both theoretically and experimentally. Also, the experimental results indicated that the concentration gradients changed with time in the diffusion layer around the particles, implying that the actual reaction-rate determining step was not the particle diffusion but film diffusion. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Optical beam deflection; Single particle; Ion exchanger; Reaction; Microscope
1. Introduction Characterization of a single particle is becoming more and more important in many fields, such as in studies on aerosols [1–3] and polymerization kinetics [4,5]. Several methods, including single particle-mass spectroscopy [1–4], small-angle neutron scattering [6], and light scattering [7] techniques, *
Corresponding author. Tel.: +81 776 27 8610; fax: +81 776 27 8767. E-mail address: [email protected] (X.-Z. Wu).
have been used for characterization of a single particle. Confocal raman microscopy has been applied for monitoring chemical reactions on single optically trapped, solid-phase support particles [8]. Laser trapping microspectroscopy and confocal fluorescence microspectroscopy have been used for in situ measurements of ion-exchange processes in a single polymer particle [9–12]. The optical beam deflection method, which is based on probing a refractive index gradient (mirage effects), [13] has been used extensively in photothermal spectroscopy, where a temperature gradient is
1381-5148/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.10.002
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X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
generated by nonradiative relaxation of the photoenergy adsorbed from a powerful excitation light source [13,14]. On the other hand, the OBD method has also been applied for exploring a concentration gradient [15–19] in chemical and electrochemical systems. The OBD method also can be used for exploring a concentration gradient in flow systems [20–22]. Recently, we have explored chemical reaction heat-induced OBD and concentration gradient-induced OBD in chemical reactions in aqueous solution [23–30]. In principle, any chemical reaction occurring at a single particle in aqueous solution will generate concentration changes or gradients of chemical species, and a reaction heat-induced temperature gradient around the particle. Therefore, the OBD method should be applicable to the study of chemical reactions occurring at a single particle. Here, we explore the possibility of applying the chemical reaction-induced OBD for studying chemical reactions occurring at a single particle. Ionexchange reactions occurring at a single particle of ion exchangers were used as model systems.
a
2. Experimental As shown in Fig. 1a, a probe beam from a diode laser (3 mW, 670 nm) was introduced into a microscope. The probe beam was reflected inside the microscope, and then focused (spot size, 5 lm) to the vicinity of a particle of ion-exchange resin (Amberlite IR-120 B, IRA-400) or a glass bead by an objection lens (magnification 4). The focused beam passed just beside the particle surface (a very small portion of the focused beam directly stroked the surface of the particle). A part of the resin particle and of the glass bead was glued onto a slide glass with epoxy glue, and an O-ring (diameter 1 cm) was also fixed on the slide glass as a reaction cell (Fig. 1a). A position sensor (Hamamatsu) was used for detection of the deflection of the probe beam. Most of the O-ring was covered by a cover glass for preventing water evaporation during experiments. A capillary syringe was used for introducing or removing aqueous solution (the volume was about 100 ll) to/from the O-ring.
Microscope
Diode laser Capillary Syringe Cover glass
Resin part particle ~ 400 μm) m) Glass beads Slide glass
O-ring
θ Epoxy glue
Position sensor
c
Immersed in 1 M NaCl solution for one day
Introducing and withdrawing water twice Introducing and withdrawing sample soon twice Introducing samples Monitoring of OBD signal
R-B + A
R-A + B Concentration or Temperature
b
CB
T CA
x
Distance from the particle surface
Fig. 1. Illustrations of the experimental setup (a), the experimental procedure for OBD measurement of the ion-exchange reaction between R–Na and Ca2+, and the concentration distribution (c) of ions in an ion exchange reaction occurring at a single particle of an ion exchanger.
X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
Commercial particles of ion-exchange resin (Amberlite IR-120 B and IRA-400) were Na-type cation ion exchangers (R–Na, with a particle size of about 400 lm), and Cl-type anion ion exchanges (R 0 -Cl, with a particle size of about 200 lm), respectively. The experimental procedure used for the ionexchange reaction between Ca2+ and R–Na is illustrated in Fig. 1b. First, 1 mol/L NaCl solution was added into the O-ring for 1 day before the start of the experiments so that the cation ion exchangers could completely become Na-type. Then, the NaCl solution in the O-ring was withdrawn, and water was added and withdrawn twice to wash away any remaining film of NaCl solution from the surface of the particle. Third, a sample solution of 1 mol/L CaCl2 was added and quickly withdrawn to wash away any water film on the surface of the particles (the length of time for which the sample solution remained in the reaction cell was kept to less than about 2 s, since an ion-exchange reaction also occurred in this washing process). Fourth, the sample solution was added to the O-ring, and the OBD signal was monitored. In the experiments for other ion-exchange reactions, the experimental procedures were similar to those described above. In experiments with the IR-120 B particle, all cation solutions were prepared from salt of chloride. In experiments with the IRA-400 particle, all anion solutions were prepared from salt of sodium. All chemicals were of analytical grade and from Wako Chemicals (Tokyo, Japan). The water used in the experiments was distilled deionized water. 3. Results and discussion As illustrated in Fig. 1c, when a solution of ion B was added to the vicinity of a resin particle in R–A form (R and A represent an ion exchanger and exchangeable ion in the exchanger, respectively), an ion exchange reaction between R–A and –B occurs at the interface of the particle/liquid. The exchanged ion A from the particle diffuses toward bulk solution from the interface, while ion B exchanged to the resin particle will diffuse to the inside of the particle. Meanwhile, ion B in the bulk solution will diffuse toward the interface. The concentration distributions of A and B illustrated in Fig. 1c are expected in the vicinity of the particle. That is, concentration gradients of A and B exist in the vicinity of the particle. Also, the reaction heat of the ion-exchange reaction will cause a temperature gradient. Both the concentration gradients
115
and temperature gradient will induce refractive index gradients, which in turn will deflect a probe beam passing through the vicinity of the particle. Deflection angle h of the probe beam induced by the concentration and temperature gradients is expressed as follows:
dn dx dnA dC A dnB dC B dn dT ; þ þ ¼k dT dx dC A dx dC B dx ð1Þ
h¼k
where k is a constant determined by the optical pathlength and refractive index n of the reaction medium; dn/dx is the refractive index gradient; dnA/dCA and dnB/dCB are concentration coefficients of n; dn/dT is the temperature coefficient of n; and dCA/dx, dCB/dx, and dT/dx are concentration gradients of A, B, and the temperature gradient, respectively. Since dn/dc (on the level of 102 M1)[17] is much larger than dn/dT (on the level of 104 K1 in aqueous solution) [31], the third term in Eq. (1) can be ignored except for special cases with large temperature change due to the reaction heat. Therefore, the deflection angle can be expressed as dn h¼k dx dnA dC A dnB dC B ¼k þ : ð2Þ dC A dx dC B dx In a certain concentration range, dnA/dCA and dnB/dCB could be approximated as a constant. Therefore, the deflection signal depends on dCA/dx and dCB/dx. Fig. 2 shows typical OBD signals for ionexchange reactions occurring at a particle of cation rechangers (IR–B). In order to prevent the possibility of liquid film remaining on the particle surface when replacing solution in the reaction cell, the particle was washed with water and sample solution as illustrated in Fig. 1b before the OBD signal was monitored. Fig. 2a shows the change of the OBD signal with time for the ion-exchange reaction between R–Na and Ca2+. It can be seen that the OBD signal decreased with time. On the other hand, the OBD signal increased with time for the reverse ion-exchange reaction between R–Ca and Na+ (Fig. 2b). In comparing Fig. 2a with Fig. 2b, it is clear that the change tendency of the OBD signal with time was opposite for the reverse cation
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OBD signa l (A.U.)
CaCl2 -
0
a2+
3
-
3
+
a
-0.25 -0.5 -0.75 -1 100
0
200
400
300
Time (sec)
NaCl -
3Ca
Na+
-
3Na
Ca2+
OBD signa l (A.U.)
0.75 0.5 0.25 0 -0.25
0
100
200
300
400
Time (sec) Fig. 2. Experimental results for the ion-exchange reaction occurring at a particle of a cation ion exchanger. In (a), 1 mol/L CaCl2 solution was added to the vicinity of the particle of an R–Na type ion exchanger. In (b), 1 mol/L NaCl solution was added to the vicinity of the particle of an R–Ca type ion exchanger.
ion-exchange reaction. Fig. 2 also shows that the OBD signal changed quickly at the beginning of the ion exchange reaction, and more slowly thereafter. This means that the concentration gradients near the particle/water interface also changed quickly at the beginning and then more slowly. The change of OBD signal was caused by the change of the concentration gradients dCNa/dx and dCCa/dx, which were related with the ion exchange reactions. In principle, both of these concentration gradients can be obtained from their diffusion equations and initial and boundary conditions (here one-dimensional diffusion was considered for purpose of simplicity). oC Na =ot ¼ DNaðsolutionÞ ðo2 C Na =ox2 Þ; oC Ca =ot ¼ DCaðsolutionÞ ðo2 C Ca =ox2 Þ:
ð3Þ
Here, DNa(solution) and DCa(solution) are diffusion coefficients of Na+ and Ca2+ in solution, respectively. The initial conditions for experiments in Fig. 2A are as follows: C Na ðx; tÞ ¼ 0 C Ca ðx; tÞ ¼ C 0Ca
at
t¼0
ð4Þ
where C 0Ca is the concentration of Ca2+ in sample solution (the sample solution was assumed to be homogeneously distributed in the solution before the ion-exchange reaction occurred). The boundary conditions were as follows: C Ca ðx; tÞ ¼ f ðr; DCaðresinÞ ; DCaðsolutionÞ Þ C Na ðx; tÞ ¼ f 0 ðr; DNaðresinÞ ; DNaðsolutionÞ Þ x ¼ 0 DCaðsolutionÞ dC Ca ðx; tÞ=dt ¼ 2 DNaðsolutionÞ dC Na ðx; tÞ=dt;
ð5Þ
where r, DCaðresinÞ , and DNaðresinÞ are the reaction rate of the ion-exchange reaction, and the diffusion coefficients of Na+ and Ca2+ in resin, respectively. The first two expressions in Eq. (5) represent that the concentrations of both Na+ and Ca2+ at the surface of the particle depends on r, DCaðresinÞ , and DNaðresinÞ . The third expression in Eq. (5) indicates that the exchanged amount of Ca2+ was stoichiometrically equal to that of Na+ per unit time. As illustrated in Fig. 1B, the direction of dCNa/dx was opposite to that of dCCa/dx in the ion-exchange reactions. For experiments in Fig. 2A, the deflection signal is expressed as follows:
X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
dn h¼k dx dC Ca dC Na dnCa dnNa ¼k dx dC dx ; dC Ca Na
117
Fig. 3 shows a typical OBD signal when the probe beam passed through the vicinity of a particle of anion exchangers (IRA-400). In comparing Fig. 3A and B, it is clear that the tendency of change in the OBD signal with time was also reversed for reverse anion exchanger reactions. Also, the change of the OBD signal with time is different from that in Fig. 2. This means that the change in the concentration gradients with time in the vicinity of the resin particles was different for the cation and anion ion exchangers. This reflects the difference both in the reaction rate and diffusion coefficients between the anion and cation ion-exchange reactions. If the OBD signal could be analyzed quantitatively, information on the reaction rate and diffusion coefficients could be obtained. In the study of ion-exchange reaction kinetics, the ion-exchange reaction is usually considered to be fast. Thus two potential rate-determining steps exist. The first one is the particle diffusion, i.e., diffusion of ions within the ion exchanger. The second one is the film diffusion, i.e., diffusion of ions in the adherent film (or diffusion layer) around the particles [32]. In the first case, the concentration profiles
ð6Þ where jdCCa/dxj and jdCCa/dxj are magnitudes (absolute values) of dCCa/dx and dCNa/dx, respectively. On the other hand, the direction of both dCNa/dx and dCCa/dx in the experiments in Fig. 2b were the opposite of those in Fig. 2a. Therefore, the deflection signal in Fig. 2b was expressed as dn h¼k dx dC Ca dC Na dnCa dnNa ¼k dx þ dC dx : dC Ca Na ð7Þ Eqs. (6) and (7) theoretically show that the tendency of change in the deflection signal is the reverse for reverse ion-exchange reactions. This was experimentally confirmed as shown in Fig. 2a and b.
NaCl
OBD signa l (A.U.)
-
3 3
4
+
Cl-
-
3 3Cl
24
0.7 0.4 0.1 -0.2 0
300
200
100
400
Time (s)
b
Na2SO4
OBD signa l (A.U.)
0.2
-
3 3
4
2-
-
3 3
4
-
-0.1
-0.4
-0.7
0
100
200
300
400
Time (s) Fig. 3. Experimental results for the anion ion-exchange reaction occurring at a single particle of an anion ion exchanger. In (a), 1 mol/L NaCl solution was added to the vicinity of the particle of an R 0 –SO4 type ion exchanger. In (b), 1 mol/L Na2SO4 solution was added to the vicinity of the particle of an R 0 –Cl type ion exchanger.
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or gradients of ions in the solution do not change with time, while the concentration profiles or gradients of ions inside the particles do change [32]. If the rate-determining step in the present experiments was the particle diffusion, concentration gradients of ions would be constant in the vicinity of the particle and thus no change in OBD signal could be detected. However, Figs. 2 and 3 show changes of the OBD signal with time. Therefore, the rate-determining step was not the particle diffusion. On the other hand, in the case of film diffusion the concentration gradients of ions in the diffusion layer change with time until the ion-exchange reaction reaches equilibrium. Since the thickness of the adherent film is on the order of 10–100 lm [32], the probe beam was passing through the adherent film or diffusion layer in these experiments. The results in Figs. 2 and 3 indicate that the concentration gradients of ions in the diffusion layer changed with time during the ion-exchange reaction. Therefore, the rate-determining step was the film diffusion for both the cation and anion ion-exchange reactions. Fig. 4 shows the OBD results for a glass bead when CaCl2 and NaCl solutions were added. No change in OBD signals was observed after addition of either CaCl2 or NaCl solution. Because no chemical reaction occurred on the surface of the glass
bead, no concentration gradients existed around the glass bead. Therefore, no change in OBD signals was observed. 4. Conclusion In conclusion, the OBD method was here shown to be applicable for the study of chemical reactions occurring at a single particle. This method can be used for non-invasive in-situ monitoring of the change in concentration gradient around a particle. Because it requires only a probe beam and detection of the concentration gradient, the method is very simple and can be applied to any chemical reaction, unlike other optical methods such as the absorbance and fluorescence methods where spectroscopic change is required. Information on the dependence of concentration gradients on the distance from the particle surface could be further obtained by adding a micro-stage to the microscope. By using the dependence of concentration gradients on the distance and a quantitative model to describe the film diffusion, it would be possible to obtain the spatial dynamic change of the concentration profile in the vicinity of the particle, as well as the fluxes of ions during a chemical reaction. This work is being done and will be reported later.
CaCl2 OBD signal (A.U.)
0.45 0.15 -0.15 -0.45
0
100
200
300
400
Time (sec)
OBD signal (A.U.)
NaCl 0.45 0.15 -0.15 -0.45 0
100
200 Time (sec)
300
400
Fig. 4. Experimental results for a glass bead. In (a) and (b), 1 mol/L CaCl2 or NaCl solution was added to the vicinity of a glass bead, respectively.
X.-Z. Wu et al. / Reactive & Functional Polymers 67 (2007) 113–119
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