Micellar properties of a mixed surfactant system constituted by n-octyl-β-D-thioglucopyranoside and sodium dodecyl sulphate

A fluorescence probe study on micellization of the binary surfactant system formed by n-octyl-beta-D-thioglucopyranoside (OTG) and sodium dodecyl sulphate (SDS) in 0.1 M NaCl aqueous solutions has been carried out. The CMC experimental data, as obtained by the pyrene 1:3 ratio method, have been analyzed in the context of the pseudophase separation model. The interaction parameter 012) determined by using the regular solution theory (RST) was found to be negative and independent on the solution composition. In addition, the experimental data on micellar composition showed a good agreement with those predicted by RST. Data reported by a treatment based on the Gibbs-Duhem equation showed a reasonable agreement with those obtained by RST, suggesting that the mixed system behaves as a regular solution. From the mixing thermodynamic function values can be inferred that the electrostatic repulsions between the sulphate groups of SDS control the stability of the mixed micelles. Size of micelles was examined through the micellar aggregation number as obtained by the static quenching method. It was found that the aggregation number initially increases, giving a maximum value at low content of the ionic component, remaining almost constant at larger content of this one. This behaviour was interpreted on the basis of the role played by the electrostatic repulsions between the headgroups of SDS in the stabilization of the mixed micelles. Finally, studies based on both pyrene 1:3 ratio index and intramolecular excimer forming of 1,3-dipyrenylpropane solubilized in the micellar phase, reported information on the microenvironmental properties of mixed micelles. The results obtained were rationalized on the basis of two effects: the electrostatic repulsions between the headgroups of the ionic component and the micellar hydration. (c) 2005 Elsevier B.V. All rights reserved.