Langmuir 2007, 23, 6519-6525
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Alkaline Degradation of the Organophosphorus Pesticide Fenitrothion as Mediated by Cationic C12, C14, C16, and C18 Surfactants Xiumei Han,† Vimal K. Balakrishnan,‡ and Erwin Buncel*,† Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, Canada K7L 3N6, and National Water Research Institute, EnVironment Canada, Burlington, Ontario, Canada L7R 4A6 ReceiVed December 5, 2006. In Final Form: March 21, 2007 The effect of varying surfactant chain length (C12, C14, C16, C18) on the alkaline hydrolysis of the organophosphorus pesticide fenitrothion was determined for the following series of inert counterion cationic surfactants: dodecyltrimethylammonium bromide (DTABr), tetradecyltrimethylammonium bromide (TTABr), hexadecyltrimethylammonium bromide (CTABr), and octadecyltrimethylammonium bromide (OTABr). Plots of kobs versus [surfactant] at constant [KOH] showed saturation behavior at low total [Br-], and (constrained) S-shaped curvature was observed at high total [Br-]. kobs values increased with increasing surfactant chain length but decreased with added KBr. For systems exhibiting saturation behavior, further analysis of the results using the PPIE treatment as modified to account for HO-/Br- exchange allowed the evaluation of substrate binding constants, KS, and micellar rate constants, k2m. The binding constants increased with chain length (hydrophobicity), but ionic strength had no effect on KS. Meanwhile, because of the increased KS values as the surfactant chain length increased, the rate enhancements observed for fenitrothion degradation correspondingly increased. However, rate enhancements decreased with ionic strength because reactive counterions could not compete against the bromide anion for micellar binding sites. Low k2m/k2w ratios revealed that the observed rate enhancements were due to the so-called concentration effect rather than true catalysis. Finally, where the PPIE model failed (displaying S-shaped curvature), our results support the intervention of sphereto-rod transitions that are favored at high ionic strength (>0.01 M Br-) and lower temperatures as the cause of the S-shaped curvature.
Introduction Organophosphorus (OP) pesticides are widely used in agriculture for field crop and fruit tree protection against a variety of insects. The acute lethality of OP compounds against pests may be attributed to the inhibition of acetylcholinesterase (AChE), and this also leads to toxicity with respect to humans and other mammals. Because of concerns related to their broad-ranging toxicity, numerous studies have been carried out to examine their long-term behavior in the plant-soil-water environment.1-6 These have included studies of microbial and chemical degradation processes, both those that occur naturally and those that can be enhanced or inhibited by human intervention. Recent studies of the degradation of fenitrothion (1), [O,O-dimethyl-O-(3methyl-4-nitrophenyl) phosphorothioate], a broad-spectrum insecticide, with nucleophiles have revealed that surfactants could be used to accelerate its hydrolytic degradation.7,8 Surfactants generally form spherical micelles in aqueous solutions above the critical micelle concentration, cmc. The micelles can affect the rate of reaction by concentrating the * Corresponding author. E-mail:
[email protected]. Phone: (613) 533-2653. Fax: (613) 533-6669. † Queen’s University. ‡ Environment Canada. (1) Greenhalgh, R.; Dhawan, K. L.; Weinberger, P. J. Agric. Food Chem. 1980, 28, 102-105. (2) Wan, H. B.; Wong, M. K.; Mok, C. Y. Pestic. Sci. 1994, 42, 93-99. (3) Ohshiro, K.; Kakuta, T.; Sakai, T.; Hirota, H.; Hoshino, T.; Uchiyama, T. J. Ferment. Bioeng. 1996, 82, 299-305. (4) Lartiges, S. B.; Garrigues, P. P. EnViron. Sci. Technol. 1995, 29, 12461254. (5) Lacorte, S.; Barcelo´, D. EnViron. Sci. Technol. 1994, 28, 1159-1163. (6) Durand, G.; Mansour, M.; Barcelo´, D. Anal. Chim. Acta 1992, 262, 167178. (7) Balakrishnan, V. K.; Han, X.; vanLoon, G. W.; Dust, J. M.; Toullec, J.; Buncel, E. Langmuir 2004, 20, 6586-6593. (8) Balakrishnan, V. K.; Buncel, E.; vanLoon, G. W. EnViron. Sci. Technol. 2005, 39, 5824-5830.
reactants and providing a reaction environment that is different from that of the aqueous solution.9 Thus, rate acceleration or inhibition of organic reactions in micellar solutions will result from two factors: (a) different rates of reaction in the micellar phase and in bulk solution and (b) the altered distribution of the substrate between these two phases (i.e., a concentration effect).10 The first theoretical approach to this interionic competition is the pseudophase model developed by Menger and Portnoy,11a which suggests that substrate sorption takes place on the highly charged surface of surfactant aggregates. Thereafter, Quina and Chaimovich developed a framework for the quantitative dissection and analysis of the influence of the charged micellar pseudophase on reactions that involve exchangeable ionic species.11b The firstorder rate constant in the framework of this model is given by eq 1, where subscripts m and w indicate micellar and aqueous pseudophases, respectively.
kobs )
k2w[Nu]w + k2m[Nu]m Ks(CT - cmc) 1 + Ks(CT - cmc)
(1)
Scheme 1 illustrates the transport from the aqueous phase into the micellar phase via the Stern layer of a hydrophobic substrate (fenitrothion) and any introduced ionic species (HO-, Br-, hydrophilic). KS is the association constant of the substrate with the micelles, and k2w and k2m are second-order rate constants. [Nu-]m is the molar concentration of the ionic reactant within the micellar (9) Gra¨tzel, M., Kalyanasundaram, K., Eds. Kinetics and Catalysis in Microheterogeneous Systems; Surfactant Science Series; Marcel Dekker: New York, 1991; Vol. 38, Chapter 2. (10) Rubingh, D. N., Holland, P. M., Eds. Cationic Surfactants: Physical Chemistry; Surfactant Science Series; Marcel Dekker: New York, 1991; Vol. 37, Chapter 7. (11) (a) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 46984703. (b) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844-1850.
10.1021/la063521u CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007
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Scheme 1. Schematic Representation of the Pseudophase Ion Exchange (PPIE) Model of Micellar Catalysis
decompose fenitrothion.15 Herein, we extend our previous investigations to address the dual issues of cation size and competitive ion exchange between inert and reactive counterions in the micelle-catalyzed alkaline hydrolysis of fenitrothion in the presence of DTABr, TTABr, CTABr, and OTABr. KBr was used to maintain a constant inert counterion concentration throughout the reaction. The reaction is summarized in Scheme 2.
Results
pseudophase and can be expressed as in eq 2, where Vm is the molar volume of the micelles and θNu- is the fractional association of the nucleophile with the micelles.
[Nu-]m )
θNuVm
(2)
According to the PPIE model, it is assumed that the micellar surface is saturated with counterions and that counterion binding, β, is constant for inert counterion surfactants, as given by eq 3.12 The selectivity of the micellar surface toward different counterions can be described by an ion exchange constant KNU X , and competition between reactive and inert anions (Nu- and X-) is given by eq 4:12a
β ) θNu + θX KNu X )
[Nu]m[X]w [Nu]w[X]m
(3) (4)
Inert counterions reduce the rate of micellar-assisted reactions of reactive counterions with nonionic substrates as a result of competition at the micellar surface.11 Bunton et al.13 found that the rate constants for the reaction of p-nitrophenyl diphenylphosphate with hydroxide ions showed a maximum when the surfactant (CTABr) concentration was varied at constant concentrations of substrate and hydroxide ion. The appearance of a rate maximum was considered to be the consequence of two counteracting effects: (a) a rate enhancement due to the enhanced concentration of the reagents on the micelle surface and (b) a rate decrease due to the replacement of the reactive counterion by the inert micelle counterion with increasing surfactant concentration. Tee and Fedortchenko14 studied the hydrolysis of p-nitrophenyl alkanoates in CTABr micelles while maintaining a constant concentration of both bromide and hydroxide ion concentrations, thus ensuring that bromide/hydroxide anion exchange remained constant. This approach eliminated the rate maxima observed in earlier work, producing saturation-style curves. If the extent to which reactive counterions are bound to the micelle (θNu) is determined, then the saturation-style curves could readily yield the desired kinetic parameters (KS and k2m). Previously, we investigated the important role played by anionic micellar counterions in the degradation of fenitrothion, showing that R-nucelophilic counterions could be effectively used to (12) (a) Mittal, K. L.; Lindman, B. Surfactants in Solution; Plenum Press: New York, 1982; Vol. 2. (b) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357-364. (13) Bunton, C. A.; Robinson, L. J. Org. Chem. 1969, 34, 773-780. (14) Tee, O. S.; Fedortchenko, A. A. Can. J. Chem. 1997, 75, 1434-1438.
Second-order rate constants have been measured for the reaction of fenitrothion with hydroxide ion in water at 25.0 °C, with k2w ) 2.15 × 10-3 M-1 s-1. Reactions were carried out under pseudofirst-order conditions by monitoring the appearance of the leaving group, 3-methyl-4-nitro phenoxide, at 398 nm. The initial concentration of fenitrothion was 7.57 × 10-5 M for all reactions. To investigate the effect of surfactants of different chain length on hydrolysis, the reaction of fenitrothion with KOH (0.02 M) in the presence of the following cationic, inert counterion surfactants was studied at 25.0 °C. Detailed results are provided as Supporting Information: dodecyltrimethylammonium bromide (DTABr) (Tables 4S-6S), tetradecyltrimethylammonium bromide (TTABr) (Tables 7S-10S), hexadecyltrimethylammonium bromide (CTABr) (Tables 11S-14S), and octadecyltrimethylammonium bromide (OTABr) (Tables 15S-16S). The reaction of fenitrothion with hydroxide ion in the presence of DTABr was also studied at 40.0 and 60.0 °C (Tables 17S-18S) in order to investigate possible reasons for the S-shaped curvature observed at high salt concentrations (vide infra). In all cases, a constant inert counterion concentration was maintained with KBr, allowing the analysis by the PPIE model, for systems exhibiting saturation behavior. In the absence of added salts, the cmc was determined by electrical conductivity method using a conductivity meter. In the presence of salts, UV-absorption spectroscopy based on the tautomerism of benzoylacetone (BZA)16 was used to determine the cmc. The cmc values are given in Table 1S.
Discussion Below we analyze in detail the results of varying surfactant chain length (C12, C14, C16, C18) on the degradation rates obtained for the hydrolysis of fenitrothion at constant hydroxide ion concentration. kobs values exhibited a nonlinear dependence on surfactant concentration and also on bromide ion concentration, varying between saturation behavior at low total [Br-] to S-shaped curvature at high [Br-]. Insight into these characteristic rate behaviors is obtained through the PPIE model, which allowed the determination of the substrate-micellar surface binding constant (KS) and the secondorder rate coefficient (k2m) as well as other parameters (β, θOH-). Of special interest is evidence of a sphere-to-rod transition at high surfactant concentration and high added [Br-]. First, we briefly consider the overall effect of surfactant chain length and factors such as [Br-] on reaction rates and cmc. (15) (a) Han, X.; Balakrishnan, V. K.; vanLoon, G. W.; Buncel, E. Langmuir 2006, 22, 9009-9017. (b) Shirin, S.; vanLoon, G. W. Can. J. Chem. 2004, 82, 1674-1685. (c) Shirin, S.; Buncel, E.; vanLoon, G. W. Can. J. Chem. 2003, 81, 45-52. (d) Eneji, I. S.; Buncel, E.; vanLoon, G. W. J. Agric. Food Chem. 2002, 50, 5634-5639. (e) Balakrishnan, V. K.; Dust, J. M.; vanLoon, G. W.; Buncel, E. Can. J. Chem. 2001, 79, 157-173. (f) Omakor, J. E.; Onyido, I.; vanLoon, G. W.; Buncel, E. J. Chem. Soc., Perkin Trans. 2001, 2, 324-330. (g) Sha’ato, R.; Buncel, E.; Gamble, D. G.; vanLoon, G. W. Can. J. Soil Sci. 2000, 80, 301307. (h) Annandale, M. T.; vanLoon, G. W.; Buncel, E. Can. J. Chem. 1998, 76, 873-883. (i) Buncel, E.; vanLoon, G. W. Can. Chem. News 2005, 57, 18-20. (16) Domı´nguez, A.; Ferna´ndez, A.; Montenegro, L. J. Chem. Educ. 1997, 74, 1227-1231.
Alkaline Degradation of Fenitrothion
Langmuir, Vol. 23, No. 12, 2007 6521 Scheme 2
Effect of [Br-] on the Rate. The effect of [Br-] on kobs for the reaction of fenitrothion with DTABr, TTABr, CTABr, and OTABr is found in Figures 1-4 and Table 3S (Supporting Information). Clearly, Br- has a retarding effect on both the rate and on the observed rate enhancement. By contrast, Brinchi et al. found that Br- accelerates reactions of methyl naphthalene2-sulfonate in cetyltrialklyammonium bromide solutions.17 However, in the reactions with methyl naphthalene sulfonates, Br- acts as a nucleophile whereas bromide is inert toward phosphorus centers and serves merely as competition for the reactive species, HO-. Thus, increased [Br-] competes with HOfor binding sites at the micellar Stern layer, resulting in a decreased concentration of micellized nucleophile, thereby reducing the observed rate. Impacts on cmc. In the absence of added salts, the increasing chain length of the surfactant causes a decrease in cmc. However, the addition of Br- caused a significant reduction in cmc for each of the surfactants (Table 1S, Supporting Information). The formation of micelles is affected by both the hydrophobicity of the surfactant and the polarity of the bulk solution. The relative hydrophobicity of the surfactants increases with chain length, resulting in the thermodynamic threshold required for the formation of micelles (i.e., the formation of two pseudophases) to be reached at lower surfactant concentrations with longerchain-length surfactants. Meanwhile, the addition of bromide
Figure 1. Reaction of fenitrothion with 0.02 M KOH as a function of [DTABr] at 25 °C. Each curve represents the reaction at different [Br-]TOT through the addition of KBr.
Figure 2. Reaction of fenitrothion with 0.02 M KOH as a function of [TTABr] at 25 °C. Each curve represents reaction at different [Br-]TOT through the addition of KBr.
ions effectively shifts the equilibrium from monomeric surfactant units toward more aggregated species, thereby reducing the surfactant concentration required for the formation of micelles. Effect of Chain Length on the Rate. Varying the surfactant chain length has a large effect on kobs values for the reaction of fenitrothion with KOH. As an illustration, Figure 5 shows the effects of TTABr and CTABr on the rate at 0.02 M KOH with total [Br-] ) 0.006 M. Clearly, the rate with CTABr is much higher than that with TTABr. Indeed, the relative impact of chain length on rate was found to proceed as DTABr < TTABr < CTABr < OTABr. The effect of chain length on rate enhancement w (km obs/kobs) is given in Table 2S (Supporting Information), from which it is evident that the rate enhancement increases with surfactant chain length. The rationale for these observations becomes clearer upon application of the PPIE model of micellar catalysis, as given below. Application of the PPIE Model for Data Treatment. Our work has shown an interesting transformation in the dependence of kobs on surfactant concentration as mediated by the bromide ion concentration (Figures 1-4): at low [Br-], relatively good saturation plots are obtained, whereas pronounced S-shaped curvature was observed at high [Br-]. Meanwhile, at intermediate [Br-], the kinetic profiles neither conformed to saturation plots nor displayed pronounced S-shaped curvature; instead, these plots could be considered to be display constrained S-shaped curvature.
Figure 3. Reaction of fenitrothion with 0.02 M KOH as a function of [CTABr] at 25 °C. Each curve represents reaction at different [Br-]TOT through the addition of KBr.
Figure 4. Reaction of fenitrothion with 0.02 M KOH as a function of [OTABr] at 25 °C. Each curve represents reaction at different [Br-]TOT through the addition of KBr.
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Figure 8. Fit of corrected rate constants to the PPIE equation for the reaction of fenitrothion with 0.02 M KOH in the presence of CTABr at 25.0 °C. CBr- ) 0.003 M, and KS ) 850. R2 ) 0.9965.
Figure 5. Reaction of fenitrothion with 0.02 M KOH as a function of [surfactant], [Br-] ) 0.006 M, for TTABr (b) and CTABr (2) at 25 °C.
Figure 9. Fit of corrected rate constants to the PPIE equation for the reaction of fenitrothion with 0.02 M KOH in the presence of OTABr at 25.0 °C. CBr- ) 0.0006 M, and KS ) 685. R2 ) 0.9991. Figure 6. Fit of corrected rate constants to the PPIE equation for the reaction of fenitrothion with 0.02 M KOH in the presence of DTABr at 25.0 °C. CBr- ) 0.02 M, and KS ) 140. R2 ) 0.9916.
Figure 7. Fit of corrected rate constants to the PPIE equation for the reaction of fenitrothion with 0.02 M KOH in the presence of TTABr at 25.0 °C. CBr- ) 0.006 M, and KS ) 300. R2 ) 0.9978.
Figure 10. Variation of θOH- with the concentration of CTABr at different total bromide concentrations for the reaction of fenitrothion with 0.02 M KOH at 25.0 °C.
The corrected rate constants (kcorr) in the micellar phase were determined by taking into account three parameters: (1) the second-order rate constant in the micellar phase, k2m; (2) the binding constant, KS, for fenitrothion to micelles; and (3) the hydroxide concentration in the micellar phase, θOH-/Vm. Among
these three parameters, the first two arise directly from the curve fitting whereas the third parameter is determined empirically. θOH- is typically determined using standard conductivity or absorbance techniques in reactive counterion solutions where no competitive ions are present. However, in the current work, the presence of Br- precludes the direct determination of θOH-. Instead, having directly determined β (eq 3) and given the ionexchange constant, KOH/Br, between the reactive and inert anions at the micellar surface, θOH- was calculated as shown in the Appendix (Supporting Information). According to this treatment, θOH- was found to increase with increasing [CTABr] (Figure 10). However, the increase in θOH- was not uniform and varied with [Br-]TOT. Meanwhile, the volume element, Vm, is treated as being independent of surfactant or counterion concentration. Bunton et al.18 assumed that the reaction occurs within the Stern layer of the micelle and that the volume of 1 mol of micellized C16 surfactant is ca. 0.14 L. An alternative assumption made by Cuccovia et al.19 was to treat the reaction as if it occurred
(17) Brinchi, L.; Di Profio, P.; Germani, R.; Marte, L.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 2001, 243, 469-475.
(18) Bunton, C. A.; Hamed, F. H.; Romsted, L. S. J. Phys. Chem. 1982, 86, 2103-2108.
The reaction data for DTABr, TTABr, CTABr, and OTABr (Figures 6-9) at low [Br-] were described well by the PPIE model of micellar catalysis, as expressed by their fit to eq 5 (Appendix, Supporting Information).
kcorr ) kobs -
k2w[Nu]w
) 1 + KS(CT - cmc) θOH- k2mKS(CT - cmc) (5) Vm 1 + KS(CT - cmc)
( )
Alkaline Degradation of Fenitrothion
Langmuir, Vol. 23, No. 12, 2007 6523
Table 1. Impact of [Br-] on the Kinetic Parameters (KS and k2m) under the PPIE Model of Micellar Catalysis in the Alkaline Hydrolysis of Fenitrothion at 25 °C (at [HO-] ) 0.02 M)a surfactant
[Br-]total (M)
KS (M-1)
103k2m(M-1 s-1)
k2m/k2wb
DTABr
0.02 0.03
140 ( 22 123 ( 19
3.38 ( 0.08 2.09 ( 0.11
1.57 0.97
TTABr
0.006 0.01
300 ( 21 258 ( 19
3.16 ( 0.08 1.13 ( 0.09
1.47 0.53
CTABr
0.003 0.006
850 ( 61 550 ( 97
1.61 ( 0.06 0.91 ( 0.04
0.74 0.43
OTABr
0.0006 0.001
685 ( 60 720 ( 51
3.50 ( 0.06 2.26 ( 0.05
1.63 1.05
a Kinetic parameters were obtained by nonlinear regression analyses of eq 5 performed using GraphPad Prism software. a k2w ) 2.15 × 10-3 M-1 s-1.
throughout the global volume of the micelle, yielding Vm ) 0.37 L mol-1. However, Berr et al.20a used small-angle neutronscattering measurements to measure the total micellar volume of C16 surfactants to be 0.60 L mol-1. Furthermore, Leibner and Jacobus,20b in their study concerning micelle shape and size, presented a model that permitted the calculation of the total micelle volume according to eq 6, where n is the number of carbons in the chain.
Vm ) 0.0274 + 0.0269n
(6)
In the present study, eq 6 was employed to calculate the total molar volume of the micelles for DTABr, TTABr, CTABr, and OTABr. The values obtained for KS and k2m upon application of eq 5 are given in Table 1, and the significance of these values is discussed below. Fitting Parameters from the PPIE Model. Table 1 shows the k2m and KS values for the reaction of fenitrothion in the presence of DTABr, TTABr, CTABr, and OTABr at different total Br- concentrations. The k2m and KS values for plots that exhibited S-shaped curvature are not given in Table 3 because the reaction data did not satisfactorily fit the PPIE equation. As can be seen from Table 1, the binding constants obtained for the reaction of fenitrothion in the inert counterion surfactant solutions increase with increasing chain length except for OTABr, which has a binding constant similar to that of CTABr. The increased binding constants observed with increasing chain length are ascribed to the increased hydrophobic interaction of fenitrothion with the surfactant. It is well known that the solubility of organic solutes in micelles increases with surfactant chain length.21 This was illustrated in the study of Romsted and Cordes22 concerning the effect of various n-alkyltrimethylammonium bromides on the hydrolysis of p-nitrophenyl hexanoate and p-nitrophenyl dodecanoate, where the rates of ester hydrolysis were found to be greater for longer-chained esters and surfactants. Furthermore, the observation that the binding constants obtained for a given surfactant were constant within experimental error suggested that ionic strength has little effect on the binding of fenitrothion to micelles. w As seen from Table 1, the rate enhancement (km obs/kobs) observed at constant [KOH] for the degradation of fenitrothion in the presence of inert counterion surfactants increases with chain length, with values ranging from 1.4 to 11.7. We attribute (19) Cuccovia, I. M.; Schroter, E. M.; Monteiro, P. M.; Chaimovich, H. J. Org. Chem. 1978, 43, 2248. (20) (a) Berr, S.; Jones, R. R. M.; Johnson, J. S., Jr. J. Phys. Chem. 1992, 96, 5611-5614. (b) Leibner, J. E.; Jacobus, J. J. Phys. Chem. 1977, 81, 130-135. (21) Abraham, M. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 153-181. (22) Romsted, L. R.; Cordes, E. H. J. Am. Chem. Soc. 1968, 90, 4404-4409.
this behavior to the increased binding between fenitrothion and micelles (Table 1). By contrast, the ratio k2m/k2w ranges from 0.33 to 1.57 (Table 1). Taken together, these comparisons suggest that the major source of the rate acceleration observed for the reaction of fenitrothion with hydroxide ion in inert counterion surfactants is derived from the concentration of the reactants into the smaller volume of the micelles and is not strictly “true catalysis”. Balakrishnan et al.8 arrived at similar conclusions in a study of the reaction of fenitrothion in C16 surfactants with different counterions. It is also noted that there are variations observed in the k2m values obtained for any given surfactant at different [KBr], with increased salt concentrations leading to apparently decreased k2m values. This can be explained from the perspective that according to the PPIE model, Vm is treated as a constant (derived by eq 6). However, Vm actually decreases with increasing [KBr], leading to an underestimation of k2m values. Despite applying the corrections to the PPIE model of micellar catalysis to account for variations in θOH- as well as differences in Vm as described above, the fit to eq 5 remained poor at high [Br-], indicating that additional factors were responsible for the S-shaped curvature shown in Figures 1-4. Therefore, we concluded that the PPIE model for micellar catalysis was inadequate for data treatment under high salt conditions, leading us to consider an alternative explanation for our observations. Failure of the PPIE Model at High Salt Concentrations: Impact of the Sphere-to-Rod Transition. Numerous studies have provided compelling evidence that micelles can change shape from spherical to rodlike as the concentration of the surfactant and the ionic strength increase.23-25 This change in shape is also affected by the chain length of the surfactant, the temperature, and the addition of specific counterions. It has been reported that chain length can have a major influence on the sphere-to-rod transition process and, specifically, that longer tails with more carbons will favor rodlike micelles.25,26 This is also reflected in the systems studied here; a good fit to the PPIE equation is obtained for the reaction in the presence of DTABr at 0.02 M total Br-; however, the fit is poorer for the reaction in the presence of CTABr micelles at 0.006 and 0.015 M Br-. This observation is consistent with the view that, in contrast to DTABr, the longer-chained CTABr can form rodlike micelles at lower surfactant concentrations and with lower added salt concentrations. Temperature can also play a significant role in the transition of micelles from sphere to rodlike.25,27 As seen in Figure 3, the plot for the reaction of fenitrothion with KOH in DTABr at 0.05 M total [Br-] does not show saturation behavior but is essentially linear. To investigate the existence of the sphere-to-rod transition in this system, the reaction of fenitrothion with KOH in DTABr at 0.05 M total [Br-] was carried out at three different temperatures: 25.0, 40.0, and 60.0 °C (Figure 11). The plots show clear saturation behavior with increasing temperature, which is in accordance with the sphere-to-rod transition at higher surfactant concentration being favored at lower temperatures.27 Thus, we conclude that changes in micellar structure arising from the sphere-to-rod transition may cause the failure of the PPIE model of micellar catalysis at high total CBr-, which is manifested as an S-shaped curve observed in this study. (23) Dorshow, R.; Briggs, J.; Bunton, C.; Nicoli, D. J. Phys. Chem. 1982, 86, 2388-2395. (24) Imae, T.; Kamiya, R.; Ikeda, S. J. Colloid Interface Sci. 1985, 108, 215225. (25) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1983, 87, 1264-1277. (26) Vethamuthu, M. S.; Almgren, M.; Karlsson, G.; Bahadur, P. Langmuir 1996, 12, 2173-2185. (27) Zielinski, R. Pol. J. Chem. 1999, 73, 1819-1826.
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Experimental Section
Figure 11. Reaction of fenitrothion with 0.02 M KOH as a function of [DTABr] at (A) 25.0, (B) 40.0, and (C) 60.0 °C. CBr- ) 0.05 M.
Conclusions The effect of four inert counterion surfactants of varying chain length (DTABr, TTABr, CTABr, and OTABr) on the alkaline hydrolysis of fenitrothion was investigated in aqueous solution at 25.0 °C at ionic strengths ranging from 0.0006-0.05 M total Br-. Our results lead to the following conclusions: (1) The PPIE model can be extended systematically and successfully to encompass micellar systems containing inert competitive ions by first determining the relative amount of the reactive ion (HO-) associated with the micelle. (2) Binding constants for the reaction of fenitrothion with inert counterion surfactants generally increased with chain length (hydrophobicity), but ionic strength was found to have no effect on fenitrothion binding. (3) Because of the increased binding of fenitrothion with increasing chain length, the rate enhancement (RE ) kobs in the presence of surfactant/kobs in the absence of surfactant) observed for fenitrothion degradation in the presence of inert counterion surfactants also increased with increasing chain length but decreased with ionic strength because reactive counterions could not compete for micelle binding sites with the inert bromide anion. (4) The low k2m/k2w values (0.33 to 1.57) suggest that the observed rate enhancements are not true catalytic effects but are instead due to the increased concentration of the reactants within the micelles (i.e., a concentration effect). (5) The PPIE model breaks down and displays S-shaped curvature at higher ionic strengths ([Br-] > 0.01 M). However, at higher temperatures, a good fit to the kinetic model is obtained. We attribute this contrast in behavior to a change in micellar structure from spherical to rod-shaped that is favored at longer chain lengths, higher ionic strengths, and lower temperatures, which the PPIE model is as yet unable to resolve.
Materials. Reagents and solvents were commercial products used as received unless otherwise specified. Octadecyltrimethylammonium bromide (OTABr) was recrystalized from anhydrous ethanol.28 Reagent-grade 1,4-dioxane was refluxed over anhydrous stannous chloride under nitrogen, followed by distillation to remove peroxides. The distillate was then refluxed over sodium metal to remove water, followed by distillation and storage in the freezer under nitrogen.28 Distilled deionized water was boiled in a three-necked roundbottomed flask and degassed with nitrogen during cooling. Fenitrothion, [O,O-dimethyl-O-(3-methyl-4-nitrophenyl) phosphorothioate], was a gift from Sumitomo Chemical Company. It was purified by column chromatography using 20% diethyl ether-80% chloroform as the eluent. The purity of the product was checked by 1H NMR and 31P NMR.29 DTABr, TTABr, CTABr, and OTABr stock solutions were prepared by the same method. The required amounts of DTABr, TTABr, CTABr, and OTABr were weighed into a 100 mL volumetric flask to give the desired concentration in water. Potassium bromide was used to keep the total bromide concentration constant during the hydrolysis of fenitrothion in the presence of inert counterion surfactants. They were prepared by dissolving the required amount of KBr in 100 mL volumetric flasks to give the desired concentration in water. Determination of Micellar Parameters. In the absence of added salts, the cmc was determined by the electrical conductivity method using a conductivity meter (constructed at Queen’s University). In the presence of salts, UV absorption spectroscopy based on the tautomerism of benzoylacetone (BZA) was used to determine the cmc.16 Kinetic Measurements. Kinetic experiments were performed by UV-visible spectrophotometry using a Varian Cary 3 double-beam spectrophotometer, a Hewlett-Packard 8452A diode array spectrophotometer, or a Perkin-Elmer Lambda-20 spectrophotometer. Temperature was controlled in each of the spectrophotometers via a circulating water bath (HP 8452A and Lambda-20) or a Peltier device (Cary 3). All kinetic runs were followed under pseudo-firstorder conditions in which the concentration of nucleophiles was at least 10 times greater than the initial concentration of fenitrothion. The addition of solvent and nucleophile solution to the reaction cuvettes was done under nitrogen using a gastight syringe. The cuvettes were allowed to equilibrate thermally (25.0 ( 0.1 °C) in the cell holder for 40 min. After temperature equilibration, 20 µL of fenitrothion stock solution (9.54 × 10-3 M) was added to each cuvette, and the kinetic run was started. The initial concentration of fenitrothion in the cuvettes was 7.57 × 10-5 M. Initial repetitive scanning between 200 and 800 nm to follow the hydrolysis of fenitrothion in each of the surfactant solutions was carried out using the HP 8452A spectrophotometer. This gave information about the disappearance of fenitrothion, the appearance of product, and the isosbestic behavior of the reaction. The presence of isosbestic points indicates that no long-lived intermediates exist in the reaction pathway. The product of the degradation of fenitrothion with the nucleophiles studied here is 3-methyl-4-nitrophenoxide. It shows a maximum absorption at 398 nm (λmax); after λmax was found, the reaction of the nucleophile with fenitrothion was monitored at a fixed wavelength corresponding to the λmax determined using the Cary 3 or Lambda20 spectrophotometer. Generally, each reaction was followed for 3 half-lives, and the infinity absorbance value (Ainf) was taken after 10 half-lives. The pseudo-first-order rate constants (kobs) were obtained from linear plots of log(A∞ - At) versus time. The second(28) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed; Pergamon Press: New York, 1988. (29) Han, X. M.Sc. Thesis, Queen’s University, Kingston, ON, Canada, 2002. (30) Bartet, D.; Gamboa, C.; Sepulveda, L. J. Phys. Chem. 1980, 84, 272275. (31) Bonilha, J. B. S.; Chericato, G.; Martine-Franchetti, S. M.; Ribaido, E. J.; Quina, F. H. J. Phys. Chem. 1982, 86, 4941-4947. (32) Palepu, R.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1990, 86, 1535-1538.
Alkaline Degradation of Fenitrothion order rate constants (k2w) in the aqueous phase were determined from linear plots of kobs versus HO- concentration. The secondorder rate constants (k2m) in the micellar phase were determined from fitting the reaction data to eq 5.
Acknowledgment. Financial support of this research from the Natural Science and Engineering Research Council of Canada (NSERC) and Queen’s University is gratefully acknowledged.
Langmuir, Vol. 23, No. 12, 2007 6525
Supporting Information Available: Pseudo-first-order rate constants and cmc values as well as an appendix showing the calculation of θOH- have been provided as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. LA063521U
