Langmuir 2003, 19, 1927-1928
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Smart Aqueous Reaction Medium D. Martin Davies* and Estelle L. Stringer Division of Chemical Sciences, School of Applied Sciences, Northumbria University, Newcastle upon Tyne NE1 8ST, United Kingdom Received December 24, 2002 Smart materials show a nonlinear response to a stimulus. In aqueous solution, the reactions between methyl p-tolyl sulfide and peroxomonosulfate or m-chloroperbenzoic acid show the expected linear dependence of the logarithm of the measured rate constant on the reciprocal temperature. This constitutes Arrhenius behavior. In the presence of 5 or 15 g L-1 of the thermoresponsive poloxamer, P104, H(OCH2CH2)27(OCH(CH3)CH2)61(OCH2CH2)27OH, which forms micelles as the temperature is increased, anti-Arrhenius behavior or hyper-Arrhenius behavior is observed. Anti-Arrhenius behavior occurs when the organic sulfide partitions into the thermally induced poloxamer micelles while the peroxomonosulfate anion remains in the bulk aqueous phase, causing a decrease in rate. Hyper-Arrhenius behavior occurs when both the organic sulfide and the m-chloroperbenzoic acid partition into the thermally induced micelles, causing a much greater increase in rate with temperature than in the absence of poloxomer. These two different types of smart behavior of aqueous P104 are discussed.
Recently, smart catalysts and substrates that have thermoresponsive polymers covalently attached to them have been described. These phase separate as the temperature is raised, causing anti-Arrhenius behavior in which their reactions slow considerably with increasing temperature.1 This communication describes the unusual effects of temperature on bimolecular reactions taking place in dilute aqueous solutions of a thermoresponsive polymer where both anti-Arrhenius behavior and hyper-Arrhenius behavior are observed. The reactions considered, Scheme 1, are the oxidation of p-tolyl sulfide by either peroxomonosulfate or m-chloroperbenzoic acid. The thermoresponsive polymer is the poloxamer, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, P104.
H(OCH2CH2)27(OCH(CH3)CH2)61(OCH2CH2)27OH P104 Poloxamers, also known as Pluronics, are commercially available surfactants with a highly temperature dependent critical micelle concentration (cmc). Micelle formation is induced by raising the temperature, due to the pronounced decrease in solubility of the hydrophobic poly(propylene oxide) moiety.2-4 The physical properties of dilute aqueous poloxamer solutions and of added probe molecules that partition into the micelle show an abrupt change as the temperature is increased, marking the onset of micelle formation at the critical micelle temperature (cmt). There follows a transition region where the proportion of micellized surfactant increases with temperature until a second inflection occurs, above which virtually all the polymer is micellized.2,3 The effects of surfactant micelles on reaction rates are well-established, and we have previously considered their effects on bimolecular reactions involving peroxides in terms of a multiple pseudo(1) (a) Bergbreiter, D. E.; Zhang, L.; Mariagnanam, V. M. J. Am. Chem. Soc. 1993, 115, 9295-6. (b) Bergbreiter, D. E.; Caraway, J. W. J. Am. Chem. Soc. 1996, 118, 6092-3. (c) Bergbreiter, D. E.; Case, B. L.; Liu, Y.-S.; Caraway, J. W. Macromolecules 1998, 31, 6053-62. (2) Chu, B.; Zhou, Z. In Nonionic surfactants: polyoxyalkene block copolymers; Nace; V. M., Ed.; Marcel Dekker: New York, 1996; pp 67143. (3) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414-25. (4) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145-59.
Scheme 1. Oxidation of Methyl p-Tolyl Sulfide by Peracids
phase model.5,6 When one reactant remains in the bulk aqueous phase and the other partitions into the micellar pseudophase where there is insignificant reaction, then micellar inhibition occurs. The measured rate constant depends on the rate constant in water and the reciprocals of the surfactant concentration and the micellar association constant of the reactant. When both reactants partition into the micelle, there is initially an increase in the measured rate constant with increasing surfactant concentration as the reactants become confined to the restricted volume of the micellar pseudophase. Once both reactants are completely removed from the bulk aqueous phase, the measured rate constant (which now depends on the apparent micellar association constant of the transition state and the reciprocals of the surfactant concentration and the micellar association constants of the reactants) decreases. This is because, with increasing surfactant concentration, the two reactants are essentially being diluted in the micellar pseudophase. Figure 1 shows the effect of temperature on the reaction of methyl p-tolyl sulfide and peroxomonosulfate at two different concentrations of P104. At low temperatures, where the poloxamer is not micellized, it has a negligible effect on the rate constant. Then the rate constant starts to drop in the presence of poloxamer, and this occurs at a lower temperature for the higher concentration of P104. This is commensurate with the lower cmt found at higher poloxamer concentrations.2,3 The extent of the drop in rate (5) (a) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698-703. (b) Yatsimirski, A. K.; Martinek, K.; Berezin, I. V. Tetrahedron 1971, 27, 2855-68. (c) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622-8. (6) (a) Davies, D. M.; Gillitt, N. D.; Paradis, P. M. J. Chem. Soc., Perkin Trans. 2 1996, 659-66. (b) Davies, D. M.; Gillitt, N. D. J. Chem. Soc., Dalton Trans. 1997, 2819-23. (c) Davies, D. M.; Foggo, S. J. J. Chem. Soc., Perkin Trans. 2 1998, 247-51. (d) Davies, D. M.; Foggo, S. J.; Paradis, P. M. J. Chem. Soc., Perkin Trans. 2 1998, 1597-602.
10.1021/la027067f CCC: $25.00 © 2003 American Chemical Society Published on Web 01/30/2003
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Langmuir, Vol. 19, No. 6, 2003
Figure 1. Rate constants for the reaction of 1 × 10-5 mol dm-3 methyl p-tolyl sulfide with 1.88 × 10-3 M peroxomonosulfate in the absence of poloxamer (circles) and the presence of 5 g L-1 (squares) and 15 g L-1 (diamonds) P104. Reactions were carried out in water buffered with 0.1 M sodium acetate and 0.1 M acetic acid.
Letters
Figure 3. Rate constants for the reaction of 1 × 10-5 M methyl p-tolyl sulfide with 1.88 × 10-4 M m-chloroperbenzoic acid. See Figure 1 for symbols and conditions.
Figure 4. Arrhenius plots for the reaction of methyl p-tolyl sulfide with m-chloroperbenzoic acid. See Figure 1 for symbols and conditions. Figure 2. Arrhenius plots for the reaction of methyl p-tolyl sulfide with peroxomonosulfate. See Figure 1 for symbols and conditions.
constant after the transition region is approximately proportional to the reciprocal P104 concentration, indicating that the reaction that occurs above the transition region is predominantly that between the aqueous peroxomonsulfate and the residual organic sulfide remaining in the aqueous solution. Figure 2 shows the Arrhenius plot of the data in Figure 1. The straight line represents the best fit in the absence of poloxamer. The lines joining the other points are a guide for the eye. It is clear that the slopes of these Arrhenius plots above the transition region are quite similar to that in the absence of poloxamer, and this is consistent with a relatively temperature independent micellar association constant of the sulfide. Figure 3 shows the effect of temperature on the reaction of the sulfide and m-chloroperbenzoic acid in solutions of P104. Again, at low temperatures, where the poloxamer is not micellized, it has no effect. Then the rate constant starts to increase above that in the absence of poloxamer, and this occurs at a lower temperature for the higher concentration of P104, again reflecting the lower cmt in this case. At the higher concentration of P104, the rate constant goes through a maximum and drops to a value below that at the lower poloxamer concentration. The drop is due to the dilution of the reactants in the micellar pseudophase after their essentially complete removal from the bulk aqueous phase. Figure 4 shows the corresponding
Arrhenius plots. The slopes of these above the transition temperature are only a little less than in the absence of poloxamer. This shows that the activation energy of the micelle-mediated reaction is only a little less than that in the bulk aqueous phase and hence the rate acceleration is predominantly due to the concentration of the two reactants in the micellar pseudophase. Smart materials show a nonlinear response to a stimulus. In the absence of poloxamer, the relationship between the logarithm of the rate constant and inverse temperature is linear. The poloxamer solution represents a smart medium that responds differently depending on the nature of the reaction. When one reactant is hydrophobic and the other hydrophilic, behavior is seen reminiscent of Bergbreiter’s smart catalysts and substrates. These would control an exothermic reaction by turning off a hot reaction mix until it cooled.7 On the other hand, with two hydrophobic reactants hyper-Arrhenius behavior holds and the reaction shows a large jump in rate constant with increasing temperature. These are explosive conditions for an exothermic reaction. However, providing enough surfactant is present when all of the poloxamer is micellized to effectively dilute the reactants in the micellar pseudophase, then a slight maximum followed by something of a plateau ensues. Hence a reaction proceeding at the maximum rate is easily slowed by a small change in the temperature. LA027067F (7) Bergbreiter, D. E. J. Polym. Sci. 2001, 39, 2351-63.
