Tl+/Na+ competitive binding at the surface of dodecylsulfate micelles in water-urea mixtures

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. (C) 1996 Academic Press, Inc.