4. Nitrosation in xylene/AOT/water microemulsions

The kinetic study of the nitroso group transfer from MNTS to the secondary amines piperazine (PIP) and N-methylbenzylamine (NMBA) in water/AOT/xylene was carried out. For PIP we used only o-xylene/AOT/water microemuslions. For NMBA we used o-xylene, m-xylene and p-xylene as continuous medium. Previously it was shown that these systems are non-percolative. In microemulsions where isooctane provides the continuous medium, W can be as large as 80 at a surfactant concentration of 0.5 M (see Figure 5). In contrast, W can barely reach 20 at identical surfactant concentrations in water/AOT/xylene microemulsions at 25°C. Raising the temperature slightly increases the solubility of water in the microemulsion. The kinetic study was conducted at 50°C, above the percolation temperature for isooctane/AOT/water and isooctane/AOT/SDS/water microemulsions.

The influence of the nature of the continuous medium on various properties of a wide range of water-in-oil (w/o) microemulsions was studied in our laboratory [43]. <sup>1</sup> H NMR spectroscopy allowed to determine the properties of water in the nanodroplet and the way they are affected by the bulk solvent. Changes in interfacial polarity were examined from the 13C NMR signals of the surfactant molecule AOT. The variation of the carbon chemical shift as a function of the water content (W) was used as a measure of polarity changes at the interface. The reactions of solvolysis of anisoyl chloride were studied in oil/AOT/water microemulsions. The AOT-based microemulsions involved various continuous media, including trichloromethane, tetrachloromethane, alkanes (n-heptane, isooctane, and n-dodecane), cycloalkanes (cyclopentane, cyclohexane, and cycloheptane), and aromatic hydrocarbons (toluene as well as o-xylene, m-xylene and p-xylene). It was found that the solvolysis rate constants of anisoyl chloride depend on the penetration of the oil into the interface.

The kinetic study of PIP reactivity was conducted using o-xylene as the continuous medium in the microemulsions, W values over the range 7.4–15.7 and variable AOT concentrations from 0.1 to 0.7 M. PIP and MNTS concentrations were kept constant at 5 <sup>10</sup><sup>2</sup> M and 2 <sup>10</sup><sup>3</sup> M, respectively. The value of pseudo-firstorder rate constants (ko) increase by increasing total surfactant concentration and decreases by increasing the water content (W).

A figure very similar to that used for PIP in the AOT/SDS quaternary system (Figure 8) describes the distribution of PIP and MNTS between the three phases of the microemulsion: water, AOT and oil (xylene). Therefore, the pseudo-first-order rate constant will be given by Eq. (5) by substituting [AOT] + [SDS] for [AOT]. Kinetic (ki) and equilibrium constants (K1 and K4) were obtained from fitting experimental data to Eq. (5). The bimolecular rate constant (k2 i ) is estimated from ki according to Eq. (9) and assuming a value of 0.37 M<sup>1</sup> for VAOT . Table 2 lists the 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 supports the validity of the model employed.

constant between water and PIP, K1, exhibits a different behavior; thus, an increase in temperature facilitates the migration of PIP to the pseudo-reaction phase in water/AOT/isooctane microemulsions. This result also seems to confirm the presence of an amount of water at the interphase that increases as the temperature is raised. However, no effect was observed on K1, K2 and K4 for isooctane/AOT/SDS/

The relative bimolecular rate constant for PIP in o-xylene microemulsions is lower (0.66) than in isooctane microemulsions. This may be a result of a less polar interface by effect of the penetration of the continuous medium. The reaction between MNTS and NMBA was found to be more markedly inhibited in the xylene microemulsions. For NMBA the reaction was 2.1–2.5 times faster in the isooctane microemulsions than in the o-xylene, m-xylene and p-xylene microemulsions. The polarity of the interphase is a function of the amount of oil and water present. Structural evidence suggests that penetration of the continuous medium into the AOT film is easier for the aromatic medium than for isooctane. NMR and kinetic experiments confirm this prediction [43]. Highly polarizable solvents such as benzene, toluene, and tetrachloromethane penetrate the amphiphilic layer presumably up to the water core boundary. The resonance signals for the water H atoms in toluene, m-xylene, and p-xylene microemulsions are suggested an increasing penetration into the oil. The resonance signals for the water H atoms suggest that penetration is slightly deeper with o-xylene than with the other aromatic solvents. The oil penetration increased in the sequence o-xylene > m-xylene > p-xylene [43]. The relative biomolecular rate constants for the reaction between MNTS and NMBA

in o-xylene, m-xylene and p-xylene are 0.40, 0.43 and 0.47 respectively.

by MNTS in AOT-based microemulsions. The reactivity of traditional

water microemulsions than in isooctane/AOT/water traditional systems. The substitution of isooctane by xylene gives rise to nonpercolated

the inhibitory effect of isooctane/AOT/water and xylene/AOT/water

surfactant, and continuous medium to be separate continuous phases.

continuous medium and the water.

33

microemulsions. Xylene/AOT/water microemulsions showed a remarkable effect in the reactivity. The ratio between the bimolecular rate constants allowed to compare

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

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,

In this chapter we have reviewed the reactions nitrosation of secondary amines

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.

Nitrosation of Amines in AOT-Based Microemulsions DOI: http://dx.doi.org/10.5772/intechopen.80947

5. Conclusions

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 interphase, k2 i , 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 parameters for the microemulsions reviewed in this chapter.

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

#### Figure 12.

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 microemulsions ( ).

#### Nitrosation of Amines in AOT-Based Microemulsions DOI: http://dx.doi.org/10.5772/intechopen.80947

constant between water and PIP, K1, exhibits a different behavior; thus, an increase in temperature facilitates the migration of PIP to the pseudo-reaction phase in water/AOT/isooctane microemulsions. This result also seems to confirm the presence of an amount of water at the interphase that increases as the temperature is raised. However, no effect was observed on K1, K2 and K4 for isooctane/AOT/SDS/ water microemulsions.

The relative bimolecular rate constant for PIP in o-xylene microemulsions is lower (0.66) than in isooctane microemulsions. This may be a result of a less polar interface by effect of the penetration of the continuous medium. The reaction between MNTS and NMBA was found to be more markedly inhibited in the xylene microemulsions. For NMBA the reaction was 2.1–2.5 times faster in the isooctane microemulsions than in the o-xylene, m-xylene and p-xylene microemulsions. The polarity of the interphase is a function of the amount of oil and water present. Structural evidence suggests that penetration of the continuous medium into the AOT film is easier for the aromatic medium than for isooctane. NMR and kinetic experiments confirm this prediction [43]. Highly polarizable solvents such as benzene, toluene, and tetrachloromethane penetrate the amphiphilic layer presumably up to the water core boundary. The resonance signals for the water H atoms in toluene, m-xylene, and p-xylene microemulsions are suggested an increasing penetration into the oil. The resonance signals for the water H atoms suggest that penetration is slightly deeper with o-xylene than with the other aromatic solvents. The oil penetration increased in the sequence o-xylene > m-xylene > p-xylene [43]. The relative biomolecular rate constants for the reaction between MNTS and NMBA in o-xylene, m-xylene and p-xylene are 0.40, 0.43 and 0.47 respectively.
