J. Phys. Chem. 1991,95, 5147-5149
5147
Incorporation of a Chemical Oscillator into a Liquld-Crystal System
D.Balasubramanian* Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad 500 007, India
and C . A. Rodley Department of Pharmacy, The University of Sydney, Sydney, NSW 2006. Australia (Received: November 9, 1990; In Final Form: January 28, 1991)
The Belousov-Zhabotinskii (BZ) chemical oscillator has been incorporated into a ternary water/isooctane/bis(2-ethylhexyl) sodium sulfosuccinate (AOT) liquid-crystal system. For particular ranges of concentrations of the oscillator's components and those of the host liquid crystal, sharp and sustained oscillations have been observed. The results indicate that, while basic features of the oscillations match those of the water-only oscillator, encapsulation within the liquidcrystal matrix introduces new features. These are quite sensitive to the proportibns of the components, indicating the importance of diffusional factors. At the optimal region of oscillator-liquid-crystal composition the binary system evolves to give relatively constant periods of oscillation.
It is now well established that certain chemical systems can exhibit periodic changes, or oscillations, in the concentrations of one or more components with time. The best known system in the Belousov-Zhabotinskii (BZ) oscillator. Chemical oscillation is also exhibited by biological systems (e.g., the glycolytic cycle'J) and is thought to be central to the process of integration, ordering, and the turnover in living organisms. In this connection, it is of interest to assess the degree to which the process of chemical encapsulation can aid further ordering of the chemical components, especially those which already display oscillatory behavior. Thus we have been studying the behavior of chemical oscillators in organized amphiphilic imposing spatial order on a temporally ordered system. Our earlier studies of the BZ reaction in reverse micelles of Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate, or AOT) show that the characteristics of the oscillations are substantially different from those of the BZ reaction in water alone?s4 In the present study, we have explored the effect of incorporating the BZ oscillator in the liquid-crystalline phase (LC phase) of the AOT-isooctanewater system.>' We find that, upon optimizing the composition of the BZ oscillator sequestered within such a membranelike phase, relatively constant oscillations are obtained.
Experimental Section The phase diagram of the AOT/oil/water system has been described by Eickdfi and reviewed by Luisi and Magid.' We have chosen to use isooctane as the oil phase, in order that comparison can be made with the behavior of the BZ oscillator in the AOT reverse micellar system in the same solvent.' One region in the liquid-crystal phase was chosen and, after some searching, a composition of the BZ oscillator was found which gave clearly observed bulk oscillations. A range of compositions was tried, which were liquid crystalline, as confirmed by using crossed polarizers, both in the presence and absence of the BZ reagents. Further variation of composition was made to more fully characterize the properties of the BZ/LC system. AOT is inert with respect to components of the BZ oscillator,' and thus the observed changes of oscillatory behavior are only related to changes in the composition. The standard procedure for preparing the binary systems is as follows: (1) A solution of AOT in isooctane (4 g in 2 mL) in the reaction vessel (an open neck vial of dimensions 5 cm high and 2.5 cm in diameter) is placed in a constant temperature bath (T = 298.5 K) and covered with a piece of expanded polystyrene, through which a pair of platinum electrodes and a stirrer are inserted. (2) The BZ mixture is prepared separately by combining 5-mL aliquots of each stock solution of KBr03 and H2S04(which Address correspondence to this author.
0022-3654191 12095-5147$02.50/0
contained Mn2+) for the "higher" bromate runs. Five-milliliter samples consisting of 2.5 mL of the stock bromate solution, diluted with 2.5 mL of water, were used for "lower" bromate runs. Solid malonic acid is then added to give the appropriate concentration together with 0.1 1 mL of a stock solution of 0.025 M ferroin. Typically, 1.6 mL of this mixture is then injected into the AOT/isooctane solution. Stirring (1 200 rpm in most experiments) is initiated and continued throughout the time of periodic oscillations. Small amounts of the preparation are removed at various stages in several runs and tested for the presence of for the liquid-crystal phase, with two pieces of polaroid material used in the "crossed" position. Oscillations in the electrical potential of the mixtures are recorded by direct connection of the platinum electrodes to a recorder set a t a 100-mV full-scale position. The experimental details are given in our earlier paper.' The oscillations of reference BZ mixtures in aqueous solution are studied by preparing the reaction mixtures in the above manner and transferring 6 mL of the 10-mL samples directly to the reaction vessel. A slower (about 400 rpm) rate of stirring is used in these cases.
Results and Discussion Two particular parameters of interest in the BZ reaction on which we focus attention on are (i) the length of time that oscillations can be maintained and (ii) the oscillation period itself and its constancy with time. We had shown earlier that incorporating the oscillator in water pools of AOT reverse micelles affects both these parameters significantly. We now turn our attention to the effects of incorporating the oscillator in a liquid-crystalline phase. In interpreting the results, we have used the following main concepts: (i) Any system that compartmentalizes the oscillator into defined water pools or channels might also involve phase coupling of such individual oscillators, so that bulk oscillations would arise. (ii) Sequestering of one or more key reactants or products away from the site of chemical oscillation would, in effect, increase the "openness" of the system by providing reservoir-flow or flow-sink features. A basic liquid-crystalline composition of 4 g of AOT in 2 mL of isooctane and 1.6 mL of aqueous component was chosen. For ( I ) Pye, K.; Chance, B. Proc. Nod. Acad. Sci. USA 1966, 55, 868. (2) Has, B.; Brand, K.; Pye, K. Biochem. Biophys. Res. Commun. 1966,
23, 102. (3) Balasubramanian, D.; Rodley, G.A. J . Phys. Chem. 1988,92, 5995. (4) Gonda, 1.; Rodley, G.A. J. Phys. Chem. 1990, 91, 1516. (5) Eicke, H.F. Chimio 1982, 36, 241. (6) Eickc, H.-F. Top. Curr. Chem. 1980, 87, 85. (7) Luisi. P. L.; Magid, L. J. CRC Crir. Reu. Biochem. 1986, 20, 409.
0 1991 American Chemical Society
Balasubramanian and Rcdley
5148 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
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Figure 1. Electrical potential change traces for BZ/LC systems: (a) [H2S04] 0.75, [KBrOj] = 0.09, [MA] 0.175; (b) [H$304] = 0.75, [KBrOJ = 0.09, [MA] = 0.1 15; (c) [H2S04]= 0.75, [KBrO,] = 0.18, [MA] = 0.175; (d) [H2S04] 0.75, [KBrO,] 0.18, [MA] = 0.115. In all cases [Mn2+] = 0.005. AI1 values are in molarity units.
Figure 2. Plots of the change in period with time for various BZ/LC and related water-only BZ oscillators: (a) BZ/LC with [H2S04]= 0.75, [KBrO,] = 0.09, [MA] = 0.06; (b) BZ/LC with [HZSO,] = 0.75, [KBrOJ = 0.09, [MA] = 0.175; (c) BZ/LC with [H$304] = 0.75, [KBrO,] = 0.09, [MA] = 0.170; (d) BZ/LC with [H$304] = 0.75, [KBr03] = 0.18, [MA] = 0.115; (e) water-only BZ with [H$304] = 0.75, [KBr03] = 0.18, [MA] = 0.115; (f) water-only BZ with [H$301] = 0.75, [KBrO,] = 0.09, [MA] = 0.06. In all cases, [Mn2+] = 0.005. All concentrations are in molarity.
oscillator. The observed oscillations presumably relate to a particular optimal composition, and excess of any one or more of the reagents may be sequestered to nonoscillating regions of the overall system. This is comparable to the situation found for appropriate BZ compositions used in this LC phase, and upon the corresponding reverse micelle system.' rapid stirring (usually 1200 rpm), we could easily observe bulk In general, four general kinds of subsequent oscillatory behavior color changes, indicating the progress of the periodic reactions. are observed in the liquid-crystalline host system: These color changes are accompanied by sharp electrical potential 1. The oscillations may cease after the initial period; this was changes (several millivolts), which are easily and precisely meaobserved in a case where the bromate concentration was reduced sured with a pair of Pt electrodes, as described earlier.3 Figure beyond the values for those systems shown in Figure 2. In this 1 shows the periodic changes in the potentials for a range of case, there must be a critical deficiency in at least one of the different BZ compositions used with this AOT LC phase. reagents. Repeat studies of systems of the same overall composition 2. The period of oscillations may reduce significantly (Figure showed that, although the general pattern is the same, the fine 2, plots a and b). This may be due to an increased diffusional details of the oscillations vary somewhat. The pattern in Figure capability the system has at that point, as a result of the sela is the simplest, while that in Figure Id is the best developed questering of components that has previously occurred. This and lasts the longest as well. Frequently, as shown especially in resembles the situation observed for the reverse micellar binary Figure IC,each pattern of oscillation contains a range of detailed ~ystem.~.~ The reason could be the same, namely that the reservoirs features. Double, overlapping patterns often appeared and rather of reagents within the system are such that increasingly rapid sharp changes of patterns were sometimes observed. The systems diffusion enhances the ease with which a cycle of oscillation may usually displayed relatively long induction times (about 10 min) be achieved. and in some cases color changes occurred in isolated regions of 3. The oscillations may become irregular. As seen in Figure the sample before bulk color changes developed. The rate of 1, curve c, the period suddenly changes between two values. stirring appears to be important, since slower rates (400 rpm) 4. The oscillation period may show relatively small changes generate less well-defined oscillations. All of these features, with time as observed for (d) in Figure 1. Again this matches including (a) reduced or (b) more-close-to-constant values of the what was observed in one instance for the reverse micellar system.' period toward the end of the oscillations, are in marked contrast Normal aqueous BZ systems may show this at earlier stages of to those observed for the reference water-only BZ systems. Details oscillation, probably due to ready availability of primary reacting of the changes in period with time for some of the LC and species (as shown from plots e and f, Figure 2). However, when aqueous-only systems are given in Figure 2. depletion of reactants occurs, the period changes to longer values The results show that encapsulation of the BZ oscillator within more rapidly. For these BZ/LC systems the opposite situation a liquid-crystal system markedly alters the characteristics of the occurs. Rapid changes in period initially occur (presumably oscillations (Figure 2), in comparison with what is seen in because the host matrix imposes wme limitation on the immediate water-only systems as well as in reverse micelle^.^ Unlike the availability of reactants). However, when a certain stage is water-only system, the liquid-crystalline system shows a decrease reached, the overall concentrations apparently become compatible in period toward the end of oscillation. The same feature was previously observed for the BZ/AOT reverse micelle ~ y s t e m . ~ . ~ and the system adopts a relatively constant period of oscillation. This could also indicate that some sequestering of components However, in contrast with the latter, all liquid-crystalline systems has occurred, giving, in this case, available reservoirs of reactants, showed an initial lengthening of the period of oscillation. What thereby making the system more 'open" than would otherwise appears significant is that the same general initial pattern of be the case. It is very likely that the common oscillatory pattern oscillation (for about 30 min, see Figure 2) is observed for a wide seen initially in these systems indicates that some spatial adrange of different BZ compositions. This indicates that the LC justment of concentrations occurs. host imposes diffusional and compositional constraints on the BZ
5149
J. Phys. Chem. 1991.95, 5149-5159
be optimized by appropriate compositional changes. Such observations may provide an indication as to how the constancy of the period of oscillation of the naturally occurring glycolytic oscillator is achieved. In this connection, it has been suggested, however, that natural systems are not “batch” systems, but closer to CSTRs.
These results indicate that sequestering may develo somewhat more readily in LC systems than in reverse micelles. Both the period and the sustenance time in the LC system are significantly shorter than what are seen in the reverse micellar systems. In the case of the latter, the oscillation period generally decreased throughout the total time of oscillation and only in one instance was a constant period of oscillation observed.’ By contrast, the LC systems invariably produce some region of relatively constant oscillatory behavior (at the transition from lengthening to decreasing of the period of oscillation). Moreover, this effect may
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Acknowledgment. G.A.R. thanks the C.C.M.B., Hyderabad, and the Department of Pharmacy, University of Sydney, Australia, for providing facilities.
Photodlssoclatlonof Acrolein and Propynal at 193 nm in a Molecular Beam. Prlmary and Secondary Reactions B.-M. Haaq T. K. Minton,? P. Felder, and J. Robert Huber* Physikalisch- Chemisches Institut der Universitiit Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland (Received: November 9, 1990; In Final Form: February 1 1 , 1991)
The translational energy distribution and the fragment anisotropy for the 193-nm photodissociation products of acrolein H2C=CHCH0 and propynal H W C H O have been measured with a molecular beam time-of-flight apparatus. For both aldehydes, three distinct dissociation processes have been established: The molecular channel leading to CO + H2C= CH2/HC=CH, the radical channel creating HCO + H2C--CH/HC=C, and the hydrogen channel involving the aldehyde C-H bond fission with the products H + H2C-CHCO/HCEC-C0. Furthermore, the secondary decay processes of the hot photofragments HCO H + CO, C2H3 C2H2+ H, H2C=CHC0 C2H3 + CO, and H W - C O H C = C + CO have been identified, as well as the secondary photodissociations C2H3+ hu C2H2+ H and C2H + hu C2 + H, which take place within a laser pulse time of 10 ns.
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1. Introduction The gas-phase photochemistry of small aldehydes has been the subject of numerous investigations and has been reviewed by several Propynal HC=CCHO and acrolein H2C= CHCHO are of particular interest since they are the two smallest unsaturated carbonyl compounds. Compared to formaldehyde, the interaction of the carbonyl group with the carbon-carbon r-systems shifts the electronic spectra to longer wavelengths and increases the intensity of the lowest transition, SI So. The UV spectrum of propynal shows a very weak absorption extending from about 41 4 nm toward shorter wavelengths and a weak system starting at approximately 382 nm. They were A* transitions from the ground So to the first assigned to n triplet and singlet states (TI and SI),respectively. At shorter wavelengths a strong and diffuse absorption peaks at about 206 nm. Watson4 has assigned it to two partly overlapping A A* transitions localized on the ethynyl group (weaker, at longer wavelengths) and on the conjugated carbonyl group (stronger, at shorter wavelengths). This interpretation was confirmed by semiempirical SCF MO calc~lations.~ The spectrum of acrolein exhibits a very weak ahsorption system starting at 412 nm, followed by a stronger one at 387 nm. As in propynal, they were assigned to n A* transitions from So to the TI and SIstates, respectively. At shorter wavelengths a strong and diffuse region of absorption is observed, having a maximum near 193.5 nm.6 On the basis of ab initio SCF calT * transition. c ~ l a t i o n s ~this - ~ band was attributed to a 7 Furthermore, by examining the temperature dependence of band intensities, Alves et al.IOshowed that ground-state acrolein at 20 OC is present to 96% as the s-trans conformer with slightly decreasing probability at higher temperatures. Much effort has been devoted to the investigation of the photodissociation of formaldehyde, the smallest member of the car-
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TABLE I: Standard Enthalpies of Formation for Some Substraces at 0 K
species H
co H2CO HCO H m H HzC
