Rate of exchange of surfactant monomer radicals and long chain alcohols between micelles and aqueous solutions. A pulse radiolysis study

Abstract

Pulse radiolysis has been used to measure the rate at which sodium lauryl sulphate monomer surfactant units enter into and exit from surfactant micelles. The free radical of the monomer is generated via OH attack following radiolysis and its reaction with ferricyanide noted. The rate constant for this reaction is 3.2 × 10⁸ dm³ mol^–1 s^–1 at 25°C. In micellar solution two distinct rate processes are observed corresponding to the previous reaction, which is in competition with monomer radical entering the micelle. Radicals in the micelle react slowly with ferricyanide due to like charge repulsion of the micelle and this ion. The rate constants for exit, k^–, and entry, k⁺, are k^–= 1.8 × 10⁵ s^–1 and k⁺= 1.5 × 10⁹ dm³ mol^–1 s^–1. These data agree well with those obtained by other methods. The site of OH attack on the lauryl chain is discussed. Similar studies of the interaction of pentyl alcohol and cyclohexanol with sodium lauryl sulphate micelles are reported and discussed in the context of micellar equilibria. The equilibrium constant for pentyl alcohol/micelle interaction is K/N= 15 dm³ mol^–1 and for cyclohexanol/micelle, K/N= 25 dm³ mol^–1.