Physicochemical properties of mixed anionic–non-ionic micelles: effects on chemical reactivity

Abstract

Addition of the non-ionic surfactant C₁₆E₂₀(C₁₆H₃₃[OCH₂CH₂]₂₀–OH) to an aqueous acid solution of alkyl nitrites decreases its rate of acid hydrolysis. Kinetic data are analysed by using the pseudophase model, the partitioning of the alkyl nitrites between water and C₁₆E₂₀ micelles being the main reason for the observed inhibiton. The kinetic results provide for a quantitative estimate of the association constants.

The observed rate constant of aqueous acid hydrolysis of alkyl nitrites increases in the presence of sodium dodecyl sulfate (SDS) micelles: first-order rate constants increase with increasing [SDS], go through maxima, and decrease at still higher concentrations of SDS. Addition of C₁₆E₂₀ to an aqueous acid solution of SDS strongly decreases the maximum for the observed rate constant, until at high [C₁₆E₂₀] : [SDS] ratio the observed rate constant decreases monotonically as the total surfactant concentration increases. Kinetic data are analysed by using the simple pseudophase ion-exchange model under the assumption that formation of mixed micelles in the binary mixed surfactant system (C₁₆E₂₀ and SDS) would occur. The properties of mixed micelles seem to be very different from those of pure surfactants. Binding constants and rate constants for the reaction in the micellar pseudophase, obtained from the kinetic analysis, are reported. Conductivity measurements were also taken.