Micellar medium effects on the hydrolysis of phenyl chloroformate in ionic, zwitterionic, nonionic, and mixed micellar solutions

Abstract

The spontaneous hydrolysis of phenyl chloroformate was studied in various anionic, nonionic, zwitterionic, and cationic aqueous micellar solutions, as well as in mixed anionic-nonionic micellar solutions. In all cases, an increase in the surfactant concentration results in a decrease in the reaction rate and micellar effects were quantitatively explained in terms of distribution of the substrate between water and micelles and the first-order rate constants in the aqueous and micellar pseudophases. A comparison of the kinetic data in nonionic micellar solutions to those in anionic and zwiterionic micellar solutions makes clear that charge effects of micelles is not the only factor responsible for the variations in the reaction rate. Depletion of water in the interfacial region and its different characteristics as compared to bulk water, the presence of high ionic concentration in the Stern layer of ionic micelles, and differences in the stabilization of the initial state and the transition state by hydrophobic interactions with surfactant tails can also influence reactivity. The different deceleration of the reaction observed in the various micellar solutions studied was discussed by considering these factors. Synergism in mixed-micellar solutions is shown through the kinetic data obtained in these media.

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

“…Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions . Another indication is the failure of 3 , a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. , …”

“…1 Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions. 16 Another indication is the failure of 3, a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, 1 as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. 25,26 If the E T (30) probe is a good micropolarity reporter, the micropolarity of the micellar Stern region is considerably lower than expected on the basis of the concentrated salt solutions used as mimics of the Stern region.…”

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

“…Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions . Another indication is the failure of 3 , a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. , …”

“…1 Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions. 16 Another indication is the failure of 3, a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, 1 as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. 25,26 If the E T (30) probe is a good micropolarity reporter, the micropolarity of the micellar Stern region is considerably lower than expected on the basis of the concentrated salt solutions used as mimics of the Stern region.…”

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

“…45,86 In our model, the effects of ionic strength and direct ionic interactions are modeled by NMS. As argued above, the rate-retarding effects exerted in solution 2 by NMS on hydrolysis of probes 1aÀf cannot be attributed to a contribution of NMS to the decrease in polarity.…”

“…Because NMS does not contribute significantly to the decreased polarity of solution 2 compared to bulk water, the kinetic effects of the lower polarity of solution 2 on 1a – f hydrolysis are dominated by the effect of 1.8 mol dm –3 1-propanol. The lower polarity of the micellar pseudophase compared to the polarity of pure water contributes to the decrease in the rate of hydrolysis reactions in the presence of micelles. , The usual interpretation of this observation involves the higher polarity of the transition states (Scheme ) compared to the polarity of the substituted substrates: a less polar medium is less favorable for hydrolysis. Therefore, the lower polarity of the Stern region of SDS micelles is responsible for part of the rate-retarding effects on the hydrolysis of 1a – f .…”

“…An important merit of the present model solutions over the simple salt solutions and aqueous alcohol solutions used previously for mimicking the Stern region of SDS micelles , is that solution 1 and solution 2 have the ability to reproduce both ionic and hydrophobic interactions experienced by a probe bound in the micellar interfacial region. The importance of using both headgroup and tail mimics is illustrated by the observation that the significant rate retardations can be accounted for only on the basis of interactions with the surfactant headgroups, as modeled by NMS, whereas the change in Hammett ρ value can be accounted for only through the lower local polarity and hydrophobic interactions with the surfactant tails, as mimicked by 1-propanol.…”

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

“…Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions . Another indication is the failure of 3 , a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. , …”

“…1 Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions. 16 Another indication is the failure of 3, a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, 1 as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. 25,26 If the E T (30) probe is a good micropolarity reporter, the micropolarity of the micellar Stern region is considerably lower than expected on the basis of the concentrated salt solutions used as mimics of the Stern region.…”

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

“…45,86 In our model, the effects of ionic strength and direct ionic interactions are modeled by NMS. As argued above, the rate-retarding effects exerted in solution 2 by NMS on hydrolysis of probes 1aÀf cannot be attributed to a contribution of NMS to the decrease in polarity.…”

“…Because NMS does not contribute significantly to the decreased polarity of solution 2 compared to bulk water, the kinetic effects of the lower polarity of solution 2 on 1a – f hydrolysis are dominated by the effect of 1.8 mol dm –3 1-propanol. The lower polarity of the micellar pseudophase compared to the polarity of pure water contributes to the decrease in the rate of hydrolysis reactions in the presence of micelles. , The usual interpretation of this observation involves the higher polarity of the transition states (Scheme ) compared to the polarity of the substituted substrates: a less polar medium is less favorable for hydrolysis. Therefore, the lower polarity of the Stern region of SDS micelles is responsible for part of the rate-retarding effects on the hydrolysis of 1a – f .…”

“…An important merit of the present model solutions over the simple salt solutions and aqueous alcohol solutions used previously for mimicking the Stern region of SDS micelles , is that solution 1 and solution 2 have the ability to reproduce both ionic and hydrophobic interactions experienced by a probe bound in the micellar interfacial region. The importance of using both headgroup and tail mimics is illustrated by the observation that the significant rate retardations can be accounted for only on the basis of interactions with the surfactant headgroups, as modeled by NMS, whereas the change in Hammett ρ value can be accounted for only through the lower local polarity and hydrophobic interactions with the surfactant tails, as mimicked by 1-propanol.…”

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

“…Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions . Another indication is the failure of 3 , a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. , …”

“…1 Similarly, the hydrolysis of phenyl chloromethanoate is not nearly as much retarded in salt solutions as in micellar solutions. 16 Another indication is the failure of 3, a hydrolytic probe that is particularly sensitive to hydrophobic interactions, to show a relation between the charge of the Stern region and the rate-retarding effect, 1 as was also observed by others for the hydrolysis of 4-nitrophenyl 2,2-dichloropropanoate. 25,26 If the E T (30) probe is a good micropolarity reporter, the micropolarity of the micellar Stern region is considerably lower than expected on the basis of the concentrated salt solutions used as mimics of the Stern region.…”

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

“…45,86 In our model, the effects of ionic strength and direct ionic interactions are modeled by NMS. As argued above, the rate-retarding effects exerted in solution 2 by NMS on hydrolysis of probes 1aÀf cannot be attributed to a contribution of NMS to the decrease in polarity.…”

“…Because NMS does not contribute significantly to the decreased polarity of solution 2 compared to bulk water, the kinetic effects of the lower polarity of solution 2 on 1a – f hydrolysis are dominated by the effect of 1.8 mol dm –3 1-propanol. The lower polarity of the micellar pseudophase compared to the polarity of pure water contributes to the decrease in the rate of hydrolysis reactions in the presence of micelles. , The usual interpretation of this observation involves the higher polarity of the transition states (Scheme ) compared to the polarity of the substituted substrates: a less polar medium is less favorable for hydrolysis. Therefore, the lower polarity of the Stern region of SDS micelles is responsible for part of the rate-retarding effects on the hydrolysis of 1a – f .…”

“…An important merit of the present model solutions over the simple salt solutions and aqueous alcohol solutions used previously for mimicking the Stern region of SDS micelles , is that solution 1 and solution 2 have the ability to reproduce both ionic and hydrophobic interactions experienced by a probe bound in the micellar interfacial region. The importance of using both headgroup and tail mimics is illustrated by the observation that the significant rate retardations can be accounted for only on the basis of interactions with the surfactant headgroups, as modeled by NMS, whereas the change in Hammett ρ value can be accounted for only through the lower local polarity and hydrophobic interactions with the surfactant tails, as mimicked by 1-propanol.…”