Reactivity in Quaternary Water in Oil Microemulsions. 2. Different

6618

J. Phys. Chem. B 2000, 104, 6618-6625

Reactivity in Quaternary Water in Oil Microemulsions. 2. Different Distribution of the Reagents Changing from Three- to Four-Component Microemulsions1 L. Garcıá-Rıó* and J. Ramo´ n Leis Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de Santiago, 15706 Santiago, Spain ReceiVed: March 2, 2000; In Final Form: April 18, 2000

A study was carried out on the nitrosation of piperazine (PIP) and N-methylbenzylamine (MeBzAm) by N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) at 25 °C in quaternary tetradecyltrimethylammonium bromide (TTABr)/1-hexanol/isooctane/water microemulsions, ensuring that the relationship [1-hexanol]/[TTABr] ) 4 remained constant. In order to interpret the experimental results, we extended the formalism of the micellar pseudophase to microemulsions thereby considering the distribution of the alcohol throughout the pseudophases of the microemulsion and the change in the volume of the interface. The volume of the interface, calculated from the molar volumes of the surfactant and the alcohol, was included in the kinetic model to quantify the dilution of the reagents associated at the interface. The application of the developed kinetic model to the two systems studied has shown that the presence of alcohol in the continuous medium increases its hydrophilicity. Hence, unlike in the case of the tertiary AOT/isooctane/water microemulsions, it is necessary to bear in mind that the piperazine is distributed between the three pseudophases of the microemulsion. A comparison of the results obtained in TTABr/1-hexanol/isooctane/water microemulsions with those of AOT/ isooctane/water shows that the incorporation of the alcohol into the interface increases its hydrophobicity by displacing water molecules and hence reducing the bimolecular rate constant for the reaction at the interface.

Introduction Microemulsions are thermodynamically stable, macroscopically homogeneous, and optically isotropic colloidal dispersions of either “oil” domains in water (o/w) or water domains in oil (w/o), stabilized by interfacial layers of amphiphiles. The type of dispersion, that is, whether o/w or w/o, is determined by the distribution of the amphiphiles between water and oil. If an amphiphile is more soluble in water than in oil, one finds o/w; if it is more soluble in oil than in water, one finds w/o microemulsions.2,3 Water in oil microemulsions are generally described as nanometer-sized water droplets dispersed in an apolar solvent with the aid of a surfactant monolayer. The interest in these complex fluids arises from the fact that they can be used in oil recovery,4 as solubilizing media for proteins5 or amino acids,6 in enzymatic reactions,7 as an alternative to phase transfer catalysts,8 and in the preparation of monodisperse colloid size particles.9 This array of fields in which microemulsions can be used is due to the fact that these aggregates possess three solubilization sites: the apolar continuum, the micellar interface, and the intramicellar water pool. Some surfactants cannot form w/o microemulsions on their own and need a cosurfactant (usually an alcohol) for their stabilization, but the ternary systems formed by surfactant, water, and apolar solvent are the simplest and so are the most widely studied.10 The alcohol has two predominant effects, one of which being to change the effective hydrophilicity of the amphiphilic mixture (surfactant + alcohol). Alcohols should be considered as cosolvents that distribute between the pseudophases of the microemulsion (depending on the carbon number of the alcohol, that of the oil, and the properties of the amphiphile) thereby decreasing the effective hydrophilicity of the amphiphile.11 The other effect is that the addition of alcohol to the ternary system water/alkane/surfactant increases the efficiency (or solubilization capacity) of the amphiphilic mixture,12 while at the same time

distorting the three-phase region of the microemulsion system. An increased efficiency can generally be related to a decrease in interfacial tension between bulk water- and oil-rich phases.13 The molecular origin of the decreased interfacial tension is difficult to explain: Strey and Penders12 suggest that on addition of medium-chain alcohols the amphiphilic film might pack more tightly, because the smaller alcohol molecules might fill in the interstitial positions that the surfactant molecules leave in the film. This would result in a better shielding of the water-oil contact and thereby a decrease in interfacial tension. These conclusions would be in accordance with earlier findings of Rosen and Murphy.14 The great capacity of solubilization, together with the specific properties of the trapped water in the microemulsions, has stimulated a great surge in kinetic studies in these new reaction media. The greater simplicity of the AOT microemulsions (sodium bis(2-ethylhexyl) sulfosuccinate), which do not need the addition of a cosurfactant to form stable microemulsions, has meant that the majority of kinetic studies have been carried out on AOT/alkane/water microemulsions.15 However, the number of kinetic studies carried out on four-component microemulsions is very small16 and few of these studies have been quantitative. The cationic surfactant cetyltrimethylammonium bromide has been known for several years to be capable of forming w/o microemulsions in various solvents such as n-hexanol, chloroform, and dichloromethane.17 There is infrared18 and 1H NMR spectral17e,19 evidence for a change of the properties of water in these cationic w/o microemulsions with an increase in W. These changes parallel those observed in AOT microemulsions.20 To carry out a quantitative interpretation of the kinetic results obtained in quaternary w/o microemulsions, it is necessary not only to know the local concentrations of the reagents and their rate constants in the different phases but also to know the variations in the volumes of the different phases,

10.1021/jp000822i CCC: $19.00 © 2000 American Chemical Society Published on Web 06/27/2000

Reactivity in Quaternary Water in Oil Microemulsions

J. Phys. Chem. B, Vol. 104, No. 28, 2000 6619

as the microemulsion composition varies as a consequence of the solubilization of the cosurfactant. In this study, we extended the kinetic models applied in tertiary microemulsions15c,d to also cover quaternary w/o microemulsions, using microemulsions formed by tetradecyltrimethylammonium bromide (TTABr)/1-hexanol/isooctane/water. In all the experiments, the molar relationship [1-hexanol]/[TTABr] was equal to 4. To carry out the study, we chose a reaction with which we are particularly familiar in our laboratory, the nitroso group transfer from N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) to secondary amines,21 in particular to piperazine (PIP) and N-methylbenzylamine (MeBzAm) (Scheme 1). The choice of these amines is based on their different SCHEME 1 Figure 1. Influence of TTABr concentration upon kobs for the nitrosation of piperazine by MNTS at constant W. [piperazine]tot ) 5.45 × 10-2 M; (O) W ) 10, (b) W ) 15, (0) W ) 20, (9) W ) 30, and (2) W ) 40. Lines are drawn for clarity. T ) 25 °C.

solubilizationpropertiesinAOT/isooctane/watermicroemulsions.15c Hence, the comparison of the results obtained in quaternary TTABr microemulsions with those obtained previously in ternary AOT microemulsions will allow us to obtain more information about the influence of the cosurfactant on the reactivity in these media. The model proposed in this study will consider first the concentrations of alcohol in the different pseudophases of the microemulsion. We will then calculate how the volume in the interface (the zone in which the reactions which are being studied will take place) changes with the microemulsion composition due to the incorporation of the cosurfactant. Finally, the kinetic model will be developed, bearing in mind these volume changes. Hence it is important to point out that the presence of alcohol, as a cosurfactant, does not only alter the properties of the interface but also those of the continuous medium. In fact, this study will show that the changes in the properties of the continuous medium can mean that we would have to propose distribution equilibria for the reagents, which would be different from those that are shown in the three component microemulsions. Experimental Section Tetradecyltrimethylammonium bromide (TTABr) was supplied by Sigma and used without further purification. 1-Hexanol and isooctane were Aldrich products of the maximum level of purity commercially available. Piperazine (PIP) (Merck) was used without further purification. N-Methylbenzylamine (MeBzAm) (Aldrich) was distilled under argon and used shortly afterwards. N-Methyl-N-nitroso-p-toluenesulfonamide (MNTS) was supplied by Merck. Microemulsions of the desired compositions were prepared from stock water/surfactant/isooctane/alcohol microemulsions by the addition of appropriate amounts of isooctane and/or water. In all cases, we worked in conditions where the molar relationship [1-hexanol]/[TTABr] remained constant and equal to 4. Densities were measured with a pycnometer. The transnitrosation reactions were carried out using a Varian Cary 500 Scan UV-vis-near-IR spectrophotometer fitted with thermostated cell holders (all experiments were carried out at 25 °C). Kinetic measurements were carried out following the

disappearance of the absorbance at 260 nm due to MNTS consumption for nitrosation of PIP at an initial MNTS concentration of 1.91 × 10-4 M. In the case of N-methylbenzylamine, its high molar absorptivity precluded us from studying the reaction at 260 nm; it was therefore necessary to follow the reaction at 392 nm using an initial MNTS concentration of 2.0 × 10-3 M. The kinetic data always fitted the first order integrated rate equation satisfactorily (r >0.999); in what follows, kobs denotes the pseudo-first-order rate constant. Results MNTS reacts with secondary amines by transnitrosation to give carcinogenic N-nitrosamines.21 The rate equations obtained for the reactions in microemulsions were in all cases similar to those in water, with first-order terms in MNTS and in total amine concentration (figure not shown). We worked under conditions in which ionization of the amines in water is negligible. Competing hydrolysis of MNTS by the small amount of hydroxyl ions liberated by this ionization is insignificant. UV-vis spectra at the end of the reaction indicated quantitative N-nitrosamine formation in every case. Nitrosation of Piperazine. We carried out a series of experiments at 25 °C and fixed amine concentration and variable surfactant concentration (typically between 0.10 and 0.80 M) with the [H2O]/[TTABr] ratio, W, fixed at values ranging from 10 to 45. This wide range of W values was used so as to be able to compare reactivity and partition coefficients obtained under conditions differing significantly as regards the physical state of the water in the medium. At W ) 10, most of the water is probably solvating surfactant heads or bromide ions and hence has a highly rigid, microviscous “icelike” structure, whereas at W ) 45 most of the water present has the properties of bulk water. At constant W, the observed rate constant, kobs, fell slightly as the TTABr concentration increased, while for constant TTABr concentration the reaction rate fell with increasing W (although the increase from W ) 10 to 45 reduced the rate no more than 3-fold); see Figure 1 for representative series of experiments. This behavior is analogous to that previously observed in the nitrosation of PIP by MNTS in tertiary AOT/isooctane/water microemulsions.15c In tertiary microemulsions, the variation of kobs with the microemulsion composition has been interpreted bearing in mind that the amine is distributed between the aqueous pseudophase and the interface, and the MNTS due to

6620 J. Phys. Chem. B, Vol. 104, No. 28, 2000

Garcıá-Rıó and Leis SCHEME 2

The alcohol, 1-hexanol, in principle will be distributed between the three pseudophases of the microemulsion through and K ROH defined as the distribution constants K ROH oi wi

K ROH ) oi Figure 2. Influence of TTABr concentration upon kobs for the nitrosation of N-methylbenzylamine by MNTS at constant W. [Nmethylbenzylamine]tot ) 0.10 M; (O) W ) 10, (b) W ) 25, and (0) W ) 45. Lines are drawn for clarity. T ) 25 °C.

its hydrophobicity will be distributed between the continuous medium and the interface. Under the experimental conditions used in this work, the amount of alkoxide from ionization of the alcohol (cosurfactant) is negligible. Because of that we can neglect the possibility that alcohol take part in the reactions. Nitrosation of N-Methylbenzylamine. We carried out series of experiments at 25 °C and fixed amine concentration. The influence of surfactant concentration, [TTABr], was studied, ensuring that the relationship W remained constant for values of W between 10 and 45. Figure 2 shows the experimental behavior observed. The observed rate constant, kobs, increases as the surfactant concentration and W do so. This behavior is the opposite of that observed experimentally in the nitrosation of PIP by MNTS but is analogous to that observed previously for the nitrosation of MeBzAm by MNTS in AOT/isooctane/ water microemulsions. Since N-methylbenzylamine, unlike piperazine, does not dissolve satisfactorily in water, the amount of amine dissolved in the aqueous phase should be negligible, and the reaction would in principle be limited to two of the three pseudophases present, i.e., the continuous phase and the interface. Discussion Microemulsions and other type of surfactant aggregates such as monolayers, micelles, and vesicles all have an interfacial region that separates the oil and water regions and is composed of surfactant headgroups, associated counterions and co-ions, and any added polar additives. Determining the compositions of aggregate interfaces is an active area of research because interfacial compositions, not stoichiometric concentrations of components, reflect the balance of forces controlling aggregate structure and stability.22 Before interpreting the kinetic results, it is necessary to know the microemulsion composition and, evaluate whether the changes in composition have any repercussions on the change in the volumes of the different pseudophases. Alcohol Partition between the Microemulsion Pseudophases. Alcohols should be considered as cosolvents that distribute between the aqueous and the oil-rich bulk phases, and the interfacial layer, thereby decreasing the effective hydrophilicity of the amphiphile as well as the effective hydrophobicity of the oil. To simplify the analysis of the kinetic data, it is interesting to transform the microemulsion from four components into one of pseudo three components. Scheme 2 shows the distribution of the alcohol throughout the different pseudophases of the microemulsion.

[ROH]i [ROH]o

Z

K ROH wi )

[ROH]i [ROH]w

W

(1)

where [ROH]o, [ROH]i, and [ROH]w refer to the alcohol concentrations in the continuous phase, interface, and water, respectively, referred to the total volume of the microemulsion. The parameters Z and W are defined in their usual way as the molar relationships Z ) [isooctane]/[TTABr] and W ) [H2O]/ [TTABr]. Because of the very low solubility of 1-hexanol in water, we can transform the four-component microemulsion into a pseudoternary one in which the three pseudophases (continuous, interface, and aqueous) would be made up of isooctane + 1-hexanol, TTABr + 1-hexanol, and water, respectively. The partition coefficient, K wo , of 1-hexanol between isooctane and water (estimated as K ow ) 0.059 on the grounds that it must be close to the coefficients for octane/water, decane/water and dodecane/water1,23) allows us to relate K ROH and K ROH as oi wi w ROH ROH ROH K o ) K oi /K wi . The distribution constant K wi was taken ) 550 for from published data for direct micelles1,24 as K ROH wi afford a value TTABr micelles; these values of K wo and K ROH wi ) 32.4. This value can be directly comparable with of K ROH oi obtained by Romsted and Yao22b for the values of K ROH oi association of 1-butanol and 1-hexanol to quaternary cetyltrimethylammonium bromide, CTABr, microemulsions. The values obtained by Romsted and Yao, K ROH ) 23.1 for the oi ) 17.8 for 1-hexanol, are compatible with 1-butanol and K ROH oi ) 32.4 for the incorporation of those calculated for K ROH oi 1-hexanol into TTABr microemulsions. Hence, the shorter length of the hydrocarbon chain of the TTABr in comparison with the CTABr can be translated as a lower degree of hydrophobicity in the interface and consequently a greater distribution constant between the isooctane and the interface. The transformation of a quaternary microemulsion into a pseudoternary one means that we have to redefine the composition parameters of the system. Given that the alcohol can be considered as solubilized at the interface, we will apply the Stilbs approximation25 considering that the surfactant, TTABr, is the only component that determines the properties of the interface. Therefore, the composition parameter W, W ) [H2O]/[TTABr], is not altered. However, the composition of the continuous medium is altered by the presence of the alcohol and therefore we will redefine the Z parameter, Z ) [isooctane]/[TTABr], as

Z* )

[isooctane] + [ROH]o [TTABr]

)Z+

Z[ROH]tot (K ROH + Z)[TTABr] oi (2)

where [ROH]tot refers to the total concentration of 1-hexanol added to the microemulsion. In the experiments carried out in this study, [ROH]tot/[TTABr] ) 4.



Variation of the Volume of the Interface. The application of this formalism implies that the number of alcohol moles incorporated into the interface increases along with the surfactant concentration,1 with the consequent increase in the volume of the interface. In three-component microemulsions (such as AOT/ isooctane/water microemulsions), the volume of the interface can be considered as constant. However, in quaternary microemulsions, the incorporation into the interface of great concentrations of cosurfactant means that the reaction volume at the interface varies along with the surfactant concentration. This situation is similar to that which exists in micellar systems when a high concentration of alcohol is added to the reaction medium.26 The volume of the interface can be considered as a sum of the volumes of the surfactant and the alcohol incorporated into the interface.

Vi ) VjTTABr[TTABr]i + VjROH[ROH]i Vtot 1 ) + Vi VjTTABr[TTABr] Vj

(3)

K ROH +Z oi

ROH

K ROH oi [ROH]tot

(4)

As a molar volume of the surfactant TTABr, we used the value of VjTTABr ) 0.336 M-1 which corresponds to the complete micellar volume of the TTABr micelles.27 In normal micellar systems, there are generally discrepancies concerning the volume element of the reaction in the micellar phase. Bunton et al. propose28 using the molar volume of the Stern layer, the region of the micelle where the reaction takes place, whereas Berezin et al. propose27 using the complete micellar volume. The molar volume of the 1-hexanol, VjROH ) 0.126 M-1, was calculated on the basis of its density. Nitroso Group Transfer to Piperazine. To interpret the variation of the observed rate constant with the microemulsion composition, it is necessary to propose a distribution scheme of the reagents throughout the different phases of the microemulsion. The nitrosation of piperazine by MNTS had been studied previously in our laboratory using AOT/isooctane/water microemulsions.15c Due to the highly hydrophilic character of piperazine, a kinetic model has been proposed in which piperazine is found distributed between the aqueous phase and the AOT interface. Likewise the highly hydrophobic character of MNTS meant that MNTS was distributed between the continuous phase and the interface. Scheme 3 shows a translation of this kinetic scheme for TTABr/1-hexanol/isooctane/water microemulsions.

composition since we are dealing with a quaternary microemulsion. Considering that the total MNTS concentration will be the sum of the concentration in the continuous medium, [MNTS]o and at the interface, [MNTS]i, we can write

[MNTS]i )

K MNTS oi K MNTS + Z* oi

kobs )

k′iK MNTS oi

(7)

K MNTS + Z* oi

where k′i is the first-order rate pseudoconstant referred to the interface. This rate constant can be expressed as the bimolecular rate constant at the interface, k′2, as in29

k′i ) ki2[PIP]ii ) ki2

Vtot [PIP]i Vi

(8)

where [PIP]ii refers to piperazine concentration at the interface referred to the volume of the interface, whereas [PIP]i corresponds with piperazine concentration at the interface referred to the total volume of the microemulsion. Considering the distribution equilibrium of the piperazine shown in Scheme 3, K PIP wi , and that the total piperazine concentration will be the sum of that present in the aqueous pseudophase and in the interface, we can write

[PIP]i )

K PIP wi K PIP wi + W

[PIP]tot

(9)

whence we obtain the following expression for kobs.

kobs )

Vtot ki2 Vi

K MNTS oi

K PIP wi

K MNTS + Z* K PIP oi wi + W

[PIP]tot

[MNTS]i [MNTS]o

Z*

K PIP wi )

[PIP]i [PIP]w

W

(5)

where Z* parameter has been introduced for the microemulsion

(10)

where the relationship Vtot/Vi is calculated on the basis of eq 4. Equation 10 predicts the existence of a linear dependence between (Vtot/Vi)([PIP]tot/kobs) and Z* in accordance with eq 11 for experiments carried out at W constant.

2

In Scheme 3, the distribution constants of the reactives are defined traditionally.

(6)

Given that the reaction can only take place at the interface, the only area in the microemulsion in which the reagents can be in contact with each other, we can obtain the following expression for the pseudo-first-order rate constant

K PIP Vtot [PIP]tot wi + W ) i MNTS PIP (K MNTS + Z*) oi Vi kobs k K K

SCHEME 3

K MNTS ) oi

[MNTS]tot

oi

(11)

wi

Figure 3 shows the existence of a good linear dependence between (Vtot/Vi)([PIP]tot/kobs) and Z*. Table 1 shows the values of the intercepts and slopes according to W, as well as those of the intercept/slope quotient. In accordance with eq 11, the slope/ and should intercept relationship should be equal to K MNTS oi remain constant and independent of W. However, the data in Table 1 show that this is not the case. The presence of alcohol acting as a cosurfactant in the quaternary microemulsions has the objective of making the interface more hydrophobic. Likewise, due to the distribution of the alcohol between the interface and the continuous medium, the latter decreases its hydrophobic character. This alteration in properties in the continuous medium can modify the distribution equilibria of the reagents, so that the kinetic model


Garcıá-Rıó and Leis

Figure 4. Variation of the relationship intercept/slope of the data of Figure 3, Table 1, in accordance with eq 16 (see text).

Figure 3. Linearization of the data of Figure 1 in accordance with eq 11 or eq 15 (see text): (O) W ) 10; (b) W ) 15; (0) W ) 20; (9) W ) 30; (4) W ) 35; (2) W ) 40; and (3) W ) 45.

TABLE 1: Slopes and Intercepts at the Origin Obtained upon Fitting (Vtot/Vi)([PIP]tot/kobs) vs Z* (Figure 3) to the Experimental Kinetic Data for the Reactions of Piperazine with MNTS in Water/TTABr/1-Hexanol/Isooctane Microemulsions at 25 °C W

intercept

slope

intercept/slope

10 15 20 30 35 40 45

291 ( 3 394 ( 15 436 ( 25 575 ( 28 620 ( 43 637 ( 59 614 ( 95

26.1 ( 0.3 42.5 ( 0.8 60.9 ( 1 86.6 ( 1 101 ( 2 109 ( 3 125 ( 5

11.15 9.27 7.16 6.64 6.14 5.84 4.91

shown in Scheme 3, which is perfectly applicable to AOT/ isooctane/water microemulsions, cannot be valid for quaternary microemulsions.30 In fact, we have confirmed that piperazine is not soluble in isooctane, but it is in 1-hexanol/isooctane mixtures with low 1-hexanol contents. This result leads us to modify Scheme 3 including the possible distribution of the piperazine between the three pseudophases of the microemulsion, in accordance with Scheme 4. SCHEME 4

According to Scheme 4, piperazine will be distributed throughout the three pseudophases of the microemulsion. The first implication of this mechanicist scheme is that the reagents will be in contact at the interface and in the continuous medium, so that there will exist two reactive zones in the microemulsion. However, the rate of the nitroso group transfer reactions from alkyl nitrites or N-methyl-N-nitrosobenzenesulfonamides to amines decreases sharply along with the polarity of the medium,31 meaning that the mechanism of the reaction can even be modified. This behavior means we can almost discount the possibility that the reaction will take place in the continuous

phase.32 Bearing these considerations in mind, and on the basis of the kinetic model of Scheme 4, we can obtain the following expression for kobs.

kobs ) ki2

Vtot K MNTS oi MNTS Vi K + Z* K oi

PIP K PIP oi K wi [PIP]tot PIP oi

PIP PIP K PIP wi + K oi W + K wi Z* (12)

Equation 12 can be rewritten as

Vtot [PIP]tot ) Vi kobs W Z* 1 + i MNTS PIP + i MNTS PIP × i MNTS k2 K oi k2 K oi K wi k2 K oi K oi

{

}

(K MNTS oi

+ Z*) (13)

If it is proven that33

Z* , ki2 K MNTS K PIP oi oi

(

)

1 W 1 + i MNTS + i MNTS PIP (14) ki2 K PIP k K k K K wi oi 2 oi 2 oi

eq 13 can be simplified to eq 15.

( (

)

Vtot [PIP]tot W 1 ) i + i PIP + Vi kobs k2 k2 K wi 1 W 1 + i MNTS + i MNTS PIP Z* (15) ki2 K PIP k K k K K wi oi 2 oi 2 oi

)

This equation predicts the existence of a linear dependence between (Vtot/Vi) ([PIP]tot/kobs) vs Z* which is formally analogous to that predicted by eq 11. The results shown in Figure 3 fully satisfy the behavior predicted by eq 15. Unlike eq 11 (kinetic model in Scheme 3), eq 15 (kinetic model shown in Scheme 4) predicts a dependence on the relationship intercept/slope with W.

intercept slope

) K MNTS + K MNTS K PIP oi oi wi

1 W

(16)

Figure 4 shows how the relationship intercept/slope satisfactorily confirms eq 16. On the basis of the values of the intercept and the slope of Figure 4, we can obtain the values of K MNTS ) 3.8 oi ) 20 ( 3. ( 0.3 and of K PIP wi



Figure 5. Variation of the intercepts of Figure 3 in accordance with eq 17, Table 1 (see text).

Figure 6. Variation of the slopes of Figure 3 in accordance with eq 18, Table 1 (see text).

The kinetic model represented in Scheme 4 (eq 15) predicts the existence of a linear dependence between the intercepts of Figure 3 (data in the Table 1) and W according to the equation

intercept )

1 W + i PIP i k2 k2 K wi

(

SCHEME 5

(17)

Figure 5 shows that the values of the intercepts of Table 1 (Figure 3) fulfill the behavior predicted for the kinetic model of Scheme 4, eq 17. On the basis of the intercept, we can obtain a value of ki2 ) (4.2 ( 0.7) × 10-3 M-1 s-1. On the basis of the value previously obtained for K PIP wi ) 20 ( 3 and of the slope of the representation of Figure 5, we can obtain a value of ki2 ) 5.1 × 10-3 M-1 s-1. This value is perfectly compatible with that obtained from the intercept at the origin. Likewise, from the kinetic model, eq 15, we can obtain the existence of a linear dependence between the slopes of the representations of Figure 3 (data in Table 1) and W in accordance with the following equation

slope )

20 ( 3, it is possible to obtain a value of ki2 ) 4.77 × 10-3 M-1 s-1. This value is in agreement with those obtained on the basis of the representation of Figure 5. As a representative value of ki2, we will use the mean value of the three determinations, ki2 ) 4.69 × 10-3 M-1 s-1. The kinetic experiments have not allowed us to determine the value of the distribution coefficient K PIP oi . Because of the high hydrophilic character of piperazine, the value of K PIP oi must be very high, so that the simplification of eq 18 is correct, which impedes its determination. Nitroso Group Transfer to N-Methylbenzylamine. As we have already commented, the experimental behavior observed on studying the variation of the microemulsion composition on kobs in the nitrosation of MeBzAm by MNTS in TTABr/1hexanol/isooctane/water microemulsions is similar to that observed in tertiary AOT/isooctane/water microemulsions. Since N-methylbenzylamine, unlike piperazine, does not dissolve satisfactorily in water, the amount of amine dissolved in the aqueous phase should be negligible, and the reaction would in principle be limited to two of the three pseudophases present, i.e., the continuous phase and the interface. Thus, in the tertiary microemulsions,15c a kinetic model had been proposed which contemplated the distribution of the amine and the MNTS between the continuous medium and the interface. The observed rate constants for reaction of MNTS with N-methylbenzylamine (and other amines) in pure isooctane31 is several orders of magnitude slower than the rate constants observed in the microemulsions, showing that, as in the case of piperazine, in w/o microemulsions the reaction effectively takes place only at the interface, so that the rate of the reaction in the continuous medium can be considered negligible by comparison with the reaction at the interface. This kinetic model can be represented in Scheme 5.

)

1 1 W + i MNTS + i MNTS PIP = ki2 K PIP k K k K K wi oi 2 oi 2 oi W 1 + i MNTS PIP (18) ki2 K MNTS k K K wi oi 2 oi

Figure 6 shows the fulfillment of eq 18, confirming, therefore, the validity of the kinetic model set out in Scheme 4. Judging by the data in Figure 6, it is not possible to determine exactly the intercept. However, on the basis of the slope, and using the values previously obtained for K MNTS ) 3.8 ( 0.3 and K PIP oi wi )

On the basis of Scheme 5 and applying a similar methodology to that developed for the nitrosation of piperazine, we can obtain the following rate equation:

kobs ) ki2

K MeBzAm Vtot K MNTS [MeBzAm]tot oi oi (19) MNTS MeBzAm Vi K + Z* K + Z* oi

oi

This equation can be transformed into

kobs )

Vtot ki2 Vi

K MNTS oi

K MeBzAm [MeBzAm]tot oi

K MNTS + Z* oi

K MeBzAm + Z* oi

(20)

The application of eq 20 predicts the existence of a linear and quadratic dependence of (Vtot/Vi)([MeBzAm]tot/kobs) vs Z* independently of the W values. Figure 7 shows the existence of the aforesaid representation for values of W between 10 and 45 and values of [TTABr] between 0.1 and 0.7 confirming the validity of the kinetic model proposed. To fit eq 20 to the experimental results, we took as a constant the value of K MNTS ) 3.8 ( 0.3 previously obtained in the oi


Garcıá-Rıó and Leis extent, given that the rate of these processes is less sensitive than the previous solvolysis reactions to changes in the polarity of the medium. As we can see in Table 2, the decrease in the rate constant at the interface with respect to the rate constant in bulk water is always greater in quaternary TTABr/1-hexanol/ isooctane/water microemulsions than in tertiary AOT/isooctane/ water microemulsions. Conclusions

Figure 7. Plot of (Vtot/Vi)([MeBzAm]tot/kobs) vs Z* for nitrosation of N-methylbenzylamine by MNTS. The solid line is the theoretical line predicted by eq 20 (parameters in Table 2). [N-methylbenzylamine] ) 0.10 M. (O) W ) 10; (b) W ) 15; (0) W ) 20; (9) W ) 25; (4) W ) 30; (2) W ) 35; (3) W ) 40; and (1) W ) 45.

TABLE 2: Kinetic Parameters and Partition Coefficients Estimated by Fitting Eqs 15 and 20 to the Experimental Kinetic Data for the Reaction of PIP and MeBzAm with MNTS in Water/TTABr/1-Hexanol/Isooctane and in AOT/ Isooctane/Water Microemulsions at 25 °C amine

ki2/M-1 s-1

KMNTS oi

kamine oi

kamine wi

PIP 4.69 × 3.8 ( 0.3 20 ( 3 PIPa 5.33 × 10-3 11 9.5 MeBzAm 6.99 × 10-4 3.8 103 MeBzAma 3.1 × 10-3 11 25.6 10-3

k2/M-1 s-1b 2.98 × 10-2 2.98 × 10-2 4.10 × 10-2 4.10 × 10-2

a AOT/isooctane/water microemulsions. Data from ref 15c. b k 2 values are bimolecular rate constants in pure water. Data from ref 21b.

nitrosation of piperazine. On the basis of this adjustment we ) 103 and ki2 ) 6.99 × 10-4 M-1 obtain the values of K MeBzAm oi -1 s . Comparison of Results. Table 2 shows the values of the rate and distribution constants of the two reactions studied in quaternary TTABr/1-hexanol/isooctane/water microemulsions. As we can see, the values of the rate constant at the interface of the microemulsion, ki2, are always lower than the values found in bulk water, k2 (Table 2). This reduction of 6.4 times in the quaternary TTABr microemulsion and 5.6 times in the AOT microemulsions for the nitrosation of PIP by MNTS is compatible with the polarity of the interface of the microemulsion being less than that of bulk water. A similar behavior is observable in the nitrosation of MeBzAm by MNTS with a reduction of 58 times when a quaternary TTABr/1-hexanol/ isooctane/water microemulsion is used and an inhibition of 13 times when a tertiary AOT/isooctane/water microemulsion is used. Recently we studied the solvolysis of different hydrophobic substrates in AOT/isooctane/water microemulsions34 and in quaternary1 TTABr/1-hexanol/isooctane/water and SDS/1-hexanol/isooctane/water microemulsions. The obtained results showed that the solvolysis rate constant for benzoyl chloride was less at the interface of SDS microemulsions than in that of AOT microemulsions. This difference in reactivity was attributed to the incorporation of 1-hexanol into the interface of the SDS microemulsions being committed to solvation of headgroups and sodium ions, with the result that interfacial polarity is less in SDS than in AOT systems. Therefore, the incorporation of alcohol into the interface should reduce its polarity. This behavior is also shown when studying the nitrosation of PIP and MeBzAm by MNTS, although to a lesser

The amplification of the formalism of the micellar pseudophase was applied satisfactorily to quaternary TTABr/isooctane/1hexanol/water. However, it is necessary to carry out certain modifications with regard to its application to tertiary microemulsions. 1. In quaternary microemulsions it is necessary to consider the distribution of the alcohol throughout the pseudophases of the microemulsion. Considering the role of the alcohol to be similar to that of a solubilized substance, the quaternary microemulsion can be transformed into a pseudotertiary one by introducing a new composition parameter of the microemulsion, Z* ) [(isooctane] + [ROH]o)/[TTABr]. 2. The high alcohol concentration necessary for the formation of the quaternary microemulsion, [ROH]/[TTABr] ) 4 in our experimental conditions, means that its incorporation into the interface causes a change in the volume. This change in the volume of the interface should be taken into consideration for the formulation of the kinetic model since it gives rise to a dilution of the reagents at the interface. The change in volume of the interface was calculated from the molar volumes of the surfactant and the alcohol, for which it suffices to bear in mind the amount of alcohol incorporated into the interface. 3. The presence of the high alcohol concentrations in the microemulsion gives rise to an increase in the hydrophobic character of the interface at the same time as decreasing the hydrophobic character of the continuous medium. These changes in the properties of the continuous medium can have a significant kinetic repercussion, as shown by the nitrosation of PIP by MNTS, where the decrease in hydrophobicity of the continuous medium means that the amine is distributed between the three pseudophases of the microemulsion. 4. The incorporation of alcohol into the interface produces a displacement of water molecules and consequently a reduction in its polarity. This reduction in polarity is shown when comparing the values of the bimolecular rate constant at the interface, ki2, between tertiary and quaternary microemulsions. The reduction in ki2 with respect to the value in bulk water is greater in quaternary microemulsions TTABr/1-hexanol/isooctane/water than in tertiary AOT/isooctane/water microemulsions. Acknowledgment. Financial support from the Direccioń General de Ensenãnza Superior of Spain (project PB96-0954) is gratefully acknowledged. References and Notes (1) Part 1 is: Garcıá-Rıó, L.; Leis, J. R.; Reigosa, C. J. Phys. Chem. B 1997, 101, 5514. (2) Degiorgio, V. Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; North-Holland: Amsterdam, 1985. (3) ReVerse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984. (4) Neogi, P. In Microemulsions: Structure and Dynamics; Friberg, S. E., Bothorel, P., Eds.; CRC Press: Boca Raton, FL, 1987. (5) (a) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 209. (b) Hatton, T. A. In Surfactant Based Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989; and references therein. (6) Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1990, 94, 6400.

Reactivity in Quaternary Water in Oil Microemulsions (7) Yang, F.; Russell, A. J. Biotechnol. Bioeng. 1994, 43, 232. (8) Starks, C. M.; Liotta, C. Phase Transfer Catalysis; Academic Press: New York, 1978. (9) Robinson, B. H.; Khan-Lodhi, A. N.; Towey, T. In Structure and ReactiVity in ReVersed Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989. (10) (a) Structure and ReactiVity in ReVerse Micelles; Pileńi, M. P., Ed.; Elsevier: Amsterdam, 1989. (b) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480. (11) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1991, 95, 5344. (12) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 10313. (13) Kahlweit, M.; Strey, R.; Haase, D.; Firman, P. Langmuir 1988, 4, 785. (14) Rosen, M. J.; Murphy, D. S. Langmuir 1991, 7, 2630. (15) See for example: (a) Valiente, M.; Rodenas, E. J. Phys. Chem. 1991, 95, 3368. (b) Moya´, M. L.; Izquierdo, C.; Casado, J. J. Phys. Chem. 1991, 95, 6001. (c) Garcıá-Rio, L.; Leis, J. R.; Penã, M. E.; Iglesias, E. J. Phys. Chem. 1993, 97, 3437. (d) Garcıá-Rio, L.; Leis, J. R.; Mejuto, J. C. J. Phys. Chem. 1996, 100, 10981. (e) Casavino, F. P.; Sbriziolo, C.; Turco Liveri, M. L. J. Phys. Chem. B 1998, 102, 3143. (f) Casavino, F. P.; Sbriziolo, C.; Turco Liveri, M. L. J. Phys. Chem. B 1998, 102, 5050, and references therein. (g) Durantini, E. N.; Borsarelly, C. D. J. Chem. Soc., Perkin Trans 2 1996, 719. (16) See for example: (a) Franssen, M. C. R.; Weijnen, J. G. J.; Vincken, J. P.; Laane, C.; Van der Plas, H. C. Biocatalysis 1988, 1, 205. (b) Hilhorst, R.; Spruijt, R.; Laane, C.; Veeger, C. Eur. J. Biochem. 1984, 144, 459. (c) Hoffmann, B.; Jackle, H.; Luisi, P. L. Biopolymers 1986, 25, 1113. (d) Samana, J. P.; Lee, K. M.; Biellmann, J. F. Eur. J. Biochem. 1987, 163, 609. (e) Martin, C. A.; McCrann, P. M.; Ward, M. D.; Angelos, G. H.; Jaeger, D. A. J. Org. Chem. 1984, 49, 4392. (f) Minero, C.; Pramauro, E.; Pelizzetti, E. Langmuir 1988, 4, 101. (g) Pereira, R. D. R.; Zanette, D.; Nome, F. J. Phys. Chem. 1990, 94, 356. (h) Adhikari, S.; Joshi, R.; Gopinathan, C. Int. J. Chem. Kinet. 1998, 30, 699. (i) Buchert, P.; Candau, F. J. Colloid Interface Sci. 1990, 136, 527. (17) (a) Ekwall, P.; Mandell, L.; Fontel, K. J. Colloid Interface Sci. 1969, 29, 639. (b) Seno, M.; Sawada, K.; Araki, K.; Iwamoto, K.; Kise, H. J. Colloid Interface Sci. 1980, 78, 57. (c) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2053. (d) Lang, J.; Mascolo, G.; Zana, R.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3069. (e) Germani, r.; Savelli, G.; Cerichelli, G.; Mancini, G.; Luchetti, L.; Ponti, P. P.; Spreti, N.; Bunton, C. A. J. Colloid Interface Sci. 1991, 147, 152. (f) Ayyub, P.; Maitra, A.; Shah, D. O. J. Chem. Soc., Faraday Trans 1993, 89, 3585. (18) (a) Profio, P. D.; Germani, R.; Onori, G.; Santucci, A.; Savelli, G.; Bunto, C. A. Langmuir 1998, 14, 768. (b) Sunamoto, J.; Iwamoto, K.; Nagamatsu, S.; Kondo, H. Bull. Chem. Soc. Jpn. 1983, 56, 2469. (c) Kondo, H.; Miwa, I.; Sunamoto, J. J. Phys. Chem. 1982, 86, 4826. (19) (a) Germani, R.; Ponti, P. P.; Spreti, N.; Savelli, G.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A.; Si, V. J. Colloid Interface Sci. 1990, 138,

J. Phys. Chem. B, Vol. 104, No. 28, 2000 6625 443. (b) Germani, R.; Ponti, P. P.; Romeo, T.; Savelli, G.; Spreti, N.; Cerichelli, G.; Luchetti, L.; Mancini, G.; Bunton, C. A. J. Phys. Org. Chem. 1989, 2, 553. (20) (a) Eicke, H. F. Top. Curr. Chem. 1980, 87, 85. (b) Luisi, P. L.; Magid, L. J. CRC Crit. ReV. Biochem. 1986, 20, 409. (c) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279. (d) Goto, A.; Yoshioka, H.; Manabe, M.; Goto, R. Langmuir 1995, 11, 4873. (e) De, T. K.; Maitra, A. AdV. Colloid Interface Sci. 1995, 59, 95. (21) (a) Castro, A.; Leis, J. R.; Penã, M. E. J. Chem. Soc., Perkin Trans 2 1989, 1861. (b) Garcıá-Rıó, L.; Iglesias, E.; Leis, J. R.; Penã, M. E.; Rios, A. M. J. Chem. Soc., Perkin Trans 2 1993, 29. (22) (a) Chaudhuri, A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8362. (b) Yao, J.; Romsted, L. S. J. Am. Chem. Soc. 1994, 116, 11779. (23) Aveyard, R.; Mitchell, R. W. J. Chem. Soc., Faraday Trans 1969/ 1970, 2465. (24) (a) Abraham, M. H.; Chadha, H. S.; Dixon, J. P.; Rafols, C.; Treiner, C. J. Chem. Soc., Perkin Trans 2 1995, 887. (b) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (25) Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. (26) Bravo, C.; Leis, J. R.; Penã, M. E. J. Phys. Chem. 1992, 96, 1957. (27) Yatsimirski, A. K.; Martinek, K.; Berezin, I. V. Tetrahedron 1971, 27, 2855. (28) (a) Bunton, C. A.; Romsted, L. S.; Savelli, G. J. Am. Chem. Soc. 1979, 101, 1, 1253. (b) Bunton, C. A. Catal. ReV. Sci. Eng. 1979, 20, 1. (29) The definition of ki2 encompassing the reaction volume means that it has units of M-1s-1. (30) An alternative interpretation would be to suppose that the equilibrium constant K MNTS varies as a consequence of the 1-hexanol distribution oi between the continuous medium and the interface. If this was the case, the intercept/slope relationship, and therefore K MNTS of Table 1, could vary oi with W. However, we have recently found that the values of Koi of other highly hydrophobic substances in TTABr/1-hexanol/isooctane/wtaer microemulsions, namely benzoyl chloride, anisoyl chloride, diphenylmethyl chloride, etc. are independent of W (Garcıá-Rio, L.; Leis, J. R.; Reigosa, C. J. Phys. Chem. B 1997, 101, 5514). Besides, as we will go on to show, the results obtained on studying the nitrosation of MeBzAm by MNTS agree with the existence of a value of K MNTS which is independent of W. oi (31) (a) Garcıá-Rıó, L.; Leis, J. R.; Iglesias, E. J. Org. Chem. 1997, 62, 4701. (b) Garcıá-Rıó, L.; Leis, J. R.; Iglesias, E. J. Org. Chem. 1997, 62, 4712. (32) This approximation has been satisfactorily applied for nitroso group transfer reactions in AOT/isooctane/water microemulsions. See for example: (a) Garcıá-Rio, L.; Leis, J. R.; Penã, M. E.; Iglesias, E. J. Phys. Chem. 1993, 97, 3437. (b) Garcıá-Rio, L.; Leis, J. R.; Mejuto, J. C. J. Phys. Chem. 1996, 100, 10981. (33) The inequality is confirmed primarily by the high value of K PIP oi . (34) Garcıá-Rıó, L.; Iglesias, E.; Leis, J. R J. Phys. Chem. 1995, 99, 12318.

Reactivity in Quaternary Water in Oil Microemulsions. 2. Different

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