The Nature of the Micellar Stern Region As Studied by Reaction Kinetics. 2^,1

Abstract

The nature of rate-retarding effects of cationic micelles on the water-catalyzed hydrolyses of a series of para-substituted 1-benzoyl-1,2,4-triazoles (1a-f) and 1-benzoyl-3-phenyl-1,2,4-triazole (2) has been studied using kinetic methods. A comparison is drawn between medium effects in the micellar Stern region and in model solutions for the micellar Stern region. Simple model solutions involving concentrated aqueous solutions of a small ionic molecule resembling the surfactant headgroup, as reported before,(1) were improved. New model solutions for alkyltrimethylammonium bromide micelles contain both tetramethylammonium bromide (TMAB), mimicking micellar headgroups, and 1-propanol, mimicking hydrophobic tails. The rate-retarding effect of micelles on the hydrolysis of 1a-f and 2 is caused by the high concentration of headgroups as well as by hydrophobic tails in the Stern region where 1a-f and 2 bind to the micelle. Individual contributions of these interactions are quantified. Rate-retarding effects found for different probes, with different sensitivities for interactions as they occur when the probe binds to the micellar Stern region, as well as the micellar Stern region's micropolarity as reported by the E(T)(30) probe, are satisfactorily reproduced by new model solutions containing both TMAB and 1-propanol.

The Nature of the Sodium Dodecylsulfate Micellar Pseudophase as Studied by Reaction Kinetics

“…In addition, the observed rate constants for hydrolysis of each probe 1aÀf in water without added cosolutes at pH 5.3 (Table 1) are in excellent agreement with those for the corresponding hydrolysis in water at pH 4. 31 The local pH on the surface of SDS micelles is lower than the bulk pH and can be estimated by applying the PoissonÀBoltzmann equation 54 that relates the proton activity at the surface of a charged micelle to that in bulk water in combination with the interfacial electrostatic potential. Based on the interfacial potential of À134 mV for SDS micelles, 54 the calculated interfacial pH is approximately 3.0 for a bulk pH of 5.3.…”

“…However, the K m values for SDS are on average about 3 times larger than those for DTAB. 31 The rate-retarding effects exerted by SDS micelles on the individual hydrolytic probes are described by the micellar rate constants relative to the rate constants in water, that is, k X,mic / k X,water , and the natural logarithms of these relative rate constants, ln(k X,mic /k X,water ) ( Table 2). …”

“…The present work continues our kinetic studies of micellar effects on the water-catalyzed hydrolysis of a series of activated amides. 31 Previously, we showed that these reactions occur in the micellar Stern region, and our approach therefore leads to a description of the micellar Stern region as a reaction medium. 31,32 An analysis of the reaction medium properties of the Stern region is of wide interest because the majority of reactants used in organic reactions in micellar solutions bind in the micellar Stern region.…”

“…31 Previously, we showed that these reactions occur in the micellar Stern region, and our approach therefore leads to a description of the micellar Stern region as a reaction medium. 31,32 An analysis of the reaction medium properties of the Stern region is of wide interest because the majority of reactants used in organic reactions in micellar solutions bind in the micellar Stern region. 6,12,33À35 This preference for the Stern region probably stems from its versatility as a binding location because it contains, in addition to water molecules, highly hydrophilic surfactant headgroups and hydrophobic domains of backfolding surfactant tails.…”

“…In line with our previous study, we chose the water-catalyzed pHindependent hydrolysis of different substituted 1-benzoyl-1,2, 4-triazoles (1aÀf, Scheme 1) as the probe reaction, because the series of hydrolytic probes 1aÀf has the required differences in sensitivity to environment. 31 The reactions occur through a ' EXPERIMENTAL SECTION Chemicals. Sodium dodecylsulfate (SDS) (99%, Fluka), sodium methylsulfate (NMS) (98%, TCI), p-anisoyl chloride (99%, Acros Organics), p-toluoyl chloride (99%, Acros Organics), benzoyl chloride (99%, Acros Organics), p-chlorobenzoyl chloride (99%, Aldrich), p-(trifluoromethyl)benzoyl chloride (98%, Acros Organics), p-nitrobenzoyl chloride (99%, Fluka), 1,2,4-triazole (purum, g98%, Fluka), p-toluic acid (98%, Aldrich), p-(trifluoromethyl)benzoic acid (99%, Acros Organics), and Reichardt's E T (30) dye (90%, Aldrich) were used as received.…”