5. Conclusions

kinetic parameters for the transnitrosation reaction between PIP and MNTS in oxylene/AOT/water microemulsion. Figure 12 compares the experimental rate constants with that calculated. The satisfactory fit obtained for these experiments

To compare results, we carried out a kinetic study in isooctane/AOT/water microemulsions at 50°C. Kinetic parameters were determined by following the same procedure as used previously. As can be seen from Table 2, the partition equilibrium constant of PIP, K1, between water and the AOT interface was greater in the o-xylene microemulsions. However, the reaction rate constant at the inter-

Table 2 shows that the changes in the equilibrium constants (K1, K2 and K4) depend on their distribution between the three pseudophases. The lower values of K4 in microemulsions with xylenes, at 50°C, may be related with the increased rigidity of the surfactant film due to the penetration of the oil. Similar effect was observed for the distribution constant of the NMBA (K2). The equilibrium partition

Plot of experimental vs. calculated rate constants, at 50°C, for the following reactions: PIP + MNTS in o-xylene/AOT/water microemulsions ( ), NMBA + MNTS in o-xylene/AOT/water microemulsions ( ), NMBA + MNTS in m-xylene/AOT/water microemulsions ( ), NMBA + MNTS in p-xylene/AOT/water

, was smaller in the interface of the water/AOT/o-xylene microemulsions. The NMBA reactivity was studied with o-xylene, m-xylene and p-xylene as the continuous medium of the water in oil microemulsion. Nanodroplet size, through the W parameter, varied between 7 and 18 and surfactant concentrations over the range 0.2–0.7 M. NMBA and MNTS concentrations were kept constant at 0.103 M and 2 <sup>10</sup><sup>3</sup> M, respectively. Pseudo-first order rate constants increased markedly with increasing surfactant concentration at a constant value of W and more slightly with increase in water content at a constant AOT concentration. NMBA and MNTS are virtually insoluble in water and both are distributed between the interface and the oil. The reaction between MNTS and NMBA in xylenes was found to be very slow and, hence, was discarded. Thus, Figure 9, is valid for these systems. From the mechanistic proposal shown in Figure 9 and considering that the surfactant is only AOT, Eq. (7) can be obtained. Kinetics parameters were obtained from fitting experimental data to Eq. (7). The consistency between the experimental and predicted values (see Figure 12) confirms the accuracy of the model for xylene/ AOT/water microemulsions. Table 2 compares kinetic and thermodynamic param-

supports the validity of the model employed.

Microemulsion - A Chemical Nanoreactor

eters for the microemulsions reviewed in this chapter.

phase, k2

Figure 12.

32

microemulsions ( ).

i

In this chapter we have reviewed the reactions nitrosation of secondary amines by MNTS in AOT-based microemulsions. The reactivity of traditional microemulsions (isooctane/AOT/water) was compared with systems where the interface has been modified by the addition of a cosurfactant or by the substitution of isooctane for xylenes. The presence of SDS as a co-surfactant has important effects on the stability of the microemulsion. An increase in its relative concentration results in a significant decrease in its percolation temperature. The water solubilization capacity also decreases by the presence of SDS. Despite these important effects in the stability of the system, no significant differences are observed in the reactivity. The pseudophase model has been used to quantitatively evaluate the kinetic parameters, but the results are practically the same in isooctane/AOT/SDS/ water microemulsions than in isooctane/AOT/water traditional systems.

The substitution of isooctane by xylene gives rise to nonpercolated microemulsions. Xylene/AOT/water microemulsions showed a remarkable effect in the reactivity. The ratio between the bimolecular rate constants allowed to compare the inhibitory effect of isooctane/AOT/water and xylene/AOT/water microemulsions. Thus, the reactions of MNTS with NMBA and PIP were found to be more strongly inhibited in the xylene microemulsions than in the isooctane microemulsions. The increased inhibitory effect of the xylene microemulsions is a result of the decreased polarity at the reaction site: the interphase between the continuous medium and the water.

Finally, we highlight the validity of the kinetic model for percolated and nonpercolated microemulsions. The pseudophase model is independent of structural connotations and internal dynamics of the system and assumes the water, surfactant, and continuous medium to be separate continuous phases.
