Application of the Pseudophase Ion Exchange Model to a Micellar

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Application of the Pseudophase Ion Exchange Model to a Micellar Catalyzed Reaction in Water-Glycerol Solutions† Lavinel G. Ionescu,*,‡,§ Vera Lu´cia Trindade,§ and Elizabeth F. de Souza| Instituto de Quı´mica, Pontifı´cia Universidade Cato´ lica do Rio Grande do SulsPUCRS, Porto Alegre, RS, Brazil, Departamento de Quı´mica, CCNE, Universidade Luterana do BrazilsULBRA, Canoas, RS, Brazil, and Instituto de Cieˆ ncias Biolo´ gicas e Quı´mica, Pontifı´cia Universidade Cato´ lica de CampinassPUC-CAMPINAS, Campinas, SP, Brazil Received June 8, 1999. In Final Form: November 9, 1999 The experimental results obtained for the hydrolysis of p-nitrophenyldiphenyl phosphate (NPDPP) in the presence of sodium hydroxide (NaOH), micelles of cetyltrimethylammonium bromide (CTAB), and aqueous solutions of glycerol are described in terms of the pseudophase ion-exchange model (PPIE). The effect of glycerol on micelle formation and its influence on the reaction medium are also discussed. The results obtained indicate that the PPIE model is applicable for micellar catalysis in water-glycerol solutions.

Introduction Phosphate esters are compounds with interesting biological and pharmacological properties and are widely used as pesticides, drugs, and nerve gases. Their accumulation and their effect in the environment are of paramount importance.1,2 In previous studies we have shown that the hydrolysis of di- and trisubstituted phosphate esters is catalyzed by micelles of cetyltrimethylammonium bromide [C16H33N+(CH3)3Br-], N,N-dimethyl-N-hydroxyethyldodecylammonium bromide [DHEDAB, n-C12H25N+(CH3)2CH2CH2OHBr-], and N,N-dimethyl-N-hydroxyethylcetylammonium bromide [CHEDAB, n-C16H33N+(CH3)2CH2CH2OHBr-]3. Micelles of DHEDAB and CHEDAB are excellent catalysts for the hydrolysis of both lithium p-nitrophenyl ethyl phosphate (LiPNEP) and p-nitrophenyldiphenyl phosphate (NPDPP) in the presence of hydroxide ions, with over a 300-fold rate enhancement for the hydrolysis of the triaryl phosphate in the presence of CHEDAB. The catalytic effect and the dependence of the reaction rate on hydroxide ion concentration have been explained in terms of nucleophilic participation of the alkoxide ion of DHEDAB and CHEDAB, with pKa of 12.4 and 12.9, respectively, for the ionization of the hydroxyl groups. For reactions with fluoride ion, the hydroxy-substituted surfactants are no better catalysts than the corresponding alkyltrimethylammonium bromides, suggesting that electrophilic catalysis is relatively unimportant. Cetylpyridinium bromide [CPBr, C5H5N+C16H33Br-] has approximately the * To whom correspondence should be addressed at PUCRS, PO Box 1429-90619-900. Phone: 055 51 320 3549. Fax: 055 51 320 3612. E-mail: [email protected], LAVINEL@ MOZART.ULBRA.TCHE.BR. † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”. ‡ Pontifı´cia Universidade Cato ´ lica do Rio Grande do SulsPUCRS. § Universidade Luterana do BrazilsULBRA. | Pontifı´cia Universidade Cato ´ lica de CampinassPUC-CAMPINAS. (1) Gunther, F. A.; Gunther, J. D. Chemistry of Pesticides; Sringer Verlag: New York, 1971. (2) Salazar, C.; Souza, G. M.; Silva, C. P. D. Manual de Inseticidas e AcaricidassAspectos Toxicologicos; UFPel: Pelotas, Brazil, 1976. (3) Bunton, C. A.; Ionescu, L. G. J. Am. Chem. Soc. 1973, 95, 2912.

same effect as CTAB at low hydroxide concentration and a slightly more pronounced effect with fluoride ion. Zwitterionic surfactants such as lauryl carnitine chloride (LCCl) and palmityl carnitine chloride (PCCl) have little effect on the rate of hydrolysis of LiPNEP.4,5 The addition of primary amines increased the rate of reaction in the presence of CTAB and CHEDAB for the triaryl phosphate, but much of the increase was due to attack by amine on the aryl group. In the absence of micelles, amines increased the overall rate of the reaction by attacking the aryl group without markedly catalyzing hydrolysis.6 The micellar catalyzed oxidative cleavage of a carboncarbon bond in Dicofol7 and the micellar catalyzed dehydrochlorination of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and some of its derivatives have also been subject of our investigations.8,9 In more recent studies we have reported results obtained for the hydrolysis of p-nitrophenyldiphenyl phosphate in aqueous solutions in the presence of micelles of diethylheptadecylimidazolinium ethyl sulfate (DEHIES) and CTAB and sodium hydroxide and dimethyl sulfoxide (DMSO) and analyzed the effect of internal pressure of the medium, dielectric constant, donor number and polarity of the solvent, and the effect of DMSO on micellization.10-14 The PPIE model has been used to describe the micellar catalysis in various systems; however, it is well-known that it fails at high ion or cosolvent concentrations.15,16 This is not surprising, since the model assumes a constant micellar structure and involves partitioning of the sub(4) Ionescu, L. G.; Martinez, D. A. J. Colo.-Wyo. Acad. Sci. 1974, 7, 13. (5) Ionescu, L. G. Bull. N. M. Acad. Sci. 1973, 14, 65. (6) Bunton, C. A.; Diaz, S.; Hellyer, J. M.; Ihara, I.; Ionescu, L. G. J. Org. Chem. 1975, 40, 2313. (7) Nome, F.; Schwingel, E. W.; Ionescu, L. G. J. Org. Chem. 1980, 45, 705. (8) Nome, F.; Rubira, A.; Ionescu, L. G. J. Phys. Chem. 1982, 86, 1181. (9) Ionescu, L. G.; Nome, F. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1894; Vol. 2, p 1107. (10) Ionescu, L. G.; Souza, E. F. South. Braz. J. Chem. 1993, 1, 75. (11) Ionescu, L. G.; Souza, E. F. In Surfactants in Solution; Chattopadhyay, A. K., Mittal, K. L., Eds.; Marcel Dekker: New York, 1996; Vol. 64, p 123. (12) Ionescu, L. G.; Souza, E. F. South. Braz. J. Chem. 1995, 3, 63. (13) Ionescu, L. G.; Rubio, D. A. R.; Souza, E. F. South. Braz. J. Chem. 1996, 4, 59. (14) Souza, E. F.; Ionescu, L. G. Colloids Surf., A 1999, 149, 609.

10.1021/la990734t CCC: $19.00 © 2000 American Chemical Society Published on Web 01/06/2000

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Scheme 1

strate and the reactant between the aqueous and micellar phases and a fixed number of reaction sites. Other models, such as those proposed by Hall17 that consider the transition state and by Bunton and Moffatt18,19 that use the Poisson-Boltzmann equation to obtain a Coulombic model, take into consideration some of the shortcomings mentioned above. Bunton et al.15 explained the discrepancy between the theoretical models and the experimental results by considering an additional reaction pathway across the micellar boundary at the shear surface between the Stern and Gouy-Chapmann layers of the micelle at high electrolyte concentrations. In the presence of a cosolvent, which may also modify the Stern layer of the micelles, it is also worthy to consider the possibility of changes in their size and shape. The present paper deals with the study of the hydrolysis of p-nitrophenyldiphenyl phosphate (NPDPP) in the presence of micelles of cetyltrimethylammonium bromide (CTAB) in aqueous solutions containing glycerol (G), as illustrated in Scheme 1. Experimental Section Materials. The p-nitrophenyldiphenyl phosphate (NPDPP) was prepared using standard methods.20-22 A sample was also obtained from Professor Fred Menger, Emory University, Atlanta, GA. The surfactant cetyltrimethylammonium bromide (CTAB) was purchased from Aldrich Chemical Co. and recrystallized three times from absolute ethanol before use. Glycerol and sodium hydroxide were analytical reagent grade and were purchased from Merck Co. Kinetic Measurements. The hydrolysis of p-nitrophenyldiphenyl phosphate was studied spectrophotometrically by measuring the rate of appearance of the p-nitrophenoxide anion at 4030 Å with a Varian DMS-80 spectrophotometer equipped with a temperature-controlled cell compartment. The reaction was studied at 15, 25, and 35 °C at various concentrations of NaOH, CTAB, and glycerol. The pseudo-first-order rate constant (kΨ), in s-1, was determined from linear plots of the logarithm of absorbance versus time, and the second-order rate constants (k2m) in the micellar phase and (k20) in the aqueous phase, in s-1 M-1, were calculated from kΨ and the hydroxide ion concentration. Activation parameters such as the activation energy (Ea), the activation entalphy (∆Hq), and the activation entropy (∆Sq) were determined from experimental kΨ values measured at three different temperatures using the following equations

ln kΨ ) ln A - (Ea/R)(1/T)

(1)

∆Hq ) Ea - RT

(2)

∆Sq ) 4.576 (log kΨ - 10.753 - log T + Ea/4.576T) (3) where R corresponds to the gas constant and T to the absolute temperature. (15) Bunton, C. A.; Romsted, L. S.; Savelli, G. J. Am. Chem. Soc. 1979, 101, 1253. (16) Lapinte, C.; Viout, P. Tetrahedron 1979, 35, 1931. (17) Hall, D. G. J. Phys. Chem. 1987, 91, 4287. (18) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1985, 89, 4166. (19) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986, 90, 538. (20) Ross, A. M.; Toet, J. Tev. Trav. Chim. 1958, 77, 1946. (21) Kirby, A. S.; Jounas, J. J. Chem. Soc. B 1970, 1165. (22) Menger, F. M.; Gon, L. H.; Johnson, E.; Durst, D. H. J. Am. Chem. Soc. 1987, 109, 2802.

Figure 1. Rate profiles for the hydrolysis of p-nitrophenyldiphenyl phosphate at 25 °C in aqueous solutions containing 0.005 M NaOH in the presence of CTAB and concentrations of glycerol varying from 0 to 90 vol %.

Results and Discussion Typical profiles of the pseudo-first-order rate constants, kΨ, as a function of the concentration of CTAB for the hydrolysis of NPDPP at 25 °C in aqueous solutions containing 0.005 M NaOH and concentrations of glycerol varying from 10 to 80 vol % are presented in Figure 1. The experimental rate profiles obtained are characteristic of micellar catalyzed reactions in aqueous solutions. The addition of CTAB to the reaction medium causes an increase in the rate of hydrolysis up to a point where there is total incorporation of the substrate in the micellar phase. Subsequent addition of the surfactant causes a decrease in the reaction rate, probably due to the dilution of the reactive counterions in the Stern layer of a higher number of micelles. There is a well-defined maximum in all the rate profiles at 20 × 10-4 M CTAB. This maximum has the same value as that observed for CTAB-H2O-NaOH. This behavior is totally different from that observed for solutions containing CTAB-H2O-NaOH-DMSO (dimethyl sulfoxide) and H2O-NaOH-DMSO-DEHIES (diethylheptadecylimidazolinium ethyl sulfate), where the maximum shifts for higher concentration of DMSO and the profiles exhibit three different types of behavior, indicating changes in the mechanism of the reaction.10-14 For the system of CTAB-H2O-NaOH-G, the mechanism of the micellar catalyzed reaction apparently does not change, but the catalytic effect of CTAB (ratio of the reaction rate in the presence and absence of surfactant under the same experimental conditions) is reduced gradually by the addition of glycerol. Figure 2 illustrates a typical plot of the pseudo-firstorder rate constant for the hydrolysis of p-nitrophenyldiphenyl phosphate at 25 °C as a function of the volume of glycerol for solutions containing 0.005 M NaOH and 20 × 10-4 M CTAB. As can be observed, the pseudo-firstorder rate constant decreases exponentially as a function of the volume percent of glycerol. It is well-known that the addition of glycerol decreases the spontaneity of micelle formation in water and that the critical micellar concentration (cmc) of the surfactant in aqueous solutions increases as a function of the added

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Ionescu et al. Table 1. Activation Parameters for the Hydrolysis of NPDPP at 25 °C in Aqueous Solutions of 0.005 M NaOH in the Presence and in the Absence of CTAB and of Glycerol

Figure 2. Plot of the pseudo-first-order rate constant for the hydrolysis of p-nitrophenyldiphenyl phosphate at 25 °C in aqueous solutions containing 0.005 M NaOH and 20 × 10-4 M CTAB as a function of the volume percent of glycerol.

Figure 3. Dependence of the critical micellar concentration (cmc) of CTAB in aqueous solutions as a function of the volume percent of glycerol: (b) 25 °C; (2) 40 °C.

volume of glycerol.23-27 Figure 3 illustrates the dependence of the cmc of CTAB at 25 and 40 °C for CTAB-H2O-G solutions and clearly indicates that the cmc increases exponentially as a function of the volume of glycerol.25-27 Liquids such as formamide (F), ethylene glycol (EG), and glycerol (G) are solvents similar to water, and micelle formation in all of them is common, although it is less spontaneous than that in water. The solvophobic effect per methylene group (-CH2-) measured for acylcarnitines was -0.69 kcal mol-1 for water, -0.18 kcal mol-1 for glycerol, -0.17 kcal mol-1 for ethylene glycol, and -0.17 kcal mol-1 for formamide.23-25 The effect of these three cosolvents on micelle formation in aqueous solutions can be explained in terms of their (23) Ionescu, L. G.; Fung, D. S. Bull. Chem. Soc. Jpn. 1981, 54, 2503. (24) Ionescu, L. G.; Fung, D. S. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2907. (25) Ionescu, L. G.; Romanesco, R. S.; Nome, F. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1982; Vol. 2, p 789. (26) Probst, S. M. H. MSc. Thesis, Universidade Federal de Santa Catarina, Floriano´polis, S.C., Brazil, 1982. (27) Ionescu, L. G. Contrib. Cient. Tecnol. Santiago 1985, NS, 35.

cosolvent (% by vol)

CTAB (M × 104)

Ea (kcal mol-1)

∆H°q (kcal mol-1)

∆S°q (eu)

glycerol (10%) glycerol (10%) glycerol (10%)

15 20 18 20 30

+15.2 +11.4 +10.5 +10.3 +10.0 +9.4

+14.6 +10.8 + 9.9 +9.7 +9.4 +8.8

-22.2 -26.6 -29.9 -30.1 -31.6 -34.2

breaking up the structure of water diminishing the hydrophobic effect in the ternary systems (surfactantwater-cosolvent). The breakup of the water structure is due to formation of complexes of the type EG‚2H2O, F‚ H2O, and G‚2H2O through hydrogen bonding. Experimental studies, using various techniques including NMR, of aqueous solutions of glycerol and ethylene glycol have confirmed the existence of inter- and intramolecular hydrogen bonding and have shown that the hydrogen bonds between either one of the two and water are stronger than those among themselves.28-30 Table 1 summarizes some typical activation parameters measured for the reaction at 25 °C. As can be noted, the addition of CTAB in the form of micelles decreases the activation energy, Ea, of about 5 kcal mol-1 as compared to aqueous solutions containing only NaOH. The addition of glycerol decreases both the activation energy and the activation entropy, suggesting a more structured transition state. Most of the models proposed for micellar catalysis31-36 consider the partition coefficient for the substrate between the micellar and aqueous phase and the distribution of the reagents between the two phases. The hydrolysis of NPDPP with hydroxide ion in the presence of CTAB may be considered a bimolecular reaction of OH- ion and the substrate. Since the concentration of OH- in the micellar phase is dependent on the concentration of both bromide ions and surfactant, a quantitative treatment of the reaction rate must consider ion exchange on or near the micellar surface. For the reaction under consideration, the model proposed by Quina and Chaimovich35 reduces to eq 4, which gives the theoretical dependence of the pseudo-first-order constant, kΨ, as a function of the total hydroxide ion concentration

kΨ )

{(k2m/V)KsKOH/Br[(Br)m/(Br)w} + k20}(OH)T (1 + KsCD)[1 + KOH/Br(Br)m(Br)w]

(4)

where CD is the concentration of micellized surfactant, V is the molar volume of the reactive region at the micellar surface, kΨ is the pseudo-first-order rate constant, k2m is the second-order rate constant in the micellar phase, k20 (28) Zinchenko, V. D.; Mank, V. V.; Moisev, V. A. Ukr. Khim. Zh. 1977, 43, 371. (29) Ueda, M.; Urahata, T.; Katyam, A.; Kuroki, N. Seni Gakkaishi 1977, 32, T301. (30) Beaudoin, J. L. J. Chim. Phys. 1977, 74, 268. (31) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (32) Berezin, I. V.; Martinek, K.; Yatsimirskii, A. K. Russ. Chem. Rev. 1973, 42, 787. (33) Martinek, K.; Yatsimirskii, A. K.; Levasov, A. V.; Berezin, I. V. InSolubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 489. (34) Romsted, L. S. In Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 509. (35) Quina, F.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844. (36) Otero, C.; Rodenas, E. Can. J. Chem. 1984, 63, 2892.

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Figure 4. Plots of the experimental pseudo-first-order rate constant for the hydrolysis of p-nitrophenyldiphenyl phosphate at 25 °C in the presence of 20 × 10-4 M CTAB and 0.005 M NaOH for different concentrations of glycerol and fits of the experimental data with the pseudophase ion exchange model: (9) experimental data; (s) calculated values.

is the second-order constant in the aqueous phase, KOH/Br is the ion exchange constant, Ks is the binding constant for the substrate, (Br)m is the concentration of Br- in micellar phase, (Br)w is the concentration of Br- in aqueous phase, (OH)T is the total concentration of hydroxide ions. With substrates such as p-nitrophenyldiphenyl phosphate that are very insoluble in water and are solubilized by CTAB, the expression for kΨ can be reduced to a simpler form given by

kΨ )

k2m KOH/Br[(Br)m/(Br)w] (OH)T CDV 1 + KOH/Br(Br)m(Br)w

(5)

The concentration of Br- in the micellar and aqueous phases can be obtained using the following equations36

A1 ) CD + cmc + KOH/Br(OH)T + (1 - R)CDKOH/Br (6) (OH)m ) (-A1) ) [(A1)2 + 4(1 - KOH/Br)(OH)TKOH/Br(1 - R)CD] 2(1 - KOH/Br) (7) (Br)m ) (1 - R)CD - (OH)m

(8)

(Br)w ) RCD + cmc + (OH)m

(9)

where cmc is the critical micellar concentration, R is the degree of ionization of the micelle, and (OH)m is the concentration of OH- in the micellar phase. We have calculated the theoretical values of kΨ for the reaction discussed above using various concentrations of glycerol, ranging from 0 to 80 vol %, taking into account the different critical micellar concentrations of CTAB. The basic underlying assumption was that the nature of the reaction medium does not change drastically the reaction mechanism. The fitting parameters used to describe the kinetics results are presented in Table 2. It is important to remember that not only the micellization process itself, as indicated by the increase of the cmc in aqueous solutions

Table 2. Fitting Parameters Used To Describe the Kinetics Results glycerol (% by vol)

concn of glycerol (M)

R

V (M-1)

k2m (s-1)

0 10 20 30 40 50 60 70 80

0 1.369 2.739 4.108 5.447 6.847 8.212 9.585 10.955

0.2 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7

0.14 0.14 0.18 0.25 0.25 0.25 0.25 0.25 0.37

0.06 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

as a function of the added volume of glycerol, but also the structures of aqueous ionic micelles can be perturbed by the organic cosolvent. Incorporation of a nonionic cosolvent should decrease the micellar charge densities and the electrostatic attraction for counterions. In this case, the degree of micellar ionization, R, is increased and the corresponding degree of micellar coverage, β, is decreased. For instance, the degree of micellar ionization of CTAB increases from about 0.25 in water to 0.68 in 0.8 M 1-butanol.37,38 Taking in account the high molar concentrations of glycerol used in this work, between 1.369 and 10.995 M, we assumed a constant R of 0.7. More over, the second-order rate constants at micellar surfaces depend on the estimation of the molar volume of reaction, V. The reactive volumes at the micellar surfaces should be approximately proportional not only to the concentration of micellar headgroups but also to the cosolvent bounded. The accepted values of V range from 0.14 to 0.37 M-1, 34,35,39 therefore we assumed a volume of reaction at the micelle surface increasing from 0.14 for the micelles in aqueous medium to 0.37 for those formed within the medium with 80 vol % of glycerol. The other parameter used was KOH/Br ) 0.08 for the various glycerol-water solutions. The results obtained for the hydrolysis of (37) Bertoncini, C. A.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5875. (38) Bertoncini, C. A.; Neves, M. F. S.; Nome, F.; Bunton, C. A. Langmuir 1993, 9, 1274. (39) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213.

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p-nitrophenyldiphenyl phosphate at 25 °C in aqueous solutions containing 0.005 M NaOH, for various concentrations of CTAB and glycerol, are presented in Figure 4(a-i). As can be seen, there is a good agreement between the experimental and calculated values of the pseudofirst-order rate constants at all concentrations of glycerol. For smaller contents of glycerol, it was expected that micellar medium should be more similar to that when only water, surfactant, and sodium hydroxide were present in solution. However, the description of the experimental results is better for higher contents of glycerol, indicating that there is an effect of the bounded cosolvent upon the structure of water or upon the ion solvation within the Stern layer that is more pronounced at smaller concentrations of cosolvent. The complete disruption of the water structure by the formation of G‚2H2O complexes seems to reduce this effect. Besides that, at higher concentrations of cosolvent, the micellization process itself is less spontaneous and the micelles formed under these conditions may have sizes and shapes different from those formed in aqueous medium. The reduction of the aggregation number of sodium lauryl sufate from 65 in pure water to 22 in 0.98 M 1-butanol is an example of this effect.40 On the other hand, highly hydrophobic substrates such as PNDPP should interact with any small aggregate of surfactant ions formed even below the measured cmc. (40) Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A. Lagmuir 1994, 10, 1151.

Ionescu et al.

In this case, as the surfactant concentration is increased and true micelles are formed, the result can be seen as a kind of dilution of the micellar-bound reactants. The kinetic results obtained at higher concentrations of glycerol, where the starting concentrations of the fittings and the rates maxima are bellow the cmc, seem to be consistent with these ideas. Conclusion The good agreement between the experimental and the theoretical values of kΨ calculated with the pseudophase ion exchange model at all concentrations of glycerol for the alkaline hydrolysis of p-nitrophenyldiphenyl phosphate shows that this model is applicable for micellar catalysis in water-glycerol solutions. These results also indicate that the alterations in the micellar medium can be described by the selected fitting parameters and that the reaction mechanism in the ternary system is similar to that obtained in aqueous solutions of surfactant and sodium hydroxide. Acknowledgment. The authors acknowledge the sample of p-nitrophenyldiphenyl phosphate obtained from Professor Fred M. Menger, Emory University, Atlanta, GA. L.G.I. acknowledges the financial support from CNPq, Brazil, and Sarmisegetusa Research Group, Santa Fe, NM. E.F.S. acknowledges the financial support from FAPESP, Sa˜o Paulo, Brazil. LA990734T