Effects of Sulfobetaine−Sodium Decyl Phosphate Mixed Micelles on

Langmuir 2000, 16, 10131-10136

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Effects of Sulfobetaine-Sodium Decyl Phosphate Mixed Micelles on Deacylation and Indicator Equilibrium Byung Sun Lee and Faruk Nome* Departamento de Quı´mica, Universidade Federal de Santa Catarina, Floriano´ polis, SC 88040-900, Brazil Received July 10, 2000. In Final Form: October 3, 2000 Critical micelle concentrations were measured for mixed micelles of sodium decyl phosphate (NaDeP) and sulfobetaines (CnH2n+1N+Me2(CH2)3SO3-, SB3-n, n ) 10, 12, 14, 16), and values of pHapp were calculated from spectroscopic data on the extent of deprotonation of 1-dodecylpyridinium-3-aldoxime bromide. For 0.1 M total surfactant, rate constants of hydrolyses of 2,4-dinitrophenyl acetate and octanoate (DNPA and DNPO, respectively) and benzoic anhydride (Bz2O) were fitted quantitatively in terms of reactions of OHand decyl phosphate ion on the assumption that sulfobetaine micelles increased the reactivity of decyl phosphate ion.

Introduction Kinetic studies of micellar effects have focused largely on bimolecular reactions of ionic reagents.1 These effects on rates, and also on equilibria, are treated quantitatively in terms of pseudophase models, and rate constants of reaction at micellar surfaces can be estimated and are generally not very different from those in water.1 Recently, it has been demonstrated that mixtures of sodium dodecyl sulfate (SDS) and sodium dodecanoate (SDOD) inhibit attack of OH- on 2,4-dinitrophenyl acetate and octanoate (DNPA and DNPO, respectively) and benzoic anhydride (Bz2O)2,3 but introduce a new reaction path that proceeds via formation of a labile carboxylic anhydride by nucleophilic attack.4 The dependence of the rate constants on the mixed surfactant concentration follows a simple pseudophase model and, indeed, increases linearly as a function of the molar fraction of dodecanoate ion in the micelle.3 Examining the same reactions in mixtures of SDOD and sulfobetaines (CnH2n+1 N+Me2 (CH2)3 SO3-, SB3-n, n ) 10, 12, 14, 16) showed that, for 0.1 M total surfactant, first-order rate constants for the hydrolyses of DNPA, DNPO, and Bz2O go through maxima at mole fractions of SDOD of ca. 0.5 for DNPA and DNPO and 0.8 for Bz2O.5 The observed rate effects were ascribed to decreased hydration and ion pairing of the dodecanoate ion in the mixed micelles of SDOD and the SB3-n surfactants. Therefore, incorporation of dodecanoate ion in SB3-n slows reaction by “dilution” in the micelle, but this inhibition is offset by activation of the nucleophilic dodecanoate ion.5 Since it has recently been demonstrated6,7 that addition of benzyltrimethylammonium n-decyl phosphate (I), in * Corresponding author. E-mail: [email protected]. (1) (a) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, S. L. Acc. Chem. Res. 1991, 24, 357. (b) Bunton, C. A. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, 1991; Vol. II, p 17. (2) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (3) Marconi, D. M. O.; Frescura, V. L. A.; Zanette, D.; Nome, F.; Bunton, C. A. J. Phys. Chem. 1994, 98, 12415. (4) (a) Gold, V.; Oakenfull, D. G.; Riley, T. J. Chem. Soc. B 1968, 515. (b) Bender, M. L.; Turnquest, B. W. J. Am. Chem. Soc. 1957, 79, 1652. (c) Bunton, C. A.; Fuller, N. A.; Perry, S. G.; Shiner, V. J. J. Chem. Soc. 1963, 2918. (5) Frescura, V. L. A.; Marconi, D. M. O.; Zanette, D.; Nome, F.; Bunton, C. A.; Blasko, A. J. Phys. Chem. 1995, 99, 11494. (6) Machado, V. G.; Nome, F. J. Chem. Soc., Chem. Commun. 1997, 1917.

aqueous acetonitrile, enables its reaction with DNPA (II) by nucleophilic addition to the carbonyl group (Scheme 1) and that the rate of the reaction is strongly dependent on solvent composition, we decided to examine the effect of SB3-n/sodium decyl phosphate mixtures on overall hydrolysis of DNPA, DNPO, and Bz2O. Since these reactions are affected by pH, we used 1-dodecylpyridinium-3aldoxime bromide (DPA), a hydrophobic indicator, to measure apparent values of the pH at the surface of SB3n/sodium decyl phosphate mixed micelles. Experimental Materials. The esters8 and Bz2O9 were prepared and purified as described. Purification of the SB3-n (Sigma) surfactants has already been described.10 The N-dodecyl-3-pyridiniumaldoxime bromide (DPA) probe11 and NaDeP12,13 were prepared as previously reported. The cmc of the mixed surfactants was measured tensiometrically in 0.01 M borate buffer, pH 8.8. Methods. Surface tension measurements were carried out, as previously described for SDS/NaDeP surfactant mixtures,12 by use of a Microquimica model MQ-ST1 surface tensiometer, based on the drop weight method. Most of the experiments in the presence of surfactants were carried out at 35 °C to ensure that the decyl phosphate remained in solution at high salt concentration. Kinetics. Reactions were followed in aqueous 0.01 M borate buffer at 35 ( 0.1 °C, in the water-jacketed cell compartments of both HP UV 8452-A diode array and Shimadzu UV 210 spectrophotometers, at 360 nm for DNPA and DNPO and 244 nm for Bz2O. Measurements of the observed first-order rate constants, kobs, s-1, were generally done at a substrate concentration of 10-4 M, and substrates were added in 1,4-dioxane. The kinetic solutions contained 0.3% 1,4-dioxane. All reactions gave strict first-order kinetics over more than five half-lives. All reported first-order rate constants represent the average of at least two kinetic experiments and were estimated from the change in absorbance as a function of time for at least 90% of the reaction (7) Machado, V. G.; Bunton, C.; Zucco, C.; Nome, F. J. Chem. Soc., Perkin Trans. 2 2000, 169. (8) Chattaway, F. J. Chem. Soc. 1931, 134, 2495. (9) Al-Lohedan, H.; Bunton, C. A. J. Org. Chem. 1982, 47, 1160. (10) Brochsztein, S.; Filho, P. B.; Toscano, V. G.; Chaimovich, H.; Politi, M. J. Phys. Chem. 1990, 94, 6781. (11) Epstein, J.; Kaminski, J. J.; Bodor, N.; Enever, R.; Sowa, J.; Higuchi, T. J. Org. Chem. 1978, 43, 2816. (12) Froehner, S. J.; Nome, F.; Zanette, D.; Bunton, C. A. J. Chem. Soc., Perkin Trans 2 1996, 673. (13) Lima, C. F.; Nome, F.; Zanette, D. J. Colloid Interface Sci. 1997, 187, 396.

10.1021/la000973c CCC: $19.00 © 2000 American Chemical Society Published on Web 12/02/2000

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Lee and Nome

Scheme 1

Table 2. Critical Micelle Concentrations at Different Molar Fractions of NaDeP in NaDeP/SB3-n (n ) 10, 12, 14, 16) Mixturesa 102 cmc, M

Bz2O

pH

103 kobs,a s-1

103 kCAT,b M-1 s-1

103 kobs,a s-1

103 kCAT,b M-1 s-1

6.5 6.8 7.2 7.8 8.5

0.37 0.46 0.69 1.49 2.56

2.55 3.31 4.17 5.02 6.32

1.28 1.65 2.76 4.51 7.41

8.20 10.8 12.3 16.2 21.2

a At 35.0 °C, extrapolated to zero buffer. b Calculated from k obs vs [Phosphate] in the range 0.01-1.0 M buffer.

by using an iterative least-squares program; correlation coefficients, r were >0.999 for all kinetic runs. The second-order rate constant for catalysis by the monoanionic and dianionic forms of phosphate buffer on DNPA and Bz2O were estimated from data obtained as a function of pH and buffer concentration (0.011.00 M). Rate constants of hydrolyses of DNPA and Bz2O over a range of pH were estimated by extrapolation of rate constants to zero buffer. Indicator Measurements. Absorbances were measured at 35.0 ( 0.1 °C in an HP UV 8452-A diode array spectrophotometer. The stoichiometric concentration of the probe, DPA, was 1.66 × 10-5 M. The ratio of dissociated to undissociated forms of the probe, in 0.01 M phosphate and borate buffers, in the presence and absence of surfactant, was obtained via eq 1 from absorbances of the mixture (Amix) and the undissociated (ADPA) and dissociated (ADPA-) forms of the probe. Measurements were at λmax for the dissociated form of the probe, which is at 340 nm in aqueous solutions, at 356 nm in the presence of SB3-10, at 358 nm for SB3-12 and SB3-14, and at 360 nm for SB3-16.

[DPA]/[DPA-] ) (ADPA- - Amix)/(Amix - ADPA)

(1)

The pHapp values were calculated from Eq 2 using the reported value of 8.33 as the pKa of the probe5 and represent the average of at least three independent measurements.

pHapp ) pKa - log([DPA]/[DPA-])

SB3-10

SB3-12

SB3-14

SB3-16

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

2.911 3.020 3.436 4.130 4.613 -

0.204 0.277 0.322 0.360 0.443 0.555 0.995 1.879

0.033 0.033 0.041 0.047 0.052 0.065 0.083 0.104 0.150 0.288

0.015 0.017 0.020 0.021 0.027 0.036 0.037 0.050 0.074 0.123

a

Table 1. Hydrolyses of DNPA and Bz2O in Water at Various pH Valuesa DNPA

χNaDeP

(2)

Results and Discussion Reactions in Water. First-order rate constants, kobs, (extrapolated to zero buffer) at 35.0 °C, increased with pH > 6 for DNPA and Bz2O (Table 1), in agreement with earlier results.3,9,14 We could not examine the hydrolysis of DNPO because of its insolubility. Table 1 also includes the second-order rate constants (kCAT) for the attack of H2PO4- and HPO4-2 on DNPA, which were calculated from the dependence of the rate constant on [Buffer] at different pH values. Plots of kCAT versus χHPO4-2 allowed calculation of kH2PO4- ) 1.92 × 10-3 M-1s-1 and kHPO4-2 ) 6.28 × 10-3 M-1s-1. Also included in Table 1 are the second-order rate (14) Bender, M. L.; Turnquest, B. W. J. Am. Chem. Soc. 1957, 79, 1652.

At 25.0 oC, pH 8.8, 0.01 M borate buffer.

constants in relation to buffer catalysis, at different pH values, for the attack of H2PO4- and HPO4-2 on Bz2O, which allowed us to calculate values of kH2PO4- ) 5.71 × 10-3 M-1s-1 and kHPO4-2 ) 2.06 × 10-2 M-1s-1, respectively. In the hydrolytic reactions of both DNPA and of Bz2O, the dianionic form HPO4-2 is ca. 3 times more reactive than the monoanion H2PO4-, and this result is consistent with earlier results.3 Micelle Formation. Values of the critical micelle concentration, cmc, for mixtures of SB3-n and NaDeP were measured by surface tension (Table 2). Critical micelle concentrations increase with increasing [NaDeP], at a constant NaDeP/SB3-n ratio in all cases, the magnitude of the changes depending on both the mole fraction of χ NaDeP and the size of the chain of the sulfobetaine. Clearly, the decreases in the cmc are largest with the longer-chain sulfobetaines, and in all cases the cmc values, as χNaDeP approaches zero, extrapolate approximately to the cmc of the corresponding SB3-n. Thus, the observed chain length dependency is not surprising, since for systems that show strong interaction and a negative deviation of ideality,5,15 the lower the cmc of the SB3-n, the larger the expected effect. Indicator Measurements. Deprotonation of DPA was followed spectrophotometrically, and pH in the aqueous phase was maintained with 0.01 M borate buffer. In all experiments, the total concentration of the surfactants, SB3-n and NaDeP, was kept constant at 0.1 M, and the ratio [SB3-n]/[NaDeP] was changed. Clearly, micellar effects upon protonation equilibria are complex and depend on the extent of transfer of a particular indicator and its conjugate base between the aqueous and micellar phases and in differences in deprotonating power in the bulk and micellar media.1,3,5 For a quantitative analysis and discussion of micellar effects on deprotonation see ref 1. Due to its hydrophobicity, 1-dodecyl-3-pyridiniumaldoxime bromide is quantitatively transferred into micelles, circumventing the previously described problem and, thus, has been used previously as an indicator for pH in the micellar pseudophase of SB3-n/SDOD mixtures,5 which are structurally related to the SB3-n/NaDeP system. In the case of both SB3-n/SDOD and SB3-n/NaDeP mixtures, pHapp values were determined under the simplifying assumption that the indicator pKa is the same in both the aqueous phase and the micelles, regardless of the surfactant chain lengths.5 Using this methodology, we were able to calculate the values of pHapp that were sufficient for our kinetic work, observing the changes in extent of indicator deprotonation on transfer from water to micelles (Figure 1 and Table 3). (15) Rubing, D. N.; Holland, P. M. J. Phys. Chem. 87, 1984, 1983.

Sulfobetaine-Sodium Decyl Phosphate Mixed Micelles

Figure 1. Effect of increasing mole fraction of NaDeP at 0.1 M total surfactant ([SB3-10] + [NaDeP]) on the pHapp of mixed micelles at 35.0 °C. Table 3. Apparent pH at Different Molar Fractions of NaDeP in SB3-n/NaDeP Mixturesa pHapp χNaDeP

SB3-12

SB3-14

SB3-16

0 0.05 0.1 0.20 0.30 0.40 0.5 0.6 0.7 0.80 0.9

9.31 8.89 8.68 8.34 8.16 8.02 7.88 7.74 7.63 7.47 7.28

9.23 8.77 8.51 8.23 8.06 7.93 7.79 7.68 7.54 7.40 7.24

9.42 8.70 8.50 8.22 8.09 7.94 7.86 7.73 7.60 7.48 7.24

a At 35.0 oC, 0.010 M borate buffer, pH ) 8.8, [SB3-n] + [NaDeP] ) 0.1 M.

As can be seen in Figure 1, which illustrates the behavior of SB3-10 NaDeP surfactant mixtures, addition of NaDeP strongly affects the apparent pH of the micellar surface. The reported values in Figure 1 and Table 3 are consistent with previous observations and show that pHapp values in the pure sulfobetaine micelles are higher than in the aqueous buffer, consistent with theoretical predictions of slight concentrations of anions at surfaces of this type of micelles.5 Since for a mixed micelle of SB3-n and NaDeP the surface charge density is largely controlled by the molar fraction of NaDeP, χNaDeP, it is expected that addition of NaDeP to SB3-n micelles provokes an increase in the negative charge density at the micellar surface, and, as a consequence, a marked decrease in [OH-] and pHapp. The experimental results are consistent with this expectation, independent of the chain length of the sulfobetaine (Table 3 and Figure 1). Plots of χNaDeP against pHapp, which are similar to that in Figure 1 in all cases, extrapolate approximately to pHapp in SB3-n. Clearly, incorporation of NaDeP increases the anionic character of the mixed micelle, and [OH-] in the micellar phase decreases following the exponential of the charge density at the micellar surface.5,16 The observed effect is less pronounced than that reported for SDOD/ SB3-n mixtures,5 indicating that the phosphate surfactant

Langmuir, Vol. 16, No. 26, 2000 10133

Figure 2. Effect of increasing mole fraction of NaDeP at 0.1 M total surfactant ([SB3-n] + [NaDeP]) on the first-order rate constants of hydrolyses of 2,4-dinitrophenyl acetate (DNPA) in 0.010 M borate buffer, pH 8.8 at 35.0 °C.

may be somewhat involved in the protonation equilibria in the micelle. Indeed, as previously reported,17 sodium decyl phosphate micelles can be composed of the decyl phosphate monoanion (at pH 5.3) or the decyl phosphate dianion (at pH 12.6) or of mixtures thereof at intermediate values of pH. According to Romsted and co-workers,17 the apparent pKa of decyl phosphate monoanion (DePM), estimated from 31P chemical shift versus pH titration curves, is 7.6 for micellized DePM and 6.92 for DePM in the monomeric form. It is important to carefully consider the data reported for the pure NaDeP micelle, since in the light of previous experiments with SDOD/SB3-n surfactant mixtures,5 it should set a limit for the largest experimental pH shift expected for NaDeP/SB3-n surfactant mixtures; experimental results show that, under the experimental conditions used in this work (pH ) 8.8 in the aqueous phase), there is only a modest formation of decyl phosphate monoanion (e6%, based on 31P chemical shift versus pH titration data). Thus, the formation of DePM, although modest, may well be responsible for the difference in pH at the micellar surface, a possibility that is also consistent with the apparent pKa reported for this surfactant. Conversely, the modest change in distribution of species, with formation of 6% DePM, is far too small to account for a kinetic effect such as the one reported (vide infra). Reactions in Mixed Micelles of NaDeP and Sulfobetaine. All reactions were followed at 35.0 °C, pH ) 8.8, in 0.01 M borate buffer, and as previously reported, first-order rate constants, kobs, are unaffected by this dilute buffer.6 Values of kobs for hydrolyses of DNPA, DNPO, and Bz2O as a function of the change in χNaDeP at [SB3-n] + [NaDeP] ) 0.1 M are shown in Figures2-4. The cmc values reported in Table 2 show clearly that under these conditions the surfactant mixture is predominantly in the micellar form; also based on reported binding constants (KS ) 1500, 50, and 1500 M-1 for DNPO, DNPA, and Bz2O, (16) (a) Gunnarsson, G.; Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1980, 84, 3114. (b) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1985, 89, 4166. (17) Romsted, L. S.; Yoon, C. J. Am. Chem. Soc. 1993, 115, 989.

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Figure 3. Effect of increasing mole fraction of NaDeP at 0.1 M total surfactant ([SB3-n] + [NaDeP]) on the first-order rate constants of hydrolyses of 2,4-dinitrophenyl octanoate (DNPO) in 0.010 M borate buffer, pH 8.8 at 35.0 °C.

Lee and Nome

into sulfobetaine micelles affects rates of hydrolysis in two opposing ways: (i) indicator measurements point to the loss of OH- from the micellar surface due to an increase in the negative charge density, a factor that should result in a decrease of kobs, and (ii) the increase in molar fraction of NaDeP in the mixture should favor catalysis due to micellar-bound decyl phosphate ion, either as a general base or as a nucleophile, which should increase kobs. Consistent with expectations, in all cases, as shown in Figures 2-4, the inhibition due to the loss of hydroxide ion from the micellar surface is the dominant factor up to approximately χNaDeP ) 0.2, and then catalysis by micellarbound decyl phosphate ion becomes predominant. The shapes of the kinetic curves depends on a delicate balance between these two factors, and while a clear rate maximum is observed in the case of DNPA and a slight upward curvature in the kinetic profile (χNaDeP range of 0.2 to 0.9) is observed for DNPO, for Bz2O the rate increases almost linearly after the initial inhibition. Quantitative treatment of the rate constant-surfactant composition profiles was carried out following a simple kinetic treatment developed for SB3-n/SDOD mixtures, where the expression for the observed rate constant includes a catalytic term due to the functional surfactant and a contribution due to the hydroxide reaction, which is expressed as a function of the effective molarity of OHin the mixed micelles, OHM. The rate constant for reaction of OH- in a mixed micelle is unaffected by decyl phosphate ion and corresponds to that observed with fully bound substrate in the sulfobetaine micelle, corrected for the changes in pHapp.5 Thus, for strongly bound substrates such as DNPO and Bz2O, the contribution of the reaction in the aqueous phase can be neglected, and, therefore, kobs is given as a function of two terms describing the pathways for reaction in the mixed micellar phase:

kobs ) kOH [OH]M + kNaDePχNaDeP

Figure 4. Effect of increasing mole fraction of NaDeP at 0.1 M total surfactant ([SB3-n] + [NaDeP]) on the first-order rate constants of hydrolyses of benzoic anhydride (Bz2O) in 0.010 M borate buffer, pH 8.8 at 35.0 °C.

respectively, in SDOD and SDS),6 DNPO and Bz2O should be almost fully micellar-bound, and DNPA should partition between the aqueous and micellar phases (KS). The extent of incorporation of DNPA into the micellar phase increases with both the increase in size of the hydrocarbon chain of SB3-n and with the decrease in cmc of the mixture, since both factors contribute to an increase in concentration of the micellized surfactant mixture. The observed kinetic behavior is totally different from that reported in mixtures of SDOD and SDS, where kobs increases linearly with increasing χSDOD and shows some similarities with the behavior observed in SDOD/sulfobetaine mixtures; in these mixtures, for 0.1 M total surfactant, first-order rate constants of hydrolyses of DNPA, DNPO, and Bz2O go through maxima as a function of the mole fraction of SDOD.6 Clearly, addition of NaDeP

(3)

where kOH, M-1s-1, is the second-order rate constant in the micelles, OHM at the micelle-water interface is estimated by pHapp ) log(OHM/KW), and kNaDeP, s-1, is the second-order rate constant for reaction of decyl phosphate ion in the micellar pseudophase, with its concentration expressed as a mole fraction. The contribution of kOH [OH]M was evaluated from the first-order rate constant in the presence of the SB3-n micelle; this was multiplied by the ratio of hydroxide ion concentration in the mixed micelle to that in the pure SB3-n surfactant to correct for the decrease in pHapp (Figure 1 and Table 3). Since the molar fraction of NaDeP was known, we were able to estimate values of kNaDeP from eq 1 as a function of χNaDeP. The results are shown in Figures5 and 6 for DNPO and Bz2O, respectively. For hydrolysis of DNPA, we have to allow for incomplete substrate binding, including terms to account for the reaction in the aqueous phase (kobs,w) and the corresponding correction for substrate distribution as a function of stoichiometric surfactant concentration in micellar form, [Dn], (which corresponds to the difference between the total surfactant concentration and the cmc) at the corresponding mole fraction (Table 2). Equation 3 becomes

kobs )

kobs,w + (kOH[OH]M + kNaDeP χNaDeP) KS [Dn] 1 + KS[Dn]

(4)

Based on KS ) 55 M-1,3,5 we can see that the extent of incorporation of DNPA into the mixed micellar system varies from a maximum value of ca. 85% in the SB3-

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Langmuir, Vol. 16, No. 26, 2000 10135

with DNPA, DNPO, and Bz2O in the micelle depends on χ NaDeP, and an increase in reactivity of the anion is observed

Figure 5. Effect of increasing mole fraction of NaDeP at 0.1 M total surfactant ([SB3-n] + [NaDeP]) on the rate constant in the micellar phase for hydrolyses of 2,4-dinitrophenyl octanoate (DNPO) in 0.010 M borate buffer, pH 8.8 at 35.0 °C. Symbols represent average values for SB3-10, SB3-12, SB314, and SB3-16.

Figure 6. Effect of increasing mole fraction of NaDeP at 0.1 M total surfactant ([SB3-n] + [NaDeP]) on the rate constant in the micellar phase for hydrolyses of benzoic anhydride (Bz2O) in 0.010 M borate buffer, pH 8.8 at 35.0 °C. Symbols represent average values for SB3-10, SB3-12, SB3-14, and SB3-16.

Figure 7. Effect of increasing mole fraction of NaDeP at 0.1 M total surfactant ([SB3-n] + [NaDeP]) on the rate constant in the micellar phase for hydrolyses of 2,4-dinitrophenyl acetate (DNPA) at 0.010 M borate buffer, pH 8.8 at 35.0 °C. Symbols represent average values for SB3-10, SB3-12, SB3-14, and SB316.

16/NaDeP mixture to a minimum of ca. 70% in SB3-10, in the presence of χNaDeP ) 0.8. The values of kNaDeP calculated by means of eq 4 as a function of the mole fraction of NaDeP, following a procedure identical to that described above, are shown in Figure 7, and the kinetic behavior is closely related to that shown for DNPO in Figure 5. Similar to the observations in SB3-n/SDOD mixtures, the rate constant for the reaction of decyl phosphate ion

as the molar fraction of decyl phosphate in the NaDeP/ SB3-n micelle decreases. The difference in reactivity of the functional surfactant between the high and low molar fractions of NaDeP is greater for the esters, DNPA and DNPO, than for the anhydride. It has been reported that benzyltrimethylammonium n-decyl phosphate, in dry acetonitrile, reacts readily with DNPA by nucleophilic addition to the carbonyl group, the reaction being strongly inhibited by water,6,7 and it has been postulated that the decrease in nucleophilicity of the anion might be related to its pairing with cations18 and/or hydrogen bonding with the protic solvents.19 Thus, the increase in reactivity of NaDeP must be intrinsically related to its incorporation in the micelle, where the less polar environment favors the reaction. Similar to the results observed with SDOD/SB3-n mixtures, the analysis shows that the reactivity of decyl phosphate increases in all cases (reactions of DNPA, DNPO, and Bz2O) upon incorporation of decyl phosphate ion into a sulfobetaine micelle. These increases in rate constant offset the “dilution” of decyl phosphate ion in the mixed micelles and lead to the observed rate maxima effects. By comparison with Figures 5-7, we can see that values of kNaDeP do not depend markedly on the chain length of the sulfobetaine. The difference in shape of the reported profiles seems to be related to the magnitude of the effect of the molar fraction of NaDeP in its reactivity, and reactions which show a stronger effect (DNPA and DNPO) show a rate maximum for the plots of kobs versus χNaDeP. Our results are fully consistent with previous observations of SDOD/SB3-n mixtures, where rate effects were ascribed to decreased hydration and ion pairing of dodecanoate ion in the mixed micelle. Indeed, mixed micelles of NaDeP/SB3-n show a dependence, in terms of aggregation behavior (Table 2), which is similar to that reported for mixtures of SDOD and sulfobetaines,5 and since both systems are structurally related, it is likely that the phosphate ion moiety will not be at the micellar surface but will be in a more hydrophobic region shielded from both Na+ and OH-. It is important to note that the analysis of plots of kobs versus surfactant concentration, customarily used in micellar catalysis, are notoriously different from the kobs versus molar fraction of surfactant. Indeed, we should remark that each individual kinetically determined value in Figures 5-7, being a particular mixture of NaDeP and SB3-n, may well show a differential reactivity, since it represents a particular surface composition, with an individual surface, in terms of both structure and charge, and, in this sense, mixed micelles of sulfobetaines and reactive anionic surfactants are unique. Conclusions Comparing the reactivities of the organic substrates DNPA and DNPO in any of the studied SB3-n/NaDeP surfactant mixtures, we conclude that the reactivity of DNPA is always greater than that of DNPO. The observed difference in reactivity may well be related to different locations in the micellar phase of these organic substrates. Indeed, in the particular case of DNPA, the driving force for partitioning of the substrate is most likely provided (18) Menger, F. M. In Nucleophilicity; Harris, J. M., McManus, S. P., Eds.; American Chemical Society: Washington, DC, 1987; pp 209218. (19) Dewar, M. J. S.; Storch, D. M. J. Chem. Soc., Chem. Commun. 1985, 94-96.

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by the 2,4-dinitrophenyl group, with the acetate moiety in a more aqueous environment, whereas when DNPO is considered, the situation is the reverse, with the octanoate moiety providing the driving force for anchoring the substrate in the mixed micelle. Another significant fact, which is similar to that reported for the reactivity of dodecanoate ion in sulfobetaine micelles,5 is the increase of kNaDeP as a function of the mole fraction of the sulfobetaine for all three organic substrates. The enhancement of reactivity observed in NaDeP/SB3-n mixtures by exploiting the microenviroment is somewhat related to the observed increases in nucleophilicity of decyl phosphate ion toward 2,4-dinitrophenyl acetate in aqueous acetonitrile. The striking decrease in nucleophilic reactivity of phosphate upon addition of water to acetonitrile has been attributed to anionic stabilization by hydrogen bonding rather than to transition-state effects, where these interactions are weaker. The observed effect in mixed micellar systems is also intrinsically related to exclusion of water and Na+, partial or total. Indeed, such exclusion may well be the driving force for the increase in nucleophilicity of the phosphate moiety, which probably results

Lee and Nome

from diminishing solvation and “burying” decyl phosphate ion in the dipolar region, close to the inner cationic ring, of the zwitterionic surface of sulfobetaine micelles. This special behavior of mixtures of anionic and zwitterionic micelles provides the opportunity to improve our knowledge in relation to solvation effects in reactivity, a topic that is of paramount importance in the understanding of enzyme catalysis. Clearly, mixed micelles, by preferentially partitioning the substrate in a microenvironment where the access of water is somewhat restricted, provides a source of catalytic effects that is not normally observed in micellar catalysis. Insofar as the active site of enzymes may have low water contents, the study of reactivities in mixed micelles of this type may shed some light on observed reactivities in systems were anionic dessolvation plays a major role. Acknowledgment. We are grateful to CNPq and PRONEX (Brazil) for financial support. LA000973C