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The effects of sodium dodecyl sulfate, SDS, micelles on the reaction between a number of arenediazonium, ArN2 + , ions with hydrophilic L-ascorbic acid or vitamin C, VC, and the hydrophobic analogue 6-O-palmitoyl-L-ascorbic acid, VC16, have been investigated at different pH values. Previous results indicate that in aqueous acid solution, in the absence of surfactant, the reaction between ArN2 + and VC takes place through the rate-limiting decomposition of a diazo ether "complex" formed from the interaction of ArN2 + with the monoanion form of ascorbic acid, VC -, in a rapid pre-equilibrium step. The kinetic profiles found for the reaction of ArN2 + with VC or with VC16 together with high-performance liquid chromatography data suggest that the presence of SDS micelles does not alter the reaction pathway. Addition of SDS decreases the observed rate constant, k obs, for the reaction with VC up to a minimum after which further addition of SDS leads to a slight increase in kobs. The kobs values at the minimum are significantly above zero and much higher than those for the spontaneous thermal decomposition of ArN2 + , suggesting that at high [SDS], a fraction of VCions are present in the micellar Stern layer. At high [SDS] ) 0.8 M, saturation kinetics is observed for the reaction with VC16 upon increasing [VC16], but at moderate [SDS] ) 0.16 M only the initial linear regions of the saturation profile are observed. The reaction is, however, inhibited upon increasing [SDS] at any fixed [VC16], with kobs approaching the value for the thermal decomposition of ArN2 + . Results with VC and VC16 are consistent with the pseudophase model and are rationalized in terms of the electrostatic micellar-induced separation of reactants and co-ion (VC -) incorporation into the micellar Stern layer and by assuming that the local VC16 concentration in the micellar pseudophase decreases because of the dilution effect caused by increasing [SDS] (VC16).

The effects of 6-O-octanoyl-L-ascorbic acid, VC8, on the reaction with 3-methylbenzenediazonium, 3MBD, tetrafluoroborate were investigated in the absence and in the presence of sodium dodecyl sulfate, SDS, cetyltrimethylammonium bromide, CTAB, and tetradecyltrimethylammonium bromide, TTAB, micelles at different pHs by employing a combination of UV-vis spectroscopy, high-performance liquid chromatography, HPLC, and differential pulse polarography techniques. VC8 behaves as a typical surfactant in aqueous solution giving rise to micellar aggregates at VC8 concentrations above its critical micelle concentration (cmc). This behavior is reflected in the variation of the observed rate constant, kobs, with [VC8]; kobs values increase smoothly upon increasing [VC8] up to a breakpoint at [VC8] ≈ 8 × 10 -3 M after which further addition of VC8 makes kobs values increase sharply. Polarographic titration of VC8 with 3MBD shows that the reaction between 3MBD and VC8 takes place through an inner sphere mechanism leading to the formation of an unstable Z-diazo ether intermediate. All evidence is consistent with a competitive reaction mechanism, that is, the thermal D N + AN dediazoniation and a rate-limiting decomposition of the Z-diazo ether "complex" formed from reaction between 3MBD and VC8ions in a rapid pre-equilibrium step. In presence of SDS, CTAB, or TTAB, at fixed pH and [VC8] , cmc, kobs values increase upon increasing [VC8] up to a maximum after which further addition of surfactant leads to a sharp (SDS) or smooth (CTAB, TTAB) decrease in k obs values. Results are consistent with the predictions of the pseudophase model and are rationalized in terms of micellar-induced concentration-dilution effects (SDS) and reactant and coion (3MBD) incorporation into the micellar Stern layer of CTAB and TTAB micelles. HPLC analyses of the reaction mixtures in the presence of added surfactants show a significant micellar effect on product distribution.

The effects of tetradecyltrimethylammonium bromide, TTAB, and hexadecyl-trimethylammonium bromide, CTAB, micellar systems on the reaction of 3-methylbenzenediazonium, 3MBD, tetrafluoroborate with ascorbic acid, VC, and with the hydrophobic derivatives 6-O-dodecyl-L-ascorbic acid, VC12, and 6-O-palmitoyl-L-ascorbic acid, VC16, were investigated at different pH values by employing a combination of UV-vis spectroscopy and high-performance liquid chromatography, HPLC, techniques. Previous studies in the absence of surfactant showed that the reaction between 3MBD and VC derivatives takes place through a rate-limiting decomposition of a transient diazo ether, DE, formed from reaction between 3MBD and the monoanion form of ascorbic acid, VC-, in a rapid preequilibrium step. In the presence of a fixed [CTAB], the kinetics of the reaction of 3MBD with VC follows a saturation kinetics similar to that observed in its absence, but for the reaction with VC12 and VC16, only the first linear portions of the saturation profiles could be obtained because k(obs) values become too large. HPLC analyses of the reaction mixtures show that no unexpected products are detected, suggesting that cationic micelles do not modify the mechanism of the reaction. Analyses of the kinetic data allowed estimations of the rate constant for the decomposition of the diazo ether and of the equilibrium constant for the formation of DE in the presence of CTAB micelles, which is approximately 6 times higher than in its absence; this suggests that CTAB micelles promote diazo ether formation. At constant [antioxidant], the variations of k(obs) for the reactions with VC, VC12, or VC16 follow bell-shaped curves, with rate enhancements of up to 2-3-fold for VC with respect to the value in the absence of surfactant. The rate maximum for the reaction of 3MBD with VC is reached at [CTAB] = 0.02 M suggesting a CTAB-induced rate increase, i.e., micellar catalysis; meanwhile the rate maximum for the reaction with VC12 and VC16, which may behave as amphiphilic compounds, is reached at [CTAB] approximately 1 x 10(-4) M, a concentration about 10 times lower than its critical micelle concentration, cmc, in pure water, but only approximately 3 times lower than the cmc of VC16, suggesting the formation of reactive CTAB-VC12 and CTAB-VC16 premicellar aggregates. Kinetic and HPLC results are consistent with the predictions of the pseudophase model and are interpreted in terms of 3MBD ions sampling in the aqueous bulk phase and the micellar effects on the different equilibrium involved. The results should contribute to a better understanding of the role of compartmentalized systems on the efficiency with which hydrophilic and hydrophobic reductants such as ascorbic acid derivatives interact with potentially mutagenic and carcinogenic ArN2+ ions.

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