Pseudophase Approach to Reactivity in Microemulsions: Quantitative

5450

Ind. Eng. Chem. Res. 2003, 42, 5450-5456

Pseudophase Approach to Reactivity in Microemulsions: Quantitative Explanation of the Kinetics of the Nitroso Group Transfer Reactions between N-methyl-N-nitroso-ptoluenesulfonamide and Secondary Alkylamines in Water/AOT/Isooctane Microemulsions L. Garcı´a-Rı´o,† P. Herve´ s,‡ J. C. Mejuto,‡ J. Pe´ rez-Juste,‡ and P. Rodrı´guez-Dafonte*,‡ Facultad de Quı´mica, Departamento de Quı´mica Fı´sica, Universidad de Santiago de Compostela, Santiago de Compostela, Spain, and Facultad de Ciencias, Departamento de Quı´mica Fı´sica, Campus de As Lagoas, Universidad de Vigo, Ourense, Spain

The kinetics of the transfer of the nitroso group from N-methyl-N-nitroso-p-toluenesulfonamide to secondary alkylamines (dimethylamine, methylethylamine, methylbutylamine, methylhexylamine, methyloctylamine, and methyldodecylamine) was studied using a wide variety of water/ AOT/isooctane 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 reactivity observed in water. Introduction Microemulsions as chemical reaction media are interesting subjects of study because these media are macroscopically homogeneous and isotropic but are heterogeneous on a microscopic scale. A microemulsion is a thermodynamically stable dispersion of one liquid phase into another, stabilized by an interfacial film of surfactant. This dispersion may be either oil-in-water (o/w) or water-in-oil (w/o). In particular, w/o microemulsions contain aqueous microdroplets dispersed in a lowpolarity bulk solvent.1,2 The reagents present in the medium may be separated into 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 their similarities with biological systems.3-5 On the other hand, microemulsions have found a growing number of scientific and technological applications:6-8 they afford control over the size of synthesized microparticles; they have numerous applications in the fields of solubilization and extraction; and recent studies have demonstrated the potentiality of microemulsions to change reaction mechanisms. In the last years, great effort has been devoted to the study of aqueous microdroplets, generally in water/AOT/ alkane microemulsions, which can contain large quantities of water and in which the 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 In addition, there have been interesting reports on the kinetics of chemical reactions in micro* To whom correspondence should be addressed. Tel.: +34 988 387031. Fax: +34 988 387001. E-mail: [email protected]. † Universidad de Santiago de Compostela. ‡ Universidad de Vigo.

emulsions,16-27 but few of these studies have been quantitative. Prediction and/or interpretation of the kinetic influence of these media are 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.23-31 In the literature, the pseudophase model has been applied to interpret the reactivity in microemulsions.23-26 This model does certainly give a satisfactory, quantitative explanation of reactivity in microemulsions. In the present work, we report the kinetics of nitrosation of secondary amines by N-methyl-N-nitroso-ptoluenesulfonamide (MNTS) in water/AOT/isooctane microemulsions. The alkylamines were chosen on the basis of their hydrocarbon chain length (between 1 and 12 carbon atoms), which is correlated with their solubilities in the various components of the microemulsions. On the other hand, MNTS is poorly soluble in water, and for that reason, it will be in the oil phase as well as in the surfactant film. The chemistry of MNTS has been extensively studied in aqueous media,32 in micellar media,33-36 and in microemulsions.23 In particular, the chemistry of MNTS has drawn much attention because of the fact that this substrate reacts with secondary amines by transnitrosation to give carcinogenic N-nitrosamines. Experimental Section AOT (Aldrich) was dried for 2 days in a vacuum desiccator and used without further purification. All other reagents were supplied by Merck and Aldrich and used without further purification. Kinetic Measurements. Reaction kinetics for amine + MNTS were carried out in a Kontron-Uvikon spectrophotometer fitted with thermostated cell holders (all experiments were carried out at 25.0 ( 0.1 °C) and were

10.1021/ie0208523 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/25/2003

Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5451 Table 1. Partition Coefficients of Secondary Alkylamines between Isooctane and Water (Kw o ) at 25 °C and Kinetic Values for the Transnitrosation between MNTS and the Different N-Alkylamines Obtained from Equations 7 and 10 amine

pKa

Kw o

kw/(M-1 s-1)

ki/s-1

K1/M-1

K2/M-1

K4/M-1

ki2/(M-1 s-1)

dimethylamine methylethylamine methylbutylamine methylhexylamine methyloctylamine methydodecylamine

10.68 10.93 11.79 11.29 11.26 9.11

1.30 0.25 0.04 0.01 methylbutylamine > methylhexylamine > methyloctylamine > methylethylamine. This order parallels the basicity of the amines (see Table 1). At the AOT interface, the order of reactivity is analogous to that observed in bulk water. Finally, absolute comparison of these values of ki2 with those corresponding to the reaction in bulk water shows that the nitroso transfer reactions are 20-50 times slower at the interface of water/AOT/isooctane

5454 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 Table 2. Percolation Temperatures of AOT/Isooctane Microemulsions in the Presence of Amines: [amine] ) 0.04 M, [AOT] ) 0.5 M, W ) 22.2 (Concentration of Amine Referring to the Water Pseudophase) amine

Tp/°C

amine

Tp/°C

without additive dimethylamine methylethylamine methylbutylamine

33.6 35.1 36.2 38.4

methylhexylamine methyloctylamine methydodecylamine

40.1 43.6 46.5

microemulsions, which can be attributed to the lower polarity of the interfacial region. The transition state 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 the reaction rate.32 This explanation is consistent with the behavior observed in other reactions of nitroso compounds (as MNTS and alkylnitrites) in microemulsions23,24 and in normal micelles.36,41,42 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 dropletdroplet collisions.43,44 In water/AOT/isooctane microemulsions,45,46 percolation is not facilitated by the presence of amines (see Table 2). This phenomenon is not taken into account by the pseudophase model. However, our results for the reactions studied in this work are valid regardless of the occurrence of percolation, because these reactions are chemically controlled and have half-lives very much longer 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 chloroformate in water/ AOT/isooctane microemulsions.25 Conclusions In all of the cases considered above, the results can be interpreted by assuming that the reaction occurs at the interface between the water droplet and the 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 constant increases, decreases, or remains essentially constant as the 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, but under the conditions used, 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 at a low concentration of surfactant can the concentration of amine at the interface be considered nonnegligible, 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 the total surfactant concentration). Previous results23,24 confirm this choice. 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 allows one to estimate the reactivity of the different amines in the interfacial region, ki. Acknowledgment Financial support from Ministerio de Ciencia y Tecnologı´a (PB98-1089 and BQU2001-3799) and from Xunta de Galicia (PGIDT01PXI30106PR) is gratefully acknowledged. Nomenclature AOT ) sodium octadecyl sulfosuccinate (Aerosol OT) ki ) rate constant at the AOT film, defined in terms of mole per mole concentrations and expressed in s-1 k0 ) pseudo-first-order rate constant ki2 ) bimolecular rate constant at the AOT film k2 ) bimolecular rate constant in bulk water K1 ) partition constant of amine between the water phase (isooctane) and the AOT film in AOT/isooctane/water microemulsions K2 ) partition constant of amine between the organic phase (isooctane) and the AOT film in AOT/isooctane/water microemulsions KMNTS ) K4 ) partition constant of MNTS between the organic phase (isooctane) and the AOT film in AOT/ isooctane/water microemulsions Kw o ) partition constants of amines between the organic phase (isooctane) and water mixtures MNTS ) N-methyl-N-nitroso-p-toluenesulfonamide noa ) number of moles of amines in isooctane in the organic phase (isooctane) and water mixtures nw a ) number of moles of amines in water in the organic phase (isooctane) and water mixtures no ) number of moles of isooctane in the organic phase (isooctane) and water mixtures nw ) number of moles of water in the organic phase (isooctane) and water mixtures o/w ) oil-in-water microemulsion Tp ) temperature of percolation V h ) molar volume of AOT in the interface w/o ) water-in-oil microemulsion W ) [H2O]/[AOT] Z ) [iC8]/[AOT]

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Received for review October 28, 2002 Revised manuscript received April 7, 2003 Accepted July 16, 2003 IE0208523