J. Phys. Chem. B 2007, 111, 11437-11442
11437
Change in the Acid Hydrolysis Mechanism of Esters Enforced by Strongly Acid Microemulsions E. Ferna´ ndez, L. Garcı´a-Rı´o,* and P. Rodrı´guez-Dafonte Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de Santiago de Compostela, 15782 Santiago, Spain ReceiVed: May 7, 2007; In Final Form: July 10, 2007
A kinetic study was carried out on the acid hydrolysis of 4-nitrophenylacetate and 4-nitrophenyllaurate in water/HOT/isooctane microemulsions. The substitution of Na+ in the sodium salt of bis(2-ethylhexyl)sulfosuccinate by H+ has permitted us to obtain a functionalized surfactant (HOT) and, consequently, strongly acid microemulsions. The use of HOT-based microemulsions allows us to reach concentrations of H+ in the aqueous core corresponding to a Hammett acidity function of H0 ) -2. The rate constant at the interface and the distribution constants of the carboxylic esters throughout the different microenvironments of the microemulsion have been quantified by application of the pseudophase formalism. The results obtained show that the hydrolysis rate constant at the interface increases as the water content of the system decreases. The correlation of the rate constants at the interface of the microemulsion with the Hammett acidity function, H0 (on the basis of the Bunnett-Olsen criterion), has allowed us to confirm that the hydrolysis process takes place via an A2 mechanism for high water contents and through an A1 mechanism for values of W e 15 (W ) [H2O]/[HOT]).
Introduction Microemulsions are macroscopically homogeneous and thermodynamically stable systems of water nanodroplets dispersed into an apolar solvent in the presence of an adequate surfactant.1 The surfactant that is mostly used for the preparation of microemulsions is the sodium salt of bis(2-ethylhexyl)sulfosuccinate (NaOT or AOT). The combination of NaOT, water, and isooctane produces microemulsions of spherical geometry, where the nanodroplet size is directly proportional to the water content, W ) [H2O]/[NaOT].2,3 Three regions can be differentiated in microemulsions: the internal aqueous nanocore or water nanopool, the micellar interface, and the external organic phase.1 Just as in the case of biological systems, the properties of the confined water are very different from those of the bulk water due to the geometric size constraints of the environment and to intermolecular interactions at the micellar interface.4 This characteristic makes possible the use of microemulsions as models for biological compartmentalization.5 In addition, we must take into account the fact that the possibility of simultaneously having a polar and an apolar solvent makes the microemulsions an attractive reaction medium where a great variety of organic reactions can take place because they allow them to come into contact with each other, reagents that, normally, would be incompatible.6 The reagents will be distributed throughout the three microenvironments of the microemulsion according to their hydrophobicity. The study of organic reactivity in microemulsions will be more complex as the number of the reagents involved increases. It is possible to simplify the quantitative treatment of the chemical reactivity in microemulsions if one of the microenvironments could be simultaneously one of the reagents involved in the reaction. Following this premise, recently, our research * Corresponding author. E-mail:
[email protected].
group has studied solvolytic reactions of benzoyl chlorides and chloroformiates in microemulsions, which has permitted the assessment of the properties of the microemulsion water from a kinetic point of view.7-11 Another possibility would be the use of a surfactant as one of the reagents involved in the reaction. The substitution of the counterion Na+ by other metallic cations produces stable microemulsions even though their properties vary according to the type of counterion used, especially when transition metal dications12 are employed. In the present work, we have modified the surfactant (NaOT) replacing the Na+ counterion by H+. The resulting surfactant (named HOT) offers a range of stability similar to that of the NaOT and may act as a reagent in acid hydrolysis reactions. We have already characterized this type of systems employing the FT-IR and 1H NMR techniques.13 The use of strongly acid microemulsions has also allowed us to determine the acidity function of the system and the degree of counterion binding (β) in the microemulsion.14 The most important characteristic of these new systems is the fact that we can obtain acid concentrations in the aqueous nanodroplet highly superior to those that would be obtained by adding acid to a NaOT microemulsion. The reaction that we have studied is the acid hydrolysis of the carboxylic esters whose reaction mechanisms have been widely studied in aqueous media.15-17 The accepted mechanism involves protonation of the carbonyl-O in the first step, followed by addition of water and breakdown of the tetrahedral intermediate to products. The esters chosen were p-nitrophenyl acetate (NPA) and p-nitrophenyl laurate (NPL) due to their differing solubility. Experimental Section The NPA and NPL used were supplied at the highest level of purity commercially available by Aldrich and were used
10.1021/jp073479r CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007
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Figure 1. Influence of the [HOT] concentration on kobs for acid hydrolysis in HOT microemulsions, at 25.0 °C. Left: NPA at W ) 2 (O), W ) 5 (B), W ) 9 (0), and W ) 10 (9). Right: NPL at W ) 3 (O), W ) 7 (B), W ) 13 (0), and W ) 20 (9).
without further purification. All of them were dissolved in isooctane (Aldrich). The HOT was prepared from NaOT by ion exchange using Amberlite IR 120 (plus) resin. Other authors18 have prepared the surfactant HOT in previous studies as an intermediate step for replacing the Na+ counterion of NaOT. The second step in replacing the counterion should be the neutralization of HOT with the appropriate base. We checked the extent of Na+/H+ exchange by two methods: atomic absorption spectroscopy showed the absence of Na+ in the HOT sample (the residual Na+ content is consistent with a degree of Na+/H+ exchange greater than 99%) and acid/base titration, which also indicated that the extent of Na+/H+ exchange is greater than 99%. Mass spectra were recorded on a Bruker Microtof ESI-TOF mass spectrometer in high-resolution mode by using electrospray ionization in the negative mode. The solutions were infused into the ESI source at flow rates of 0.2 mL/min. Comparison of the mass spectra of NaOT- and HOT-based microemulsions shows no HOT decomposition after 7 days of preparation of the microemulsion. The hydrolysis reactions were followed by monitoring the UV-vis absorbance of substrate solutions, concentration range (3.0-4.0) × 10-4 M, using a Cary 500 scan UV-vis-NIR spectrophotometer fitted with thermostated cell holders. The wavelengths used for the kinetic studies fell in the range of 375-380 nm. The integrated first-order rate expression was fitted to the absorbance-time data by linear regression (r > 0.999) in all cases. The observed rate constants, kobs, could be reproduced with an error margin of 5%. All experiments were carried out at (25.0 ( 0.1) °C. Results and Discussion The influence of the microemulsion composition on the observed rate constant of the acid hydrolysis reaction of the NPA and NPL has been studied. To do that, experiments have been carried out varying the surfactant concentration and keeping constant the W parameter. These experiments have been realized for different W values and, therefore, the water content of the system has been varied from W ) 2 to W ) 30. In Figure 1 are given as an example the results obtained after studying the influence of [HOT] on the observed rate constant for the acid hydrolysis of NPA and NPL. The value of kobs increases along with the surfactant concentration. It can also be observed that the value of kobs increases on decreasing the W parameter. The obtained results are related to the variation in the concentration of the H+ ions in the microemulsion. As we have previously demonstrated, a decrease of the nanodroplet size implies an enhancement of the local H+ concentration. However,
SCHEME 1
SCHEME 2
to carry out a quantitative interpretation of the experimental results, it is necessary to know the ester concentration in different phases of the microemulsion. With this aim, we will apply the micellar pseudophase formalism modified for microemulsions. According to this model, the system is constituted by three pseudophases (oil, interface, and water), and the ester will be distributed depending on its solubility (Scheme 2). There is a controversy in the scientific community about the microstructure of water-in-oil microemulsions, mainly at very low water contents. In fact, some authors have suggested that, for low water contents, the aggregate microstructure will deviate from the droplet model. Therefore, the pseudophase model considering the existence of three pseudophases should be wrong. However, recent results from our laboratory studying the effect of NaOT-based microemulsions on the reaction between crystal violet and sulfite ion represent irrefutable evidence of the existence of three well-differentiated microenvironments in the NaOT-based microemulsions, even at very low water contents.19 1. Distribution Constants from Kinetic Parameters. Previous studies carried out in our laboratory on the basic hydrolysis and aminolysis of the NPA in NaOT-based microemulsions have demonstrated that it is distributed throughout the three pseudophases of the microemulsion.20,21 The acid hydrolysis reaction of the NPA cannot take place in the continuous medium
Change in the Acid Hydrolysis Mechanism of Esters
J. Phys. Chem. B, Vol. 111, No. 39, 2007 11439
Figure 2. Variation of 1/kobs with Z for the acid hydrolysis reaction of the NPA in HOT microemulsions at 25 °C. Left: NPA at W ) 2 (O), W ) 5 (B), W ) 9 (0), and W ) 10 (9). Right: NPL at W ) 3 (O), W ) 7 (B), W ) 13 (0), and W ) 20 (9). Data for the left figure were fitted to eq 4 and for the right figure according to eq 3.
TABLE 1: Values of the Intercepts and Slopes Obtained for Different W Values on Representing 1/kobs vs Za NPA
a
NPL
W
intercept
slope
W
intercept
slope
2 3 4 5 6 9 10 12 15 20 25 30
(2.4 ( 0.1) × 103 (3.4 ( 0.2) × 103 (4.4 ( 0.2) × 103 (5.3 ( 0.1) × 103 (6.9 ( 0.1) × 103 (7.0 ( 0.1) × 103 (9.1 ( 0.1) × 103 (11.2 ( 0.1) × 103 (12.3 ( 0.1) × 103 (16.8 ( 0.1) × 103 (20.7 ( 0.1) × 103 (24.5 ( 0.1) × 103
(1.4 ( 0.4) × 102 (1.9 ( 0.2) × 102 (2.6 ( 0.8) × 102 (3.4 ( 0.9) × 102 (4.7 ( 0.7) × 102 (4.3 ( 0.3) × 102 (5.2 ( 0.1) × 102 (6.4 ( 0.5) × 102 (7.4 ( 0.6) × 102 (9.4 ( 1.1) × 102 (12.70 ( 0.1) × 102 (14.4 ( 0.1) × 102
2 3 4 5 6 7 10 13 17 20 25 30
(9.4 ( 0.2) × 103 (14.1 ( 0.1) × 103 (17.1 ( 0.3) × 103 (19.9 ( 0.4) × 103 (23.1 ( 0.5) × 103 (29.8 ( 0.6) × 103 (41.5 ( 0.6) × 103 (44.9 ( 0.6) × 103 (48.7 ( 0.9) × 103 (52 ( 1) × 103 (57 ( 1) × 103 (59 ( 2) × 103
(4.5 ( 0.1) × 103 (4.96 ( 0.02) × 103 (7.0 ( 0.1) × 103 (10.0 ( 0.2) × 103 (12.7 ( 0.2) × 103 (13.2 ( 0.6) × 103 (19.7 ( 0.2) × 103 (25.0 ( 0.5) × 103 (27.4 ( 0.5) × 103 (28.2 ( 1.0) × 103 (32.8 ( 0.7) × 103 (31 ( 3) × 103
The significance of the intercepts and slopes is given by eq 1 for the NPA and by eq 3 for the NPL.
of the microemulsion because it is not possible to find H+ ions in the oil phase. From this kinetic scheme and taking into account that the concentrations refer to the total volume of the microemulsion, we can obtain the following rate equation:
h H2O(Kwi + W) V h HOTV 1 ) + kobs Kwi βV h H2Oki + (1 - β)V h HOTkw h H2O V h HOTV
Kwi Koi
Kwi βV h H2Oki + (1 - β)V h HOTkw
Z (1)
where Z is defined (like W) as Z ) [Isooctane]/[HOT]. Figure 2 shows the good fit of eq 1 to the experimental data, confirming the validity of the proposed kinetic model. Table 1 shows the values of the intercepts and slopes of the representations of 1/kobs vs Z for different W values. The quotients between the intercepts and the slopes of representations analogous to those shown in Figure 2 for different W values in the acid hydrolysis of the NPA should show the following dependence on W:
Koi intercept ) Koi + W slope Kwi
w isooctane, Kwo . This coefficient can be written as: Kwo ) XNPA / o w o w o XNPA = [NPA]w/[NPA]o = (nNPA/nH2O)/(nNPA/noil) = [NPA]w/ w o [NPA]o Z/W where XNPA and XNPA represents the molar fractions of NPA in the aqueous and isooctane phases, respecw o and nNPA are the number of NPA moles in the tively; nNPA aqueous and isooctane phases, and nH2O and noil are the number of water and isooctane moles in the respective phases. The concentrations [NPA]ww and [NPA]oo refer to the volume of the phase, while [NPA]w and [NPA]o refer to the total volume of the system. On the basis of the definition of the distribution NPA constants, we can write: Kwo ) KNPA oi /Kwi . Thus, we can w NPA rewrite eq 2 as intercept/slope ) Ko (Kwi + W), which can be w NPA simplified if KNPA wi . W, resulting in intercept/slope ) Ko Kwi . This relationship justifies the fact that the quotient intercept/ slope remains constant and independent of W. From the mean value of the relationship intercept/slope ) 17 and the value of Kwo ) 2.60 × 10-2, we can obtain the values of KNPA ) 654 wi
(2)
Nevertheless, Figure 3 shows that the quotient intercept/slope is independent of W and presents a mean value close to 17. This type of behavior has been previously found when studying the basic hydrolysis and aminolysis of the NPA in NaOT-based microemulsions.20,21 To explain the absence of this linear dependency, we have resorted to the previously determined distribution coefficient of the NPA between bulk water and
Figure 3. Variation of the quotient intercept/slope with W for the NPA (O) and NPL (B), respectively.
11440 J. Phys. Chem. B, Vol. 111, No. 39, 2007
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SCHEME 3
and KNPA ) 17. These values are similar to those previously oi reported20,21 for NPA distribution in water/NaOT/isooctane microemulsions, KNPA ) 996 and KNPA ) 25.9, whose diswi oi crepancies can be attributed to the different properties of the interfacial water on changing the surfactant counterion.13 Moreover, eq 2 can be rewritten as intercept/slope ) Koi(Kwi + W/Kwi) and the value of KNPA ) 654 perfectly satisfies that wi . W for all the studied W values. KNPA wi The hydrophobic character of the NPL simplifies the study of its acid hydrolysis in HOT-based microemulsions, being distributed only between the interface and the continuous medium of the microemulsion. Because reagents can only come into contact with each other at the interface (Scheme 3), we can obtain the following expression for kobs.
h HOT V h HOT V 1 + ) Z kobs kiβ kiβKoi
(3)
Equation 3 also predicts the existence of a linear correlation between 1/kobs and the Z parameter. This dependency is satisfactorily fulfilled, increasing the reliability of our experimental results (see Figure 2). Table 1 shows the values of the intercepts and slopes obtained for representations similar to those shown in Figure 2B for different W values. Figure 3 shows that the quotient intercept/slope remains constant and independent of W, as expected from eq 3 (because intercept/slope ) KNPL oi ), a mean value of KNPL ) 2 being obtained. oi 2. Intrinsic Rate Constants for the Acid Hydrolysis of Esters. From these experimental data of the acid hydrolysis of the NPA and NPL and using the corresponding equations (eq 1 and eq 3), we can obtain the value of the hydrolysis rate constant at the interface (ki) of HOT-based microemulsions. In the case of the NPA, it is feasible to assume an approximation to reduce the complexity of the equation. To do this, we will take into account the fact that the distribution constant between the water and the interface of HOT microemulsions has been previously calculated, KNPA wi ) 654, that the fraction of neutralized charge at the interface of HOT-based microemulsions22 is β = 0.92, and that the molar volume of the experimentally determined HOT is V h HOT ) 0.418 M-1. Therefore, for HOT microemulsions, we can assume that KwiβV h H2Oki . (1 - β)V h HOTkw. Thus, eq 1 would be simplified to:
Kwi V h HOT(Kwi + W) Koi 1 ) + Z kobs Kwiβki Kwi βki V h HOT
(4)
Once all the parameters necessary for the application of eqs 4 and 3 are known, we can determine the value of ki for the acid hydrolysis of the NPA and NPL in HOT microemulsions. Figure 4 shows how the value of ki decreases with the increase in the nanodroplet radius. From Figure 4, two preliminary conclusions can be inferred. First, we observe that the variation of the rate constant (ki) with W is parallel for the NPA and
Figure 4. Variation of ki with W for the reaction of the acid hydrolysis of the NPA (O) and NPL (B), at 25 °C, in microemulsions of water/ HOT/isooctane.
SCHEME 4
NPL. Therefore, it fully confirms the supposition that lead to the simplification of the eq 1. Even though the NPA could be distributed throughout the three pseudophases of the microemulsion, the hydrolysis reaction takes place solely at the interface of the system. The difference in reactivity between the two esters remains constant for the whole size range of the microemulsion, with ki always inferior in the case of the NPL, probably caused by a greater esteric impediment that this ester might have. To confirm this behavior, a kinetic study of the two compounds in 50% acetonitrile/water mixtures and with an acid medium of 1 M was carried out. The quotient between the rate constants of the NPA and NPL shows that the reaction rate is 3.5 times higher for the NPA than for the NPL. This result is compatible with the relation obtained between the rate constants of the two compounds at the interface of a HOT/ isooctane/water microemulsion. The second aspect to be considered is the decrease in the value of ki with the increase of W. As W increases, the value of the acidity function increases too and, therefore, the concentration of H+ ions in the aqueous nanocore of the microemulsion decreases. In the definition of ki, the concentration of H+ is already included, and thus we can directly relate the variation of ki with W to the variation in the medium acidity. To interpret this decrease with W, it will be necessary to study in depth the reaction mechanism. We have to analyze to what extent the reaction mechanism of the acid hydrolysis of the NPA and NPL is affected in a strongly acid medium with special aqueous properties like the HOT microemulsions. 3. Modification of the Reaction Mechanism. The reaction mechanisms of the acid hydrolysis in aqueous media have been extensively studied.15-17 An acid-catalyzed reaction of the general type as shown in Scheme 4 can take place through different reaction mechanisms. A possible reaction mechanism would imply direct conversion of the protonated substrate to the transition state. In this case, we will have a simple A1 type reaction mechanism. However, A2 or ASE2 type mechanisms are also very probable. In A2 type reactions, conversion to the transition state occurs by attack of a nucleophile on the protonated species. The intervention of water as a nucleophile
Change in the Acid Hydrolysis Mechanism of Esters
J. Phys. Chem. B, Vol. 111, No. 39, 2007 11441
Figure 5. Application of the eq 5 for the acid hydrolysis of the NPA in aqueous medium in the presence of HCl. [HCl] ) 0.5-6.0 M.
is just one of the possible examples for this type of reactions. In the ASE2 type reaction mechanism, the slow step is represented by the proton transfer (Scheme 4). Possible change in the mechanism of the acid hydrolysis of esters has been analyzed by combinations of changes in structure, acidity, and water activity.15 Thus, for example, for the NPA in aqueous dilutions of sulfuric acid, it has been observed that for a percentage of acid superior to 70% a mechanism change from A2 to A1 takes place. This mechanism change has also been confirmed by the presence of perchloric acid which, at low acidity hydrolysis, occurs by the A2 mechanism. This involves rate-determining attack of water on the conjugate acid of the ester and at high acidities. However, the conjugate acid undergoes unimolecular heterolysis to form an acylium ion and phenol.23 There are various methods that allow us to determine the hydrolysis reaction mechanism such as the Zucker-Hammett criterion24 and the methods developed by J. F. Bunnett in the 1960s.25-28 Bunnett and Olsen extended the linear free energy relationship (LFER) to acid-catalyzed reactions,29 obtaining the equation
log kobs + H0 ) Φ(H0 + log[H+]) + log
k KSH+
(5)
where the solvation parameter, Φ, represents the response of the equilibrium to the change in the medium, that is, increase in [H+] and subsequent decrease in water activity, and measures the effects brought about by the changes of hydration. Scorrano et al.30 suggest that the slope parameter Φ is a measure of the solvation requirements of the species involved in the protonation equilibrium. Therefore, it is possible to obtain information on the solvation, on the amount of charge localization, and hence
on the structure of the transition state from Φ parameter. The Bunnett-Olsen equation is still valid today because it is still used in the study of hydrolysis reactions in media of moderate or strong acidity.31-33 We have already indicated that the concentration of [H+] in the nanodroplet of the microemulsion can be estimated from the values of Hammett acidity function, H0, and varies between14 [H+] ) 0.1 and [H+] ) 5.0 M. With the aim of comparing our results in HOT microemulsions, the acid hydrolysis of the NPA has been studied in aqueous medium, in the presence of HCl, and in a similar range of concentrations. From the application of the eq 5 results a good linear correlation (Figure 5) and a value of Φ ) +0.74, indicating that the reaction mechanism is the A2 type in aqueous medium. For the application of the eq 5 in HOT-based microemulsions, we must take into account that the quotient kobs/[H+] is equal to ki and, therefore, we can obtain the Bunnett-Olsen parameter (Φ) from the following linear correlation:
log ki ) (Φ - 1)(H0 + log [H+]) + C
(6)
Figure 6 shows the results obtained as a consequence of the use of the Bunnett-Olsen criterion for the acid hydrolysis of the NPA and NPL in HOT microemulsions. In each case, two strongly differentiated regions are obtained: the first one with a clearly negative slope (-1.17 and -1.19) that corresponds with the values of the rate constant at the interface for smaller nanodroplet sizes, and the second one with a positive slope associated with the higher W values of the microemulsion. The slope values provide us the Φ values for every ester. For the NPA, we obtain a value of Φ ) +1.38 (for W > 15) and Φ ) -0.16 (for W < 15). This Φ value indicates the mechanism type and, consequently, we can visualize Figure 6 as a linear free energy relationship: an abrupt change in the slope represents a change in the reaction mechanism. The reaction mechanism for the acid hydrolysis of the NPA in HOT microemulsions is predominantly the A2 type for values of W > 15 (Φ ) +1.38), as in the aqueous dilution (Φ ) +0.74), but of the A1 type (Φ ) -0.16) for values of W < 15. Similar results are obtained for the acid hydrolysis of the NPL (Φ ) +1.74 and Φ ) -0.19), and we also obtain a change of the mechanism close to a value of W ) 15. Therefore, the water/ HOT/isooctane microemulsions cause a change in the reaction mechanisms of the acid hydrolysis of esters. A change in the acid hydrolysis mechanism from A1 to A2 have been previously reported by Cook34 on going from benzyl to methyl esters. Arcelli35 reported a change in the acid hydrolysis mechanism of the ether bond on increasing the acid concentration as a consequence of a break in the Bunnett-Olson
Figure 6. Variation of log ki with (H0 + log[H+]) for the acid hydrolysis of the NPA (left) and NPL (right) in microemulsions of water/HOT/ isooctane.
11442 J. Phys. Chem. B, Vol. 111, No. 39, 2007 plot. Humeres and co-workers36 reported a displacement from an A1 mechanism for the hydrolysis of N-alkyltioncarbamate esters at pH < 2 to an BAC2 mechanism for pH > 6.5. Moreover, the transition from an A1 to A2 mechanism have been also reported for the acid hydrolysis of thioacetic and thiobenzoic acids on decreasing the acid concentration in the reaction media.37 The reason for this change in the reaction mechanism cannot be only the increase in the acid character of the microemulsion as a consequence of a reduction in the nanodroplet size because, in the same range of [H+] concentrations in aqueous dilution, no change can be noticed (Figure 5). As previously demonstrated in our laboratory,13 the properties of the strongly acid aqueous core of the HOT microemulsions are very different from those of the bulk water. It has been noticed that the chemical displacements of the water signal of 1H NMR vary differently with the Hammett acidity function (H0) depending on if they are in aqueous dilution or in HOT microemulsions.13 These differences have been explained by taking into account that, for a given [H+], the number of water molecules available for solvating the H+ is much higher in water than in the microemulsion. For high W values, the water properties in the microemulsion are much closer to those of the bulk water and, therefore, the reaction mechanism is the A2 type. This behavior is equal to the one observed in the aqueous medium with moderate acidity, the water addition being the decisive step. As the water content of the microemulsion is reduced, the properties of the aqueous core differ more from the bulk water and the number of molecules available for solvating the acid is smaller. Consequently, the water does not intervene in the key step of the reaction and we will have an A1 type mechanism. Therefore, the HOT microemulsions allow us to model the reaction mechanism for the acid hydrolysis of esters depending on the size of the aqueous nanocore. Conclusions The use of water/HOT/isooctane microemulsions allows one to reach high concentrations of acid that would be impossible to achieve by the addition of an aqueous dilution of acid to any other type of microemulsions. The application of the BunnettOlsen method has allowed us to clarify the mechanism of the ester hydrolysis reactions in HOT-based microemulsions. A mechanism change from A1 (for W < 15) to A2 has been observed where the water molecule reacts with the protonated intermediate (for W > 15). This change of mechanism is due to the fact that, on decreasing the water content of the system, the water is associated with the surfactant counterion and presents low availability for reactions. A similar mechanism change has been observed in the presence of a high percentage of H2SO4 (superior to 70% by weight or H0 value smaller than H0 ) -5.9).38 In the presence of HOT-based microemulsions, the mechanism change is detected for a much lower acidity, H0 ) +0.3, equivalent to a H2SO4 solution of 2% by weight.38 The reason for the mechanism change to be observed at a lower acidity is due to the different availability of water to solvate in the microemulsion. The obtained results allow one to confirm that it is possible to achieve a change in the reaction mechanism of the acid hydrolysis of the carboxylic esters in HOT microemulsions varying the size of the water nanodroplet. Acknowledgment. Financial support from Ministerio de Ciencia y Tecnologı´a (Project CTQ2005-04779) and Xunta de
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