J. Phys. Chem. 1994, 98, 10550-10553
10550
Light-Induced State Transitions in the Oscillatory ClO~--C1--Iodomalonic Acid System in a Semibatch Reactor Gyula Rabai' and Ichiro Hanazakil Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Received: May 9, 1994; In Final Form: July 26, 1994@
The dynamical behavior of the oscillatory ClOz--Cl--iodomalonic acid reaction in a semibatch reactor has been found to be extremely sensitive to the UV light, while the light above 420 nm does not affect the kinetics. Continuous illumination increases the amplitude and the period. Oscillations are completely extinguished beyond a critical light power. Light-induced transitions from nonoscillatory to oscillatory state and bursting-like responses upon periodic perturbations with 370 nm light have been found. The lightaccelerated formation of a reactive iodine species from iodomalonic acid and its subsequent reaction with the chlorite ion is responsible for the light sensitivity of the dynamical behavior.
Introduction Photoresponses of chemical oscillators have been of interest as they supply important insights for understanding the mechanism.2 Furthermore, the light can be used for controlling nonlinear dynamical behavior in homogeneous solutions and for perturbing spatial structures in reaction-diffusion systems. The use of light in nonlinear chemical dynamics is a major advance because of the feasibility in varying intensity and wavelength. Another interesting point is that photosensitive chemical oscillators can provide model reactions for less tractable behavior of more complex biological systems submitted to extemal ~ t i m u l i .It~ is, however, a drawback that very few oscillators are known to be light-sensitive, and their photoresponses are usually small. Discoveries of new systems with large and quick photoresponses are, therefore, of obvious importance. A recent experimental study on the mechanism of the chlorite-iodide-malonic acid system4' has led to the discovery of a simpler oscillatory reaction between the acidic C102- and iodomalonic acid (IMA) in the presence of C1- in a semibatch reactor.6 Here diluted C102- solution is introduced at a controlled flow rate into an acidic aqueous solution of IMA containing C1-. Oscillations in redox potential are measured in the solution. In this letter, we report that this new mode of oscillations is very sensitive to light. First, the dynamical behavior in darkness is discussed. The following section contains the results of illumination experiments. We have investigated the effect of both continuous illumination and single pulse perturbations with light band with a maximum at 370 nm. In the final section, a brief discussion on the mechanism is given.
Experimental Section Materials. Iodide- and iodine-free stock solutions of 0.060 M IMA in 0.10 M H2S04 were ~ r e p a r e dand ~ , ~kept from light in a refrigerator. Other chemicals were the highest purity commercially available and were used as received from Katayama Chemical Ltd. The concentration of NaCl02 stock solution was determined by iodometric titration. Aliquots of the stock solution were added to the excess iodide solutions containing 0.025 M sulfuric acid. The iodine formed in this way was then titrated with Na2S203 solution. @
Abstract published in Advance ACS Abstracts, September 1, 1994.
0022-365419412098-10550$04.5010
Semibatch Reactor. A cylindrical-shaped, double-jacketed, black plastic vessel with a volume of 500 mL (8 cm i.d., 10 cm height) was used as a semibatch reactor. It was equipped with a 3 x 3 cm2 quartz window for irradiation. Some control experiments were performed in darkness with a all-glass vessel. The same results were obtained as in the plastic reactor. The light source was a 500 W xenon lamp (USHIO UI-501C). Diameter of the light beam and the optical path length in the reactor were 2.5 and 8 cm, respectively. Bandpass filters and neutral density filters were used to obtain light bands with desired power. The light power was measured with a calibrated photodiode at the incident window of the reactor. The integrated light power entering the reactor was divided by the volume of the irradiated solution and given in units of pWImL,. Procedure. Usually 300 mL of IMA solution containing sulfuric acid, sodium hydrogen sulfate, and sodium chloride was placed in the reactor at the start, and sodium chlorite solution was flowed in by a peristaltic pump (EYELA MP3 type) at measured rates through one inlet tube (i.d. 1.0 mm). The input point of sodium chlorite was close to the bottom of the reactor. The mixture was rigorously stirred with a 6 cm long magnetic stirrer bar. The redox potential was measured with a wire-type Pt-AgIAgC1 combination electrode (Horiba 6861-1Oc). The electrode was immersed in the solution so that the light beam passing through the reactor was not obstructed. All measurements were conducted at 20.0 & 0.2 "C. Results Dynamical Behavior in Darkness. When, in a semibatch configuration, constant input flow of diluted (10-4-10-3 M) NaC102 solution into the acidic (pH 2) IMA solution ( M) M) was maintained, long-lived containing NaCl ( lop3oscillations in redox potential were measured. Oscillations can last for many hours or even days. Note that in a semibatch reactor, oscillatory behavior may be a long-lived response, but it is necessarily a transient phenomenon leading to the thermodynamic state eventually. Therefore, in contrast to the continuous-flow stirred tank reactor (CSTR), the periodic behavior is always accompanied by monotonous changes and the elapsed time affects the state of the system. All the results presented here were obtained in the first 2 h of the experiments. To characterize the state of the system as a function of experimental constraints in a semibatch reactor, we have constructed phase diagrams that are commonly used in char0 1994 American Chemical Society
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J. Phys. Chem., Vol. 98, No. 41, 1994 10551
Light-Induced State Transitions 1
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rate (mL/min) Figure 1. Phase diagram measured in a semibatch reactor in darkness: [MA] vs in-flow rate: [HzS04] = 0.0050 M, [NaHSO4] = 0.015 M, [NaCl] = 0.0033 M, [NaC102],,,,, = 0.0010 M. Initial volume in the batch is 300 mL. inflow
acterizing CSTR systems. A phase diagram obtained in the dark is shown in Figure 1. Since, to our knowledge, this kind of phase diagram has not yet been applied for characterizing dynamical behavior of nonlinear chemical systems in a semibatch configuration, it seems to be desirable to describe it in details. Here we have fixed the following parameters: T = 20 k 0.2 OC, [H2S04] = 0.0050 M, [NaHS04] = 0.015 M, [NaCl] = 0.0033 M, [NaClO2]+,, = 0.0010 M and determined the state of the system at various values of [IMA] and input flow rates (VO) at the end of the first hour of the experiments. Oscillations in the Pt potential between 570 mV and 740 mV were measured when the flow rate was low and [ M A ] > 4.5 x M. The amplitude does not depend significantly on the in-flow rate, but the frequency does. The higher the flow rate within the region of oscillations, the higher the frequency. Stable states characterized with about 700 mV potential value were found at high vo values. At intermediate inflow rates, the system exhibits bistability between oscillatory and steady states. Starting the experiment with these intermediate flow rates, the system can reach either oscillatory or steady states: when the flow rate was close to the boundary of the oscillatory region, usually the oscillatory state was reached, in the other part of the region the steady state was favored. However, the reproducibility of whether the oscillatory or steady state was actually reached was rather poor in the bistable region. A gradual decrease in flow rate from the steady-state region always drives the system from steady to oscillatory state only when the boundary of oscillatory and bistable regions is crossed. On the other hand, the boundary of bistable and steady-state regions should be crossed in order for the system to switch from oscillatory to steady states. This chemical hysteresis is very well reproducible and served as the basis of constructing the phase diagram shown in Figure 1. In the bistable region, a temporary increase of in-flow rate forces the oscillatory system to a stable state, while a transition from nonoscillatory to oscillatory behavior can be induced either by decreasing the flow rate temporarily or by a superthreshold temporary illumination. Effect of Light. A light band with a peak at 370 nm was used because visible light (A =- 420 nm) does not affect the kinetics, and UV light with a wavelength shorter than about 330 nm initiates the undesirable decomposition of IMA. The transmittance spectrum of the bandpass filter used in the
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nm Figure 2. (a) Transmittance spectrum of the bandpass filter used in the perturbation experiment and (b) absorption spectrum of the reaction mixture in a 1.0 cm cell: [ M A ] = 0.0011 M, [H2S04] = 0.0050 M, [NaHSO4] = 0.015 M, and [NaCl] = 0.0033 M.
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Figure 3. Effect of continuous illumination on oscillations in a semibatch reactor. A solution of 5.0 x low4M NaClOz is flowed at 0.117 " i n into 300 mL of a solution containing 6.0 x M MA, 0.0025 M HzS04, 0.0175 M NaHS04, and 0.005 M NaCl. Oscillations (a) in darkness, and (b) under illumination with PO = 2.7 pW/mL. Oscillations are inhibited with PO = 5.3 pW/mL (c). perturbation experiments along with the absorbance spectrum of IMA solution are shown in Figure 2. Under continuous illumination the amplitude is higher, and the period is longer than in darkness. Oscillations can completely be suppressed with light power (PO)above a critical value (Poc,Figure 3). The relative cross section of the inhibition of oscillations (a~)'qualitatively follows the absorption spectrum of IMA indicating that IMA is the primary light absorber. More interestingly, we found that a short single light pulse can induce transitions from nonoscillatory to oscillatory state in the bistable region shown in Figure 1. The threshold of exposure appears to be very small: 10-s illumination with light power 5.3 & 0.5 yW/mL forced the bistable system from nonoscillatory to oscillatory state under the experimental conditions shown in Figure 1 (Figure 4a). When the experimental conditions support only steady state in darkness, the system responds with transient oscillations to a single pulse perturbation if POexceeds a threshold value. The most interesting phenomenon was observed when the system was kept in the stable-state region close to the boundary of the bistable region and was perturbed repeatedly with light pulses. As shown in Figure 4b, transient oscillatory responses were initiated by each 1-min stimulation with PO = 5.3 yW/ mL light power. These bursting-like responses are characterized
RSlbai and Hanazaki
10552 J. Phys. Chem., Vol. 98, No. 41, 1994
by a continuous I2 input into the oscillatory system. This finding underlines that the role of IMA may be more complex than a simple source of 12.
Discussion
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Figure 4. (a) Light-induced transition from nonoscillatory to oscillatory state. A 10 s illumination with PO = 5.3 pW/mL was applied at the arrow. (b) Bursting-like photoresponses. The arrows indicate 1 min illuminations with PO = 5.3 pW. [HzSO~]= 0.0050 M, [NaHS04] = 0.015 M, [NaCl] = 0.0033 M, [MA] = 5.0 x M, [NaClOz]hpUt = 0.0010 M, in-flow rates (a) 0.12 mL/min and (b) 0.16 " i n . Dashed lines show the trace of the reaction without perturbation.
by trains of large-amplitude oscillations separated by nonoscillatory periods. This phenomenon is well-known in biological excitable media,* and it can also be found in chemical reaction^^,'^ mostly when an excitable steady state in a flow reactor is stimulated by superthreshold injection of a reagent However, we are not aware of any previously reported photoinduced bursting in chemical reactions. While the reproducibility of this bursting-like phenomenon is good, there is some uncertainty in the number of peaks obtained in each burst. For example, repeated experiments of Figure 4b resulted in bursts consisting of either two or four oscillations in addition to the three-peak bursts shown in Figure 4b. In general, more oscillations could be obtained with the same exposure in the early than in the later stage of the semibatch experiments which underlines the significant contribution of monotonous change (consumption of IMA) to the dynamical behavior. The number of peaks increases up to a limit with increasing light exposure. Effect of Iodine. According to an earlier hypothesis: the key component reaction in the mechanism responsible for the oscillations involves a reactive species (I2 or ICl) formed from IMA and C1-. The decomposition of IMA is known to be accelerated by light.l43 l 5 In conection with this, a referee has raised an interesting question whether the addition of pure iodine has the same effect on the dynamical behavior as light. To answer this question, we repeated the experiments shown in Figure 4 by, instead of illuminating, injecting dilute aqueous iodine solution to the nonoscillatory reaction mixture in darkness. After optimizing the amount of iodine, we were able to observe the same responses as in the case of the light perturbations. We found that sustained oscillations started upon injecting 0.2 mL of 1 x M 12 to 300 mL of nonoscillatory reaction mixture (composition is given in Figure 4) when the in-flow rate of 0.001 M NaClOz was 0.12 mL/min. At 0.16 mL/min in-flow rate, addition of 0.5 mL of iodine solution initiated the same transient oscillations as the light perturbation did. From this result it was logical to assume that simultaneous inflows of diluted I2 and C102- solutions into a semibatch reactor containing some sulfuric acid and water might induce oscillatory reaction even in the absence of IMA. Despite our best efforts, however, we could not observe any oscillations using many different concentration combinations and flow rates in the range 1 x 10-5-1 x M and 0.1-1.0 mL/min, respectively. It seems to be likely that IMA cannot be replaced
We showed that a continuous slow inflow of diluted ClOzsolution into an iodomalonic acid solution initiated an oscillatory reaction. It should be emphasized that IMA is always in a large excess over C102- in the semibatch configuration. Our experiments indicate that iodine plays an important role in the oscillations. The obvious source of iodine appears to be the decomposition of iodomalonic acid. The extreme light sensitivity of the dynamical behavior suggests that some radical involvement is possible in this composite process. I' and malonyl radicals can be formed in the photocatalytic reaction (1). The radicals can initiate a reaction chain leading to the ICH(COOH), ihv
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I'
+ 'CH(COOH),
(1)
formation of 12, or they can react directly with ClOz-. An autocatalytic redox reaction between the iodine species and the incoming C102- provides the feedback that leads to the oscillatory kinetics6 The photosensitivity of IMA and its derivatives has been k n o ~ n ' ~ 9but ' ~ has not yet been investigated quantitatively. It was supposed that IMA (or diiodomalonic acid) could act as a source of iodine in the Briggs-Rauscher reaction.16-19 On the basis of the results reported here, similar role of IMA in the present oscillatory reaction is anticipated. A kinetic study of the light-induced decomposition of iodide-free IMA solution to tartronic acid, iodine, and iodide ion can shed light on the details of the mechanism of this photosensitive oscillator as well as that of the Briggs-Rauscher reaction. This work is now under way in our laboratory. In addition to leading to the above-mentioned mechanistic insight, the light perturbation of the present reaction can provide an ideal chemical experimental model for excitable biological systems submitted to external stimuli. Furthermore it can, in principle, be interesting for creating digital recording media20 with threshold-type responses to light exposure. Particular advantage of this system is that very small light power is required to induce quick and large responses, and there is a possibility of visualizing the oscillations by using redox indicators. Finally, our finding calls for an investigation of the light effect on the symmetry-breaking, reaction-diffusion spatial structures found recently in the chlorite-iodide-malonic acid system.21-x Here significant contribution of the iodomalonic acid-chlorite ion reaction to the formation of Turing structures is very likely suggesting that these structures are sensitive to light.
Acknowledgment. We thank one of the referees for suggesting us the experiments with iodine. We acknowledge fruitful discussions with Y. Mori and V. K. Vanag. References and Notes (1) Permanent address: Institute of Physical Chemistry, Kossuth Lajos University, H-4010 Debrecen, Hungary. (2) Hanazaki, I.; Mon, Y . ;Sekiguchi, T.; Rkbai, Gy., to be published. (3) Dulos, E.; De Kepper, P. Biophys. Chem. 1983,18, 211. (4)De Kepper, P.; Epstein, I. R.; Kustin, K.; Orbin, M. J. Phys. Chem. 1982,86,170. (5)Noszticzius, 2.; Ouyang, Q.; McCormick, W. D.; Swinney, H. L. J. Am. Chem. SOC. 1992,114, 4290. (6) Rkbai, Gy. J. Phys. Chem. 1994,98, 5920. (7)Hanazaki, I. J. Phys. Chem. 1992,96,5652. OR was calculated from the photon energy (hv), the optical density (D), and PoC as UR = (hvD/
Light-Induced State Transitions Poc)[l - exp(-2.303D)I-l. The absorption spectrum of IMA is shown in Figure 2 (€284 = 404 M-' cm-'). A quantitative measurement of Poc below 330 nm is hampered by the rather fast light-induced decomposition of IMA. (8) Zaikin, A. N.; Zhabotinsky, A. M. In Biological and Biochemical Oscillators Chance, B., Pye, E. K., Ghosh, A,, Hess, B., Eds.; Academic: New York, 1973; p 8 1. (9) Perkel, D. H.; Mulloney, B. Science 1974, 185, 181. (10) Marek, M.; Svobodova, E. Biophys. Chem. 1975, 3, 263. (11) Dolnik, M.; Marek, M.; Epstein, I. R. J. Phys. Chem. 1992, 96, 3218. (12) Dolnik, M.; Epstein, I. R. J. Chem. Phys. 1992, 97, 3265. (13) Dolnik, M.; Epstein, I. R. J. Chem. Phys. 1993, 98, 1149. (14) Cooke, D. 0. React. Kinet. Catal. Lett. 1975, 3, 377.
J. Phys. Chem., Vol. 98, No. 41, 1994 10553 (15) Cooke, D. 0. J. Chem. Educ. 1987, 64, 257. (16) Cooke, D. 0. Int. J. Chem. Kiner. 1982, 14, 1047. (17) Leopold, K. R.; Haim, A. In?. J. Chem. Kinet. 1977, 9, 83. (18) Vanag, V. K.; Alfimov, M. V. J. Phys. Chem. 1993, 97, 1878. (19) Furrow, S.D. J. Phys. Chem. 1989, 93, 2817. (20) Alfimov, M. V.; Vanag, V. K. Dokl. Russ. Acad. Nauk. 1992,322, 935. (21) Castets, V.; Dulos, E.; Boissonade, J.; De Kepper, P. Phys. Rev. Lett. 1990, 64, 2953. (22) De Kepper; Castes, V.; Dulos, E.; Boissonade, J. Physica D 1991, 49, 161. (23) Ouyang, Q.;Swinney, H. L. Nature 1991, 352, 610. (24) Lengyel, I.; K&&, S.;Epstein, I. R. Science 1993, 259, 493.
