Ultramicroelectrode Voltammetric Investigation of Intermicellar Interaction and Micellar Growth of Sodium Dodecyl Sulfate in Aqueous NaCl Solutions

Ultramicroelectrode voltammetric measurements have been carried out to calculate the micellar self-diffusion coefficient (D s ) and intermicellar interaction parameter (K d ) of sodium dodecyl sulfate (SDS) in aqueous NaCI solutions at 298.15 K. The hydrodynamic radius (R m h), the aggregation number, and the molecular weight of SDS micelles at various NaCI concentrations are also estimated. It is found that the D s value is a function of SDS concentrations and the electrolyte concentration dependence on D, exhibits two regions of behavior. With increasing NaCl concentration, D s initially increases because of increasing Coulombic screening and, then, decreases linearly because of a linear spherical expansion of the micelles resulting from the increasing aggregation number. K d is found to be a function of electrolyte concentrations. The stepwise decrease of K d reflects that, due to Coulombic repulsion, the electrostatic interaction is gradually screened with increasing NaCI concentration. As the linear interaction theory applies rigorously over the electrolyte concentration range from 0.20 to 1.00 mol dm - 3 , it suggests that the micellar structure remains effectively spherical in the region, the linear spherical expansion of the micelle is occurring, and no dramatic structural changes have taken place in the micellar solutions. In addition, the micellar interaction energy is calculated, which implies that it is the Coulombic nature of the interaction between micellar particles that controls the micellar self-diffusion behavior in aqueous solutions.