Abstract
The dehydrobromination reaction 2-(p-nitrophenyl)ethyl bromide + OH−was investigated in several alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br− (with s = 2, 3, 4, 5, 6, 8, 10, 12) micellar solutions, in the presence of NaOH 5 × 10−3 M. The kinetic data were quantitatively rationalized within the whole surfactant concentration range by using an equation based on the pseudophase ion-exchange model and taking the variations in the micellar ionization degree caused by the morphological transitions into account. The agreement between the theoretical and the experimental data was good in all the dimeric micellar media studied, except for the 12-2-12,2Br− micellar solutions. In this case, the strong tendency to micellar growth shown by the 12-2-12,2Br− micelles could be responsible for the lack of accordance. Results showed that the dimeric micelles accelerate the reaction more than two orders of magnitude as compared to water.
Highlights
Micellar solutions contain organized surfactant aggregates that have common structural features: hydrocarbon cores composed of surfactant tails; interfacial regions containing head-groups, counterions, and water and the surrounding aqueous phase or bulk region [1]
The rate of this process in the micellar reaction media depends on the ion-exchange equilibrium constant, KOH/Br, for the competition between the bromide and the hydroxide ions for the positively charged surface of the dimeric micelles and this study could show if changes in this magnitude with micellar growth have to be considered in order to rationalize the micellar kinetic effects
The dehydrobromination reaction 2-(p-nitrophenyl)ethyl bromide + OH− was investigated in several alkanediyl- - -bis(dodecyldimethylammonium) bromide, 12-s-12,2Br−
Summary
Micellar solutions contain organized surfactant aggregates that have common structural features: hydrocarbon cores composed of surfactant tails; interfacial regions containing head-groups, counterions, and water and the surrounding aqueous phase or bulk region [1]. They are formed by two amphiphilic moieties connected at the level of the head groups by a spacer [8,9] The interest in such surfactants arises from their physicochemical properties that are more favorable than those of conventional surfactants, such as much lower critical micelle concentrations (cmc), better wetting, greater surface tension lowering, and unusual morphologies. The rate of this process in the micellar reaction media depends on the ion-exchange equilibrium constant, KOH/Br, for the competition between the bromide and the hydroxide ions for the positively charged surface of the dimeric micelles and this study could show if changes in this magnitude with micellar growth have to be considered in order to rationalize the micellar kinetic effects. This process has the advantage that the equilibrium binding constants of the organic substrate to the dimeric micelles is experimentally accessible
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