Translational motion and association in aqueous sodium dodecyl sulphate solutions

Abstract

The translational motion of surfactant anions and water molecules has been studied at 25 °C in aqueous solutions of sodium dodecyl sulphate. Self-diffusion coefficients were determined with the open-ended capillary tube method employing radioactive labelling. Anion diffusion coefficients follow a simple model for micelle formation except that a marked decrease in translational mobility is observed well below the critical micelle concentration thus giving evidence for pre-micellar aggregation. From the anion self-diffusion coefficients micellar diffusion coefficients were calculated. These latter decreased with increasing soap concentration in a way theoretically predicted for intermicellar interactions. Addition of sodium dodecyl sulphate retards water translational motion most effectively at submicellar concentrations. A change in micellar hydration is indicated at about 70 · 10−3 mol/kg. The critical micelle concentration is found both from anion and water diffusion to be 8.5. 10−3 mol/kg. In acetate buffer the critical micelle concentration is considerably lower.