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

The critical micelle concentrations (CMC) of sodium dodecyl sulfate (SDS) a nd the degrees of counterion dissociation in a wide range of urea concentra tions 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 t han those recently published. The micellization process of SDS in different aqueous urea solutions has also been followed through the fluorescence beh aviour of pyrene-3-carboxaldehyde (PCA) solubilized in the micellar surface . From this study an attempt was made in order to extract quantitative conc lusions about changes in the micropolarity in the micellar surface induced by the presence of urea. Although no definitive conclusions could be derive d due to the desolvation of the probe caused by the urea action, the result s obtained were interpreted as evidence of a direct interaction of urea wit h the micellar surface. Changes in the structure of the hydrophobic region of SDS micelles upon addition of urea were studied by using polarized fluor escence measurements of diphenylbutadiene (DPB) solubilized in the micellar phase. The effect of urea on the hydrophobic region of SDS micelles was cl early reflected in the DPB steady-state fluorescence anisotropy measurement s, which indicated a more rigid microenvironment around the probe as urea c oncentration increased. The results obtained in this study support a direct mechanism of urea action, whereby urea participates in the solvation of th e hydrophobic chains and the polar headgroups of the amphiphile. (C) 1999 E lsevier Science B.V. All rights reserved.