Abstract
Surfactants with ionic or polar head groups and extended apolar groups form micelles in water with hydrocarbon-like interiors and ionic or polar groups at the surface and bind ionic and nonionic solutes. They act as reaction regions, i.e., as pseudophases distinct from the bulk solvent, and can accelerate or inhibit reactions, depending on rate constants and reactant concentrations in the two regions. Theoretical treatments and experimental evidence of reactant partitioning between water and micelles allow estimation of rate constants in the micellar pseudophase for both uni-and bimolecular reactions, provided that they are slower than reactant transfers between water and micelles, but some treatments apply only to dilute electrolyte. Competition between reactive and inert ions inhibits bimolecular ionic reactions and is treated by ion-exchange equations and some local ionic concentrations can be estimated by dediazonization trapping or physical methods. Ionic micelles affect rates of unimolecular and bimolecular watercatalyzed reactions because the reaction region at the micelle-water interface is less polar than water. Zwitterionic micelles have no net charge but they interact with ions, although ion-binding is weaker than with ionic micelles and there are limitations in the use of the theoretical treatments applied to ionic micelles. Micelles can control product composition, but product isolation limits use of surfactants in some reactions
Highlights
This article covers reactivity in solutions of ionic and zwitterionic micelles in water for simple reactions with known mechanisms, and covers the assumptions and approximations involved in the theoretical kinetic treatments
In early work on chemical reactions acid-base equilibria were examined by using visual indicators and apparent acid dissociation constants were sensitive to ionic micelles
The pseudophase model of micellar rate effects depends on equations with terms describing reactant partitioning between two assumed reaction regions and rate constants in each region
Summary
This article covers reactivity in solutions of ionic and zwitterionic micelles in water for simple reactions with known mechanisms, and covers the assumptions and approximations involved in the theoretical kinetic treatments. The discussion largely covers self association of chemically simple surfactants, called amphiphiles or detergents, with an ionic or polar head group and a long alkyl tail, and in water, above a critical micelle concentration, cmc,[3] they form normal micelles which are approximately spherical with the head groups in contact with water. Work on association colloids was largely on physical properties, generally in water, and involved interactions with ionic and nonionic solutes and development of treatments of surfactant association, ion binding, effects of ionic charge on micellar growth and the cmc.[3,5] Micellar aggregation numbers, cmc values, and extents of incorporation of counter-ions, which follow the Hofmeister series, were very important in subsequent kinetic work. The substrates have apolar groups which favor micellar binding and were selected so that mechanisms are the same in micelles and aqueous solvents
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