Pseudophase Approach to the Transfer of the Nitroso Group in Water

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Pseudophase Approach to the Transfer of the Nitroso Group in Water/AOT/SDS/Isooctane Quaternary Microemulsions L. Garcıá-Rıó,† P. Herve´s,‡ J. C. Mejuto,*,‡ J. Pe´rez-Juste,‡ and P. Rodrı´guez-Dafonte‡ Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago de Compostela, Santiago de Compostela, Spain, and Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Universidad de Vigo, Vigo, Spain Received April 7, 2000. In Final Form: September 20, 2000 The kinetics of the transfer of the nitroso group from N-methyl-N-nitroso-p-toluenesulfonamide to each of three secondary amines (piperazine, N-methylbenzylamine, and morpholine) was studied using a wide variety of water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/sodium dodecyl sulfate/isooctane quaternary microemulsions as reaction media. The diverse kinetic behavior of these amines can be explained quantitatively on the basis of a single model taking into account the distribution of the amine between the aqueous and isooctane phases and their mutual interface (surfactant film); the reaction always takes place at the surfactant film. The reactivities of the amines are discussed in comparison with the behavior observed in water and in AOT/isooctane/water ternary microemulsions.

Introduction Microemulsions as chemical reaction media are interesting subjects of study because they are macroscopically homogeneous and isotropic but are heterogeneous on a microscopic scale. Microemulsions contain aqueous microdroplets dispersed in a low-polarity bulk solvent.1,2 The reagents present in the medium may be separated in different microscopic phases or may share the same phase, and the kinetics of their reactions will reflect their various distributions. Apart from these considerations, microemulsions are of interest as reaction media because of similarities with biological systems.3-5 On the other hand, microemulsions have found a growing number of scientific and technological applications: they afford control over the size of synthesized nanoparticles;6 they have numerous applications in the fields of solubilization and extraction;7 and when the surfactant interface is stereoselective for certain reagents, they can be used for stereoselective synthesis. A new variety of systems are currently being studied in which two surfactants with different properties are used. Such mixed systems are used in daily applications. In the case of micellar aggregation, surfactant mixtures often perform better than a single surfactant. Detergent and cleaning formulations often include both anionic surfactants, to maximize solubilization, and nonionic † ‡

Universidad de Santiago. Universidad de Vigo.

(1) Reverse Micelles; Luisi, P. L., Straub, B. E., Ed.; Plenum Press: New York, 1984. (2) Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989. (3) Fendler, J. H. Chem. Eng. News 1984, 2, 29. (4) Thomas, J. K. Chem. Rev. 1980, 80, 281. (5) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1980, 947, 209. (6) Fendler, J. H. Chem. Rev. 1987, 87, 877. (b) Liz, L.; Lo´pez Quintela, M. A.; Mira, J.; Rivas, J. J. Mater. Sci., 1994, 29, 3797. (7) Mukerjee, P.; Ray, A. J. Phys. Chem. 1966, 70, 2144. (b) Mukerjee, P.; Cardinal, R. J.; Desai, N. R. Micellization, Solubilization and Microemulsions; Mital, K. L., Ed.; Plenum Press: New York, 1977. (c) Funasaki, J. J. Phys. Chem. 1979, 83, 1998. (d) Fernańdez, M. S.; Fromherz, P. J. Phys. Chem. 1979, 83, 1755.

surfactants, to maximize water hardness tolerance. In skin care applications, synergism in a surfactant mixture can minimize the total surfactant monomer concentration, which in turn has been shown to reduce skin irritation. With regard to mixed microemulsions, some studies about stability and internal dynamics have been published.8a Although comprehensive studies of kinetic effects of nonionic-ionic mixed microemulsions have been reported in the literature,8b we are as yet unaware of any studies on the influence of ionic-ionic mixed microemulsions upon chemical reactivity. In recent years, great effort has been devoted to the study of aqueous microdroplets, generally in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/alkane microemulsions, which can contain large quantities of water and in which droplet size is controlled by the water/AOT ratio: W ) [H2O]/[AOT].1,2,9 This research suggests that the physical characteristics of water inside a microdroplet differ from those of bulk water.9-15 There have also been interesting reports on the kinetics of chemical reactions occurring in microemulsions,16-27 but few of these studies (8) Fernańdez No´voa, A.; Quibeń, J.; Liz-Marzań, L. M. Colloid Polym. Sci. 1996, 274, 239. (b) Ikushima, Y. Recent Res. Dev. Chem. Eng. 1997, 1, 49. (9) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480. (10) Wong, M.; Gratzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1976, 98, 2391. (11) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977, 99, 4730. (12) Llor, A.; Rigny, J. J. Am. Chem. Soc. 1986, 108, 7533. (13) Keh, E.; Valeur, B. J. Colloid Interface Sci. 1981, 79, 465. (14) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409. (15) Zinsli, P. E. J. Phys. Chem. 1983, 83, 3223. (16) Menger, F. M.; Donohue, J. A.; Williams, R. F. J. Am. Chem. Soc. 1973, 95, 286. (17) Blandamer, M. J.; Burgess, J.; Clark, B. J. Chem. Soc., Chem. Commun. 1983, 659. (18) Da Rocha Pereira, R.; Zanette, D.; Nome, F. J. Phys. Chem. 1990, 94, 356. (19) Hubig, S. M.; Rodgers, M. A. J. J. Phys. Chem. 1990, 94, 1933. (20) O’Connor C. J.; Fendler, E. J.; Fendler, J. H. J. Am. Chem. Soc. 1973, 95, 600. (21) El-Soeud, O. A.; da Silva, M. J.; Barbur, L. P.; Martins, A. J. Chem. Soc., Perkin Trans. 2 1978, 331. (22) Minero, C.; Pramauro, E.; Pelizzetti, E. Langmuir 1988, 4, 101.

10.1021/la000523k CCC: $19.00 © 2000 American Chemical Society Published on Web 11/17/2000

Nitroso Group Transfer

have been quantitative. Prediction and/or interpretation of the kinetic influence of these media is relatively easy when both reagents congregate in the aqueous microdroplets which act as variable-size nanoreactors concentrating the reagents. 25-28 Less attention has been paid to reactions in which the reagents are distributed between the aqueous and apolar phases and at their interface.23-27,29 In the existing literature, the pseudophase model has been applied to interpret the reactivity in microemulsions,23-24,26 providing a satisfactory, quantitative explanation of reactivity in microemulsions. In the present work we report the kinetics of nitrosation of three secondary amines by N-methyl-N-nitroso-ptoluenesulfonamide (MNTS) in water/AOT/sodium dodecyl sulfate (SDS)/isooctane microemulsions. The amines (piperazine, N-methylbenzylamine, and morpholine) were chosen on the basis of their degrees of solubility in the various components of the microemulsions: piperazine (PIP) is practically insoluble in isooctane, N-methylbenzilamine (NMBA) is poorly soluble in water, and morpholine (MOR) has a considerable solubility in both water and isooctane. The MNTS has a very low degree of solubility in water. The chemistry of MNTS has been extensively studied in aqueous media,30 in micellar media,31-33 and in water/AOT/isooctane microemulsions.23 In particular, the chemistry of MNTS has received much attention due to the fact that MNTS reacts with secondary amines by transnitrosation to give carcinogenic N-nitrosamines.34 Experimental Section AOT (Aldrich) was dried for 2 days in a vacuum desiccator and used without further purification. SDS (Aldrich) was used as supplied. NMBA (Aldrich) was purified by distillation under an argon atmosphere. All other reagents were supplied by Merck and Aldrich and used without further purification. Reaction kinetics for PIP + MNTS and MOR + MNTS were followed by monitoring changes in absorbance at 260 nm due to MNTS consumption. In the case of NMBA + MNTS, it was necessary to follow the reaction at 280 nm. The concentration of MNTS was always much lower than the concentration of amine. The kinetic data always fitted the first-order integrated equation satisfactorily (r > 0.999); in what follows, ko denotes the pseudofirst-order constant. The equation derived from the proposed pseudophase model was fitted to the whole set of pseudo-firstorder rate constants, ko, by means to a nonlinear curve fitting program based on Marquardt’s algorithm.35 (23) Garcıá-Rıó, L.; Leis, J. R.; Penã, M. E.; Iglesias E. J. Phys. Chem. 1993, 97, 3437. (24) Garcıá-Rıó, L.; Leis, J. R.; Mejuto, J. C. J. Phys. Chem. 1996, 100, 10981. (25) Blagoeva, I. B.; Gray, P.; Ruasse, M. F. J. Phys. Chem. 1996, 100, 12638. (26) Garcıá-Rıó, L.; Iglesias, E.; Leis, J. R. J. Phys. Chem. 1995, 99, 12318. (27) Ruasse, M. F.; Blagoeva, I. B.; Ciri, R.; Garcıá-Rıó, L.; Leis, J. R.; Marques, A.; Mejuto, J. C.; Monnier, E. Pure Appl. Chem. 1997, 69, 1923. (28) Valiente, M.; Ro´denas, E. J. Phys. Chem. 1991, 95, 3368. (29) Khmelnitsky, Y. L.; Noverova, I. N.; Polyakov, V. I.; Grinberg, V. Y.; Levashov, A. V.; Martinek, K. Eur. J. Biochem. 1990, 190, 155. (30) Garcıá-Rıó, L.; Iglesias, E.; Leis, J. R.; Penã, M. E.; Rios, A. J. Chem. Soc., Perkin Trans 2 1993, 29. (31) Bravo, C.; Herve´s, P.; Leis, J. R.; Penã, M. E. J. Phys. Chem. 1990, 94, 8816. (b) Bravo, C.; Leis, J. R.; Penã, M. E. J. Phys. Chem. 1992, 96, 1957. (32) Garcıá-Rıó, L.; Leis, J. R.; Mejuto, J. C.; Pe´rez-Juste, J. J. Phys. Chem. B 1997, 101, 7383. (b) Herve´s, P.; Leis, J. R.; Mejuto, J. C.; Pe´rez-Juste, J. Langmuir 1997, 13, 6633. (33) Garcıá-Rıó, L.; Herve´s, P.; Leis, J. R.; Mejuto, J. C.; Pe´rez-Juste, J. J. Phys. Org. Chem. 1998, 11, 584. (34) Castro, A.; Leis, J. R.; Penã, M. E. J. Chem. Soc., Perkin Trans 2 1989, 1861. (35) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431.

Langmuir, Vol. 16, No. 25, 2000 9717

Figure 1. Fitting eq 5 to experimental results for the transnitrosation between MNTS and PIP: (b) F ) 0.150; (9) F ) 0.050; (2) F ) 0.010.

Results and Discussion The reactivity between MNTS and secondary amines in water is well established.30 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 total amine concentration. In our experimental conditions, the ionization of the amines in water is negligible. The competing hydrolysis of MNTS by the small amount of hydroxyl ions liberated by this ionization is insignificant because solutions of MNTS with as much as 2 × 10-2 M of HO- (referred to the total volume of solution) are stable for long periods of time (several days). UV-vis spectra at the end of the reaction indicated quantitative nitrosamine formation in every case. Reaction with Piperazine. The influence of the composition of the microemulsion on the rate of transnitrosation of PIP by MNTS was studied in a series of experiments in which [PIP]total was always 5.00 × 10-2 M. The mole ratio W was defined as W ) [H2O]/[surfactant] (where [surfactant] ) [AOT] + [SDS]) in analogy with water/AOT/isooctane microemulsions (where W ) [H2O]/ [AOT]). W was varied from series to series over the range 7.4-25.8. The concentration surfactant was varied between 0.443 and 0.544. The mole ratio F was defined as [SDS]/[AOT], and it was varied from series to series over the range 0-0.15. Properties of the medium with marked effects on reactivity, including polarity, microviscosity, interdroplet mass transfer, and the structure of water, are known to vary significantly over the range of compositions used. The value of ko increases by increasing [surfactant] for any given W and also decreases by increasing W for a given [surfactant]. This result is analogous to that observed in water/AOT/isooctane microemulsions,23 and it differs from that observed when PIP is nitrosated by alkylnitrites in water/AOT/isooctane.24 In this case, ko is unaffected by [surfactant] and for a given [AOT] it increases slightly with W. These differences can be attributed to the differences in solubility between MNTS and alkylnitrites. Figure 1 shows the results for different F values. The pseudophase model for the reaction with MNTS and PIP, which treats the microemulsion as a three-layer bulk system and ignores its actual micellar structure, is illustrated in Scheme 1. To avoid having to define the volumes of the pseudophases, the partition coefficients assumed to govern the distribution of the reagents among the three pseudophases are defined in terms of their mole

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Garcıá-Rıó et al.

Scheme 1

per mole concentrations in the pseudophases

K1 ) K4 )

[PIP]i [PIP]w

W

[MNTS]i [MNTS]o

Z

(1)

where the subscripts w, o, and i indicate quantities in water, oil, and the surfactant film, respectively, square brackets, as usual,36 indicate concentrations referred to the total volume of microemulsion, and Z is defined, in analogy with W, as the ratio [iC8]/[surfactant] ([surfactant] ) [AOT] + [SDS]). With these definitions, the model of Scheme 1 implies that the overall pseudo-first-order rate constant is given by

1 ko ) ki′ 1 + (Z/K4)

(2)

where ki′ represents the pseudo-first-order rate constants in the surfactant film. This constant is expressed in terms of the bimolecular rate constant ki as

ki′ ) ki

[PIP]i [surfactant]

[PIP]t

ki

[surfactant] (1 + (W/K1))(1 + (Z/K4))

ko )

[PIP]t

kiK1K4

[surfactant] (K1 + W)(K4 + Z)

kiK1K4 [surfactant] ko ) [PIP]t (Ki + W)(K4 + Z)

K2 )

(3)

Like the partition coefficients, to avoid having to define the volumes of the pseudophases, ki is defined in terms of mole per mole concentration in the corresponding phases; eq 2 then becomes

ko )

(the discrepancy was less than 10%). This is confirmed by the agreement with previously reported values of similar reactions in AOT microemulsions23,24 and K4 values for the association of MNTS to SDS micelles.31 The average value of K4 is also in good agreement with the value of about 11 estimated for the MNTS-(AOT + SDS) association constant by analysis of changes in the UV spectrum of MNTS at 265-275 nm. It should be borne in mind that precise spectroscopic estimation of this constant is ruled out by the impossibility of having all the MNTS in the interface. The spectroscopic estimation of K4 was therefore based on an analysis of the spectroscopic data37 using a fitting procedure with the absorbance of MNTS when fully bound to the interface as an optimizable parameter. From these experiments the value of K4 was K4 ) 11 ( 2. Reaction with N-Methylbenzylamine. The reactions of MNTS with NMBA in water/AOT/SDS/isooctane microemulsions were studied by means of series of reactions analogous to those described for MNST + PIP reactions. The influence of the composition of the microemulsion on the rate of transnitrosation of NMBA by MNTS was studied in a series of experiments in which [NMBA]total was always 0.1 M. The mole ratio W was varied from series to series over the range 7.4-25.8. [surfactant] was varied between 0.443 and 0.544. The mole ratio F was varied from series to series over the range 0-0.15. In this case, however, ko values increase significantly with [surfactant] for a given W, and only slightly with W for a given [surfactant]; similar behavior is exhibited when NMBA is nitrosated by MNTS and alkylnitrites in water/AOT/ isooctane microemulsions.23,24 Because of the poor solubility of NMBA in water, the pseudophase model for this reaction would a priori consider simultaneous reactions in the surfactant film and the isooctane pseudophase. However, the reaction rate in isooctane38 is several orders of magnitude less than those observed in this work. The pseudophase model adopted for the reaction of NMBA is therefore that shown in Scheme 2. With K2 and K4 defined as above by

(4)

(5)

(6)

The parameters ki, K1, and K4 were estimated by fitting with a multidimensional nonlinear regression program based on Marquardt’s algorithm.35 Table 1 lists the values of kinetic parameters for each value of F. Although in this case (with three parameters to optimize in the equation) there is a significant degree of correlation, the fit is good (36) Romsted, L. S. Surfactants in Solution; Lindman, B. Mittal, K. L., Eds.; Plenum Press: New York, 1984, vol. 2, p 1015.

K4 )

[NMBA]i [NMBA]o [MNTS]i [MNTS]o

Z

Z

(7)

calculations analogous to those described for PIP lead to the expression

ko )

[NMBA]t

ki

[surfactant] (1 + (Z/K2))(1 + (Z/K4))

ko )

[NMBA]t

KiK2K4

[surfactant] (K2 + Z)(K4 + Z)

KiK2K4 [surfactant] ko ) [NMBA]t (K2 + Z)(K4 + Z)

(8)

(9)

(10)

Fitting eq 10 to the experimental data (Figure 2) with the same nonlinear regression program as described above (37) Garcıá-Rıó, L.; Herve´s, P.; Mejuto, J. C.; Parajo´, M.; Pe´rez-Juste, J. J. Chem. Res. 1998, 716. (38) 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. (c) Boni, J. C.; Garcıá-Rıó, L.; Moreira, J. A. J. Org. Chem. 1999, 64, 8887.


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Table 1. Kinetic Values for the Transnitrosation between MNTS and PIP, NMBA, and MOR Obtained from Equations 6, 10, and 14a V h /M-1

F MNTS + PIP

MNTS + NMBA

MNTS + MOR

a

0.1500 0.1250 0.1125 0.1000 0.0750 0.0500 0.0250 0.0100 0.0000 0.1500 0.1250 0.1125 0.0875 0.0625 0.0500 0.0250 0.0100 0.0000 0.1500 0.1250 0.1125 0.1000 0.0875 0.0750 0.0625 0.0500 0.0250 0.0100 0.0000

K1/M-1

K2/M-1

9.5 ( 0.7 9.4 ( 0.8 9.1 ( 0.5 9.7 ( 0.6 10 ( 1 9.7 ( 0.4 9.5 ( 0.8 9.5 ( 0.8 9.6 ( 0.9

0.336 0.341 0.344 0.347 0.353 0.359 0.364 0.368 0.370 0.336 0.341 0.344 0.350 0.356 0.359 0.364 0.368 0.370 0.336 0.341 0.344 0.347 0.350 0.353 0.356 0.359 0.364 0.368 0.370

46 ( 1 47 ( 2 47 ( 1 48 ( 2 46 ( 2 45 ( 2 46 ( 3 46 ( 2 47 ( 2 46 ( 2 49 ( 3

25 ( 1 26 ( 2 25 ( 1 28 ( 2 26 ( 2 25 ( 1 24 ( 1 25 ( 2 25 ( 1 ∼600 ∼600 ∼600 ∼600 ∼600 ∼600 ∼600 ∼600 ∼600 ∼600 ∼600

K4/M-1 10 ( 1 11 ( 1 11 ( 1 9(2 12 ( 1 10 ( 2 12 ( 1 11 ( 1 12 ( 1 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b 11b

ki/s-1

k2i/M-1 s-1 10-2

(1.57 ( 0.07) × (1.64 ( 0.08) × 10-2 (1.55 ( 0.05) × 10-2 (1.56 ( 0.06) × 10-2 (1.57 ( 0.06) × 10-2 (1.50 ( 0.05) × 10-2 (1.50 ( 0.06) × 10-2 (1.47 ( 0.08) × 10-2 (1.45 ( 0.08) × 10-2 (4.4 ( 0.1) × 10-3 (4.2 ( 0.2) × 10-3 (2.8 ( 0.1) × 10-3 (3.1 ( 0.1) × 10-3 (3.7 ( 0.2) × 10-3 (4.7 ( 0.2) × 10-3 (3.5 ( 0.1) × 10-3 (4.0 ( 0.2) × 10-3 (3.1 ( 0.1) × 10-3 (8.8 ( 0.5) × 10-4 (9.3 ( 0.6) × 10-4 (8.1 ( 0.4) × 10-4 (8.8 ( 0.4) × 10-4 (8.4 ( 0.5) × 10-4 (8.3 ( 0.5) × 10-4 (8.2 ( 0.5) × 10-4 (8.8 ( 0.6) × 10-4 (8.9 ( 0.7) × 10-4 (8.8 ( 0.6) × 10-4 (6.5 ( 0.6) × 10-4

5.3 × 10-3 5.6 × 10-3 5.3 × 10-3 5.4 × 10-3 5.5 × 10-3 5.4 × 10-3 5.5 × 10-3 5.4 × 10-3 5.4 × 10-3 1.5 × 10-3 1.4 × 10-3 9.5 × 10-4 1.1 × 10-3 1.3 × 10-3 1.6 × 10-3 1.3 × 10-3 1.5 × 10-3 1.2 × 10-3 3.0 × 10-4 3.1 × 10-4 2.8 × 10-4 3.1 × 10-4 2.9 × 10-4 2.9 × 10-4 2.9 × 10-4 3.1 × 10-4 3.3 × 10-4 3.2 × 10-4 2.4 × 10-4

k2i is obtained from the ki value using eq 15. b From MNTS + PIP data.

(with K4 ) 11 as obtained in the experiments with PIP) provided the values of K2 and ki listed in Table 1. The satisfactory fit obtained for these experiments supports the validity of the model employed. Reaction with Morpholine. In experiments on the reaction between MOR and MNTS, the observed rate constant, ko, hardly changed. This is because MOR is probably present in all three pseudophases in the microemulsion, a more complex situation than any of those considered above.23,24 However, the effective reaction region is still constituted by the interface alone, because reaction in isooctane is much slower than the observed rates. Taking these considerations into account, the pseudophase model applied is that shown in Scheme 3. Calculations analogous to those described in the previous section lead to the following expressions

[MOR]i

K1 )

[MOR]w

K2 ) K4 ) ko )

ko )

[MOR]i

W

[MOR]i [MOR]o

Z

[MNTS]i [MNTS]o

Z

(11)

ki W Z Z [surfactant] 1+ + 1+ K1 K2 K4

)

(12)

[surfactant] (K1K2 + K2W + K1Z)(K4 + Z)

(13)

[MOR]i

[MOR]i [surfactant]

ko )

(

)(

kiK1K2K4 kiK1K2K4 (K1K2 + K2W + K1Z)(K4 + Z)

(14)

Scheme 2

Fitting eq 14 to the experimental data (Figure 3), with K4 fixed values obtained as described above (Table 1), yielded the values of ki, K1, and K2 that are likewise listed in Table 1. The mean discrepancy between the experimental and fitted values of ko was less that 10%. Comparison of the Results. For comparison of reactivities in the surfactant interface with the corresponding reactivities in bulk water, the ki of Table 1 (defined in terms of mole per mole concentrations and expressed in s-1) must be converted to conventional reaction rates expressed in M-1 s-1. In standard water/ AOT/isooctane microemulsions23,24 this requires some knowledge of the molar volume of AOT under the conditions prevailing at the interface, which is far from easy to ascertain with confidence since it implies a precise definition and knowledge of the volume of the interface. For the purposes of comparison with previous results, we assume that the molar volume of AOT in the interface, V h, is to be given by its density, and we accordingly define a “conventional” bimolecular rate constant, k2i for the

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Figure 2. Fitting eq 8 to experimental results for the transnitrosation between MNTS and NMBA: (b) F ) 0.112; (9) F ) 0.050; (2) F ) 0.01.

Figure 3. Fitting eq 11 to experimental results for the transnitrosation between MNTS and MOR: (b) F ) 0.150; (9) F ) 0.125; (2) F ) 0.050. Scheme 3

reaction at the interface by23,24

k2i ) kiV h

(15)

This value of molar volume of AOT was estimated as 0.37 M-1. We will consider water/AOT/SDS/isooctane to be an ideal mixture. With this assumption, the molar volume of the surfactant mixture can be written as

h AOT V h ) FV h SDS + (1 - F)V

(16)

where F is the mole ratio SDS/AOT (vide supra) and the

Figure 4. Variation of k2i for the transnitrosation reaction between MNTS and PIP (b), NMBA (9), and MOR (2) in water/ AOT/SDS/isooctane microemulsions. Solid lines represent the theoretical values.

subscripts SDS and AOT denote the molar volume of surfactants (0.14 and 0.37 M-1, respectively). The comparison of k2i and corresponding k2 in Table 1 shows that the reactions studied proceed much more slowly in the interphase due to the fact that the polarity of the interface, like the polarity of the interface of normal micelles,39 is less than that of bulk water. The order of reactivity in water30 is NMBA > PIP > MOR. The respective rate constant ratios are 8:6:1. This order parallels the basicity30,34 of the amines. At the AOT interface (in water/AOT/isooctane microemulsions)23 the corresponding rate constant ratios are 5:49:1, and these rate constant ratios are constants for each F value in water/ AOT/SDS/isooctane microemulsions. For NMBA and MOR the order is the same as that in water, but PIP is considerably more reactive than the other amines studied here. The increased relative reactivity of PIP is due to its two equivalent N atoms endowing it with a statistical advantage. This advantage may be enhanced by the mobility of the substrate molecule being reduced by entrapment in the interface. The absence of differences between the rate constant ratios in water/AOT/isooctane microemulsions and water/AOT/SDS/isooctane microemulsions can be attributed to the absence of changes in the microviscosity of the surfactant film with the inclusion of SDS molecules. Significant changes in the microviscosity will induce changes in the reactivity of PIP where, as noted above, the mobility in the interface plays an important role in the reactivity. Figure 4 shows the influence of F upon the ki values. There was no influence of SDS/AOT mole ration on the value of ki, but when F increases, the value of k2i decreases (Table 1). This decrease with the increase of SDS in the microemulsion correlates with a decrease in the polarity of the interface. The lack of any observable influence on ki can be attributed to the presence of two opposed effects when F increases: (1) a larger value of the SDS/AOT mole ratio means a lower molar volume of the surfactant film; (2) an increase in the SDS/AOT molar ratio means a decrease in the surfactant film polarity. Finally, absolute comparison of these values of k2i with those corresponding to the reaction in bulk water shows that the nitroso transfer reactions are 20-50 times slower at the interface of the microemulsions of water/AOT/SDS/ isooctane microemulsions, which can be attributed to the lower polarity of the interfacial region. The transition state (39) Iglesias, E.; Leis, J. R.; Penã, M. E. Langmuir 1994, 10, 662.


Langmuir, Vol. 16, No. 25, 2000 9721

Table 2. Changes in Percolation Temperatures of Water/ AOT/SDS/Isooctane Microemulsions for Different G Values F

δTp ) F)0Tp - F*0Tp

F

δTp ) F)0Tp - F*0Tp

0 0.0100 0.0250 0.0500 0.0625 0.0750

0.0 1.3 1.3 21.3 33.5 40.3

0.0875 0.1000 0.1125 0.1250 0.1500

>44 >44 >44 >44 >44

for the transnitrosation between MNTS and secondary amines requires a certain degree of charge separation, and reduction of the polarity will cause a decrease in reaction rate.30,34 This explanation is consistent with the behavior observed in other reactions of nitroso compounds (as MNTS and alkylnitrites) in water/AOT/isooctane microemulsions23,24 and in normal micelles.31-33,40,41 Percolation. The conditions used in some of the experiments carried out in this study were such that electric percolation may have taken place, i.e., interdroplet mass transport made possible by the fleeting formation of interdroplet channels during droplet-droplet collisons.42 In water/AOT/isooctane microemulsions,43,44 percolation is facilitated by the presence of amines. A similar effect is observed when adding SDS to water/AOT/ isooctane. An increase in the molar ratio F in water/AOT/ SDS/microemulsion corresponds with a strong decrease in the percolation threshold for these quaternary microemulsions.45 The difference between the percolation threshold of a standard water/AOT/isooctane microemulsion and a water/AOT/SDS/isooctane microemulsion is more than 40 °C (Table 2). This effect is also observed when the rate constant of matter exchange between droplets is determined. In the literature there is evidence that this constant is 1.7 times bigger in water/AOT/SDS/ heptane systems with F ) 0.1 than in water/AOT/heptane microemulsions (F ) 0, ke ) 13 × 106 M-1 s-1; F ) 0.1, ke ) 22 × 106 M-1 s-1).46 The pseudophase model does not take this phenomenon into account. However, our results for the reactions examined in this study are valid regardless of the occurrence of percolation, because these reactions are chemically controlled and their half-lives are much longer (40) Garcıá-Rıó, L.; Iglesias, E.; Leis, J. R.; Penã, M. E. Langmuir 1993, 9, 1263. (b) Garcıá-Rıó, L.; Iglesias, E.; Fernańdez, A.; Leis, J. R. Langmuir 1995, 11, 1917. (41) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans 1 1987, 83, 985. (42) Granqvist, C. G.; Hunderi, O. Phys. Rev. B 1978, 18, 1554. (43) Garcıá-Rıó, L.; Iglesias, E.; Leis, J. R.; Mejuto, J. C.; Penã, M. E. Langmuir 1994, 10, 1676. (b) A Ä lvarez, E.; Garcıá-Rıó, L.; Mejuto, J. C.; Navaza, J. M. J. Chem. Eng. Data 1998, 43, 433. (44) Garcıá-Rıó, L.; Herve´s P.; Mejuto J. C.; Pe´rez-Juste J.; Rodrı´guezDafonte P. J. Colloid Interface Sci. 2000, 225, 259. (45) Garcıá-Rıó, L.; Herve´s P.; Mejuto J. C.; Rodrı´guez-Dafonte P. Manuscript in preparation. (46) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3543.

than those of the interdroplet mass transport phenomena.47 Such kinetic and mechanistic independence of percolation has previously been reported for the hydrolysis of 4-nitrophenyl chlorofomate in microemulsions of water/ AOT/isooctane.26 Conclusions In all the cases considered above, the results can be interpreted by assuming that the reaction occurs at the interface between the water droplet and isooctane and that the dependence of the observed rate constants on reaction conditions is largely governed by the relative affinities of the amines for the different phases present in the medium. The hydrophobicity of the amine determines its distribution between the three phases and whether the rate constants increase, decrease, or remain essentially constant as droplet size increases. These results are a particularly satisfactory quantitative explanation in terms of a single theoretical model for all droplet sizes studied. This agreement with the model might seem surprising if we consider that no corrections have been introduced to take into account any increase in the volume of the interface due to the incorporation of amine,32 but under the conditions prevailing in the study, this influence must be almost negligible. The problem of the volume occupied by the amine at the interface arises only when the concentration of amine at the interface is not negligible with respect to the total concentration of surfactant. Only in experiments involving a low concentration of surfactant, the concentration of amine at the interface can be considered non-negligible, and even in these cases, the fact that the amine molar volume is much lower than the surfactant molar volume reduces the importance of such possible volume changes. Our results also seem to support our definition of partition coefficients in terms of mole ratios and not of mole fractions (such a difference will only be significant in those experiments in which the concentration of amine incorporated into the surfactant film is not negligible with respect to total surfactant concentration). Previous results23,24 confirm this. In the present study, the pseudophase model allows the thermodynamic problem of partition among the phases to be separated from the kinetic problem and therefore to estimate the reactivity of the different amines in the interfacial region, ki. Acknowledgment. Financial support form D.G.E.S. and Xunta de Galicia is gratefully acknowledged (Projects PB98-1089, PB98-1088, PGIDT99PXI3014B). P.R.D. thanks Universidade de Vigo for a Research Training Grant. J.P.J. thanks Ministerio de Cultura y Educacioń for a Research Training Grant. LA000523K (47) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10. (b) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387.

Pseudophase Approach to the Transfer of the Nitroso Group in Water

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