Field-Enhanced Dissociation, the Validity of Onsager's Theory in Surfactant Solutions

This study deals with the field dissociation effect in solutions of AOT surfactant in cyclohexane, characterized by very complex ionic aggregation. Thermodynamic information on the dissociation of this electrolyte solution from conductivity measurements, together with the kinetic parameters obtained with an electrical relaxation method, are used to derive the characteristics of the mechanism of AOT dissociation in cyclohexane. Two essential results are obtained. First, the conductance increases with the field as predicted by Onsager's theory up to 5.5X106 Vm-1, the maximum investigated field value, whatever the concentration of AOT (10-4 to 10-1 mo1-1) even in the ranges where there are reversed micelles. Second, the theoretical relation τ = ε/2 σ(E) giving the relaxation time τ of the ionic disociation equilibrium as a function of the solution permittivity σ and its conductance σ(E) is found valid, except at AOT concentrations larger than 10-3 mo1-1 with a AOT/water molar ratio lower than 1. In this case, the theory foresees that τ varies with C-1AOT and the experiments indicate a variation of τ with C-0.25AOT We analyze the influence of different factors such as permittivity, water content, solvent, electrolyte, and suggest that the deviation between experiments and theory may originate from the scheme of reactions producing charge carriers. We propose a multiple-step mechanism involving reversed micelles.