Diffusion coefficients of three organic solutes in aqueous sodium dodecyl sulfate solutions

Tracer diffusion coefficients of phenol, toluene, and benzoic acid in aqueo us solutions of sodium dodecyl sulfate (SDS) were measured by the Taylor di spersion technique. In addition, the viscosities and densities of the SDS s olutions were measured. For phenol and toluene, the effect of micelle forma tion on the diffusion coefficient is pronounced. When the SDS concentration is below the critical micelle concentration (cmc), the diffusion coefficie nts are almost independent of the SDS concentration. However, above the cmc there is a rapid decrease in the diffusion coefficients, and the apparent diffusion coefficients of the two solutes are the weighted average of free solute diffusion and the micelle diffusion. A model is presented to describ e the diffusion behavior of the two solutes in aqueous micellar solutions o f SDS. The interaction between the two solutes and the micelles has been in vestigated and the fraction of each solute that is solubilized by the micel les is estimated from the measured apparent diffusion coefficient. For benz oic acid, the diffusion coefficient is dependent on the joint contribution of the benzoic acid molecules that are solubilized by the micelles as well as the corresponding benzoate ions. The effect of micelle formation on the diffusion coefficient of benzoic acid is not as pronounced as for phenol an d toluene. (C) 2000 Academic Press.