Sodium Dodecyl Sulfate Micellar Effects on the Reaction between Arenediazonium Ions and Ascorbic Acid Derivatives

The effects of sodium dodecyl sulfate, SDS, micelles on the reaction between a number of arenediazonium, ArN 2 +, 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 ArN 2 + and VC takes place through the rate-limiting decomposition of a diazo ether “complex” formed from the interaction of ArN 2 + with the monoanion form of ascorbic acid, VC -, in a rapid pre-equilibrium step. The kinetic profiles found for the reaction of ArN 2 + 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 k obs . The k obs values at the minimum are significantly above zero and much higher than those for the spontaneous thermal decomposition of ArN 2 +, suggesting that at high [SDS], a fraction of VC - ions 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 k obs approaching the value for the thermal decomposition of ArN 2 + . 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).