On the nature of patterns arising during polymerization of acrylamide in the presence of the methylene blue-sulfide-oxygen oscillating reaction

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1 October Chemical Physics Letters On the nature of patterns arising during polymerization of acrylamide in the presence of the methylene blue-sulfide-oygen oscillating reaction Krisztina Kurin-Csorgei a, Miklos Orban a, Anatol M. Zhabotinsky b, Irving R. Epstein b,) a Department of Inorganic and Analytical Chemistry, L. EotÕos UniÕersity, P.O. Bo, H-1518 Budapest 11, Hungary b Department of Chemistry, Brandeis UniÕersity, Waltham, MA , USA Received 9 July 1998 Abstract Transient Turing-like spatial patterns ere recently reported by Munster et al. hen the components of the methylene blue oscillator are mied ith acrylamide and a polymerization initiator to form a polyacrylamide gel. We have qualitatively reproduced these results, but e demonstrate that, although the observed patterns have the appearance of Turing structures, they actually result from Rayleigh Benard convection in a miture of acrylamide, N,N X -methylene-bisacrylamide, triethanolamide, sulfide ions and ammonium peroodisulfate during the polymerization and gelation process. Neither methylene blue nor sulfite is required for patterns to develop. q 1998 Published by Elsevier Science B.V. All rights reserved. 1. Introduction The many efforts to find Turing structures 1 in chemical systems ere unsuccessful until eperi- y y ments on the CIMA ClO -I -malonic acid-starch. reaction carried out in a specially designed gel reactor demonstrated unambiguously the eistence of such patterns 4. The other oscillatory chemical system that has been reported to be capable of forming Turing-like structures in a gel matri 5 7 is the methylene blue oscillator MBO., hich consists of methylene blue Mblue., sulfide S., sul- fite SO. and oygen 8. ) Corresponding author. Tel.: q ; Fa: q In this paper e reeamine the results obtained in the study of the methylene blue oscillator-polyacrylamide gel system. Our main aim is to reproduce and analyze the previously observed patterns ith an eye toard clarifying their origin.. Eperimental.1. Chemicals All reagents ere of the highest grade commercially available. These ere: acrylamide AA., Sigma, electrophoresis reagent, G99%; N,N X -methylene-bis-acrylamide Bis., Sigma, electrophoresis reagent, G 98%; triethanolamine TEA., Fisher, r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved.. PII: S

2 K. Kurin-Csorgei et al.rchemical Physics Letters 95 ( 1998) 71 99%; ammonium peroodisulfate S O., Sigma, electrophoresis reagent, G 98%; sodium sulfide S., Merck, 5% Na S; methylene blue Mblue., Fisher, p.a. In some initial eperiments, e used sodium sulfite SO. Sigma, 99%., hich is a non-essential but suggested component of the methy 8. Since this compound pro- lene blue oscillator duced no noticeable effect on the behavior of the MBO-polyacrylamide gel system, e ceased to employ it. Recrystallization of sodium sulfide as sug 5 in order to remove polysulfide gested in Ref. impurities present in the commercial product. We observed no difference beteen the results obtained ith unpurified and ith recrystallized sulfide. Therefore instead of purification, e used nely purchased Merck product in our eperiments. We note that polysulfide, S, is formed in any case as an intermediate during the reaction beteen SO8, and S. For preparation of stock solutions, the chemicals ere dissolved in deionized ater. All stock solu- tions but NH. 4 SO8 and Na S could be stored for many days ithout decomposition. The folloing stock solutions ere made: AAs.7 M; Biss 0.1 M; TEAs1.6 M; NH. S O 4 s0.79 M; Na Ss0.16 M; Mblues0.01 M... Procedure The gel components, AA, Bis and TEA, ere mied ith Mblue and S, and NH. 4 SO8 as then added to initiate the gelation process. The final volume, composed of the stock solutions and ater, as 1.6 cm, hich formed an approimately mm thick solution layer in a 9 cm i.d. Petri dish. In most eperiments the concentration of the gel components as kept constant AA s 1.05 M, 5.50 cm of y stock solution; Bis s 4.85 = 10 M, 0.47 cm ; TEAs0.055 M, 0.47 cm.. The Mblue as eiy4 ther 4=10 M 0.5 cm. or the miture as free of this component. The time of polymerization as found to be unaffected by Mblue but changed significantly ith S and S O. For eample,. at S s1.5=10 M 1.0 cm, =10 M..40 cm and 5 = 10 M,.94 cm., and SO s 0.11 M 1.80 cm. or 0.6 M,.60 cm., gelation took about 10 min, 0 min and 4 h or 5 min, 10 min and min, respectively. After the miture became solid, the gel layer as covered either ith a solution of methylene blue, 4 = 10 y4 M, 0.5 cm stock solution plus 5.5 cm ater, hich resulted in a 1 mm thick solution layer. as suggested in Ref. 5 or ith ater 6 cm., and the system as eposed to air. Observations ere made visually from above using illumination light bo. from belo. Images ere recorded ith a video camera, and the data ere processed ith an OPTI- MAS image analysis system Bioscan... Results.1. Reproducing Munster and Watzl s results Munster et al. present a phase diagram in the Mblue vs. Na S plane shoing the concentration domains in hich colorless and blue heagons and stripes develop in the polyacrylamide-methylene blue oscillator system 5 7. The stationary patterns shon in the photographs in Refs. 5 and 6 strongly resemble the Turing structures seen in the CIMA system. We had difficulties in directly reproducing Munster et al. s observations for to reasons: i Phase diagrams ith the same eperimental points are shon in Refs. 5 and 6, but the scales of the aes i.e. the concentrations of Mblue and Na S. differ significantly, by a factor of 5 for Mblue and.1 for Na S. One of the diagrams, e believe it to be the one presented in Ref. 5, must be erroneous. ii. Assuming the phase diagram in Ref. 6 to be correct, e still had to use concentrations different from those given for the reagent S andror the initiator NH. 4 SO8 in order to transform the miture of AA, Bis, TEA, Mblue, S and SO8 to a gel state. The concentrations of sulfide and peroodisulfate ere found to influence strongly the time needed for gelation. Higher S results in longer polymerization times, and no gelation at all occurs at many compositions here a gel state and pattern formation are epected to appear according to the phase diagram in Ref. 6. With proper variation of the concentration ratio of sulfide to peroodisulfate e finally succeeded in

3 7 K. Kurin-Csorgei et al.rchemical Physics Letters 95 ( 1998) reproducing qualitatively most of the observations described in Ref. 5. Keeping S as in Ref. 6 and using a ne set of concentrations for peroodisulfate M, i.e. one order of magni. 6, e observed blue and tude higher than in Ref. colorless spots and stripes in the polyacrylamide gel 0 0 min after covering it ith Mblue solution. The patterns changed in time and persisted for many hours. Munster et al. assert that the pattern observed in the polyacrylamide gel layer is a Turing structure that arises from the reaction-diffusion interaction beteen the components of the methylene blue oscillator and the gel matri. They suggest that an adduct formed beteen methylene blue and the gel serves as the activator, hile an unspecified, rapidly-diffusing sulfur species acts as the inhibitor. Such a scheme implies that the pattern arises from the interaction of the MBO ith the polyacrylamide gel. We checked this hypothesis by submerging mm polyacrylamide gel layers in a solution containing all soluble components of the MBO ith a volume ratio of solution to gel of 100:1 under an argon atmosphere. After 1 h the gel layers ere taken from the reagent solution, rinsed ith distilled ater and placed in contact ith air. During the subsequent oidation e did not observe any spatial pattern formation. The studies reported here demonstrate that the observed pattern is not a Turing structure, and the components of the methylene blue oscillator are not required for development of the pattern seen in the polyacrylamide gel layer. In the folloing sections e describe eperiments that sho that the structures seen in the methylene blue oscillator-polyacrylamide system are of convective origin and the major factors that bring about these patterns are the heat evolved during polymerization and the interaction beteen sulfide and the gel components. M solution of Mblue, the same Mblue as in the gel. To purposes ere suggested for covering the gel surface ith Mblue solution: a. to prevent the drying out of the gel; and b. to enhance the transport of oygen, the oidizing agent in the MBO, from the air into the gel... Pattern formation in the AA-Bis-TEA-S O y - S y system In the eperiments described in Refs. 5 and 6, to components of the MBO, Mblue and S, ere mied ith the gel components prior to polymerization, and the gel layer as covered ith a 4=10 y4 Fig. 1. a Spotted pattern in the AA-Bis-TEA-S O -S gel system. Composition of the gel miture: AAs1.05 M; Biss y 4.85=10 M; TEA s0.055 M; S O s0.11 M; S s =10 M. Covering solution: ater. The domain size is 7 mm=5 mm. The average diameter of the spots is about 1.5 mm. b Striped pattern in the AA-Bis-TEA-SO8 -S gel system. All conditions and concentrations, ecept S s5= 10 Masin a.

4 K. Kurin-Csorgei et al.rchemical Physics Letters 95 ( 1998) 7 eperiments ith all components of the MBO present. The results are shon in Fig. a and b. We note that patterns can be formed in a miture of polyacrylamide components, sulfide and methylene blue hen the miture remains in a liquid state for an etended time 1 h or longer. if the concentration of initiator is lo e.g M.. Similar patterns arise hen solutions of methylene blue and sulfide are mied and spread in a Petri dish in the absence of any gel component. Such patterns, shon in Fig. a and b certainly arise from convection and are not related chemically to the patterns in Figs. 1 and. Fig.. a Spotted pattern in the AA-Bis-TEA-S O -MBO methylene blue-s -oygen. gel system. Composition of the gel y4 miture as in Fig. 1a, plus Mblue s4=10 M. The domain size is 19 mm=14 mm. Covering solution: 4=10 y4 M Mblue. b Striped pattern in the AA-Bis-TEA-SO8 -MBO gel system. All conditions and concentrations as in a ecept S s5= 10 M. In one set of our eperiments the gel as prepared ith sulfide but ithout Mblue, and pure ater instead of Mblue solution as spread on the surface of the gel. Surprisingly, the same type of structures appeared in these eperiments, and even the dependence of the character of the patterns on the concentration of sulfide resembled the results reported in Refs. 5 and 6. Fig. 1 a and b sho the spots and stripes that developed ith a single composition of gel matri containing different concentrations of sulfide. To eamine the similarity beteen these structures and the reported Turing-like patterns, e repeated our Fig.. a Convective pattern in Mblue-S solution layer e y4 posed to air. Mblue s4=10 M; S s=10 M. Picture as taken 10 min after miing the components and layering the solution into a Petri dish. b Convective pattern in Mblue-S solution layer eposed to air. Conditions as a in but picture as taken at 0 min after miing.

5 74 4. Discussion K. Kurin-Csorgei et al.rchemical Physics Letters 95 ( 1998) Acknoledgements Munster et al., ho first observed pattern formation in an MBO polyacrylamide gel layer 5,6, suggest that it occurs only if all components of the methylene blue oscillator are included in the gel. They propose that the chemistry of these species combines ith the interaction beteen methylene blue and the gel to provide the activator-inhibitor pair that leads to the formation of a reaction-diffusion based Turing pattern. Here e have shon that the chemistry of the methylene blue oscillatory system does not contribute to bringing about pattern formation in the gel. The patterns that develop in the polyacrylamide gel layer resemble Turing structures, but they are almost certainly convection-generated phenomena rather than genuine reaction-diffusion based Turing structures. It orth noting that many similar, chemically driven convective patterns ere reported earlier 9, and several of these ere initially confused ith Turing patterns. This ork as supported by the Chemistry Division of the National Science Foundation NSF., a US Hungarian cooperative grant from the NSF and the Hungarian Academy of Sciences HAS., by the HAS OTKA T , F 01707, BOr00096r98. and by a grant from MKM FKFP 0188r References 1 A.M. Turing, Phil. Trans. Roy. Soc. B V. Castets, E. Dulos, J. Boissonade, P. De Kepper, Phys. Rev. Lett I. Lengyel, S. Kadar, I.R. Epstein, Science R. Kapral, K. Shoalter Eds.., Chemical Waves and Patterns, Kluer, Dordrecht, M. Watzl, A.F. Munster, Chem. Phys. Lett A.F. Munster, M. Watzl, F.W. Schneider, Phys. Scr M. Watzl, A.F. Munster, J. Phys. Chem. A M. Burger, R.J. Field, Nature J.C. Micheau, M. de Min, M. Gimenez, BioSystems , and references cited therein.