Journal of Radioanalytical and Nuclear Chemistry, Vol. 265, No. 1 (2005) 8590 02365731/USD 20.00 AkadØmiai Kiad, Budapest ' 2005 AkadØmiai Kiad, Budapest Springer, Dordrecht Interaction of UO 2 2+ with sodium dodecyl sulfonate micelles based on fluorescence data P. Repossi, W. A. Massad, G. A. Argüello* INFIQC - Departamento de Fsico Qumica, Facultad de Ciencias Qumicas, Universidad Nacional de Crdoba, Ciudad Universitaria  5000 Crdoba, Argentina (Received September 16, 2004) The interaction between the U(VI) and a sodium dodecyl sulfonate (SDS) micelle was studied for intensity and emission lifetime. The measurements of the uranyl ion were done in a 1M H 3 PO 4 medium. The self quenching rate constant (k sq ) shows a larger value in micellar than in a SDS monomeric solution. This fact should be interpreted by micelles favoring the localized concentration of UO 2 2+ species. Dynamic and static quenching was observed in the interaction between uranyl ion and the surfactant monomer before the induced critical micelle concentration (icmc) (1 mM) yielding a value of K D = (134–2) . 10M 1 and K S = (16–2) . 10 2 M 1 for the dynamic and static quenching constant, respectively. A quantitative description of the binding was obtained by monitoring the emission lifetime of the uranyl excited state as a function of the surfactant concentration (titration curve), assuming that the observed lifetime is related to a weighted fraction of rate constants for the bound and unbound species. Introduction The photochemistry and photophysics of the uranyl ion in homogeneous solutions has been the subject of different studies 13 whereas the application of the uranyl ion as a luminescence probe in microheterogeneous systems has been a growing field in the last years. 49 In our laboratory there is a continuous interest in the study of uranyl chemistry encouraged by fundamental and applied scientific interest as well as practical demand. In a first stage, we were engaged in the development of an equipment and procedure for the trace determination of U(VI) using a blue light emitting diode (LED) as excitation source in phase-resolved luminescence spectroscopy. 10 This system and other spectroscopic techniques were used to study the photochemistry and photophysical aspects of UO 2 2+ in homogeneous 11,12 as well as microheterogeneous media. 9 At present, different studies have been done for the uranyl-SDS system, 5,9,13,14 but none of them in 1M H 3 PO 4 , which is one of the most used medium to measure the fluorescence of uranium, mainly due to its high emission quantum yield. 15 In this medium, the uranyl ions were principally found as 1 : 1 and 2 : 1 phosphateuranium complexes. 16 According to MORIYASU et al., 17 a quantitative discussion is difficult, however, for this medium it has been reasonably established that the U(VI) forms various phosphate complexes (UO 2 H 2 PO 4 + , UO 2 (H 2 PO 4 ) 2 and UO 2 (H 2 PO 4 ) 2 . H 3 PO 4 ), and nonexponential decay curves might be composed of single decay components of these complexes. This may indicate that the rate of ligand exchange should not be so fast as to attain the equilibrium between these excited species. The feature * E-mail: gerardoa@fisquim.fcq.unc.edu.ar of the decay curve approaching a single exponential function with increasing concentration of added phosphate can be interpreted as an indicative that 2 : 1 complexes are predominant, in agreement with the data stability constants. 17,18 We report here a study on the binding of uranyl complexes with SDS in 1M H 3 PO 4 . Monitoring the emission lifetime as a function of the surfactant concentration and applying the method developed by SNYDER et al. 19 for the data treatment, we obtained a quantitative description of the binding. Besides, we have studied the interaction between the U(VI) and a SDS micelle from the measurements of intensity and emission lifetime of the excited state of the uranyl ion (*UO 2 2+ or *U(VI)) in a 1M H 3 PO 4 medium. Experimental Stock solutions of uranyl ion were prepared by dissolving UO 2 (NO 3 ) 2 . 6H 2 O (Merk) in aqueous 1M H 3 PO 4 . Phosphoric acid (Cicarelli, p.a.) and sodium dodecyl sulfate (SDS) (Sigma, purity > 99.1%) were used without further purification. Absorption spectra were obtained with an Agilent 8453 UV-Visible spectrometer. A solution of [U(VI)]=3 . 10 3 M and SDS ranging from 10 4 to 0.5M was used. Static and dynamic measurements were carried out during the quenching experiments. The steady-state emission intensity was performed using a PTI QM2 spectrofluorometer, the excitation was accomplished at 337 nm and the emission spectrum was recorded between 450 and 650 nm. Luminescence lifetime measurements were obtained with a N 2 laser as