Micelle formation and microenvironmental properties of sodium dodecyl sulfate in aqueous urea solutions

Abstract

The critical micelle concentrations (CMC) of sodium dodecyl sulfate (SDS) and the degrees of counterion dissociation in a wide range of urea concentrations have been determined by the electrical conductivity method. While the CMC values are in good agreement with literature values, the values of the degree of counterion dissociation at high urea concentration are greater than those recently published. The micellization process of SDS in different aqueous urea solutions has also been followed through the fluorescence behaviour of pyrene-3-carboxaldehyde (PCA) solubilized in the micellar surface. From this study an attempt was made in order to extract quantitative conclusions about changes in the micropolarity in the micellar surface induced by the presence of urea. Although no definitive conclusions could be derived due to the desolvation of the probe caused by the urea action, the results obtained were interpreted as evidence of a direct interaction of urea with the micellar surface. Changes in the structure of the hydrophobic region of SDS micelles upon addition of urea were studied by using polarized fluorescence measurements of diphenylbutadiene (DPB) solubilized in the micellar phase. The effect of urea on the hydrophobic region of SDS micelles was clearly reflected in the DPB steady-state fluorescence anisotropy measurements, which indicated a more rigid microenvironment around the probe as urea concentration increased. The results obtained in this study support a direct mechanism of urea action, whereby urea participates in the solvation of the hydrophobic chains and the polar headgroups of the amphiphile.