The dependence of micellar rate effects upon reaction mechanism.
In the pseudophase treatment of reactivities in aqueous surfactants, water and the micelles are treated as discrete reaction media. Provided that transport of reactants between water and micelles is faster than the chemical reactions the overall reaction rate depends on local concentration(s) in water and micelles and rate constants in each medium. Only substrate binding has to be considered for spontaneous reactions, and observed first-order rate constants are interpreted in terms of reaction mechanism on the assumption that the micellar interfacial reaction region is less polar than water and has (for ionic micelles) a high electrolyte content. Rate effects on bimolecular reactions depend on second-order rate constants and concentrations of both reactants in the aqueous and micellar pseudophases. Second-order rate constants in the micellar pseudophase, relative to those in water, are also as expected for a reaction region that is less polar than water. For reactions of anionic nucleophiles and bases in cationic micelles local second-order rate constants are generally not very different from those in water, but are much lower for oxygen transfers from anionic oxidants. These rate effects are understandable in terms of properties of the interfacial region, reaction mechanism and consequent charge redistributions in formation of the transition state.
Bunton
Clifford A
CA
Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA 93106-9510, USA. bunton@chem.ucsb.edu