ABSTRACT
8-Anilinonaphthalene-1-sulfonate (ANS) was used as fluorescence probe to study the effects of urea on properties of sodium dodecyl sulfate micellar solutions. The critical micelle concentration (CMC) of the surfactant, the degree of counterion association to the micelles, and the partition constant of ANS between the micelles and the external medium in the vicinity of the CMC were obtained at different urea concentrations ranging from 0 to 8 M. Complementary results regarding the effects of urea on the micelle aggregation number, the ET(30) parameter, and the partitioning of ANS between n-octanol and water or water-plus-urea mixtures were also obtained to allow critical evaluation of the ANS fluorescence data. It is concluded that the effect of urea on ANS fluorescence at intermediate surfactant concentrations is due mainly to increased stability of the probe in the aqueous medium. Extrapolation of the data at total ANS incorporation into the micelles allows the conclusion that the intramicellar emission increases slightly on urea addition. This result is attributed to an increase in the microviscosity at the micellar surface resulting from urea-micelle interactions.
ABSTRACT
The effect of urea on the competition between Tl+ and Na+ ions to bind with the surface of dodecylsulfate micelles in aqueous solution has been studied by fluorescence techniques, employing methods based on fluorescence quenching data (FQD) and critical micelle concentration measurements (CMCM). The results obtained are interpreted in terms of selectivity coefficients K (Tl+/Na+) defined within the framework of the pseudophase ion-exchange formalism. It is found that the ion specificity observed in the absence of urea [K(Tl+/Na+) = 3.8 +/- 0.3 (from FQD), K(Tl+/Na+) = 2.5 +/- 0.3 (from CMCM] is progressively lost in the presence of increasing concentrations of urea. Typically, values of K (Tl+/Na+) = 1.4 +/- 0.2 (from FQD) and K (Tl+/Na+) = 1.2 +/- 0.2 (from CMCM) in a water-urea mixture at 8 M urea, were obtained. The results are discussed in terms of urea association to the micellar interface and changes in the characteristics of the external medium.
