Journal of Colloid and Interface Science 306 (2007) 166–174 www.elsevier.com/locate/jcis Effect of terbium(III) chloride on the micellization properties of sodium decyl- and dodecyl-sulfate solutions A.C.S. Neves, A.J.M. Valente ∗ , H.D. Burrows, A.C.F. Ribeiro, V.M.M. Lobo Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal Received 21 August 2006; accepted 21 October 2006 Available online 27 October 2006 Abstract The effect of TbCl 3 on the aggregation processes of the anionic surfactants sodium decyl sulfate (SDeS) and sodium dodecyl sulfate (SDS) has been investigated. Electrical conductivity data, combined with Tb(III) luminescence measurements suggest that the formation of micelles involving TbCl 3 and SDS occurs at concentrations below the critical micelle concentration (cmc) of the pure surfactants; the formation of these mixed aggregates was also monitored by light scattering, which indicates that the addition of TbCl 3 to surfactant concentration at values below the pure surfactant cmc results in a much greater light scattering than that found with pure sodium alkylsulfate surfactant micelles. This phenomenon is dependent upon the alkyl chain length of the surfactant. With Tb(III)/DS − , complexes are formed with a cation/anion binding ratio varying from 3 to 6, which depends upon the initial concentration of Tb(III). This suggests that the majority of the cation hydration water molecules can be exchanged by the anionic surfactant. When the carbon chain length decreases, interactions between surfactant and Tb(III) also decrease, alterations in conductivity and fluorescence data are not so significant and, consequently, no binding ratio can be detected even if existing. The surfactant micellization is dependent on the presence of electrolyte in solution with apparent cmc being lower than the corresponding cmc value of pure SDS.  2006 Elsevier Inc. All rights reserved. Keywords: Terbium(III); Sodium dodecyl sulfate; Sodium decyl sulfate; Micellization parameters; Aggregation 1. Introduction There is increasing interest in the interaction of trivalent and higher valent metal ions with surfactants in aqueous so- lutions for various reasons. The trivalent lanthanides are of particular interest because they form a homogeneous family with very similar chemical behaviour but with a wide range of spectroscopic and magnetic properties. They have been ex- tensively used as luminescent probes in the investigation of metal-binding sites in biological materials [1,2] as well as in the study of surfactant association in solution [3,4]. In addi- tion, systems of SDS micelles with lanthanide ions have been used for studying magnetic field effects on geminate pair re- combination of radicals [5], or photoinduced emulsion poly- merization [6] and are also of increasing interest in relation to the mesomorphism of lanthanide salts of anionic amphiphiles * Corresponding author. Fax: +351 239 827703. E-mail address: avalente@ci.uc.pt (A.J.M. Valente). for applications in materials [7,8]. Lanthanides incorporated in suitable mesoporous materials also show considerable potential for catalysis [9]. The Tb(III) ion is of particular interest as a luminescent probe for detection of organic analytes in batch solutions and for fluorescence labelling in clinical chemistry and molecular biology [10–12]. Terbium(III) ions can form stable complexes or chelates with various organic ligands and undergo intramole- cular energy transfer through the triplet state of the ligand to the emitting level of the lanthanide ion [13]. It has been also reported that the fluorescence intensity of Tb(III) is enhanced in the presence of cationic (e.g., cetyltrimethylammonium bro- mide) [14] or anionic (e.g., sodium dodecyl sulfate and sodium dodecylbenzene sulfonate) [10,15] surfactants. It is therefore of interest to obtain more detailed information on the interaction of this cation with surfactants. In a previous paper [16] we have shown that in the pres- ence of Eu(III) the micellization parameters of SDS change, and it was suggested that this is due to the formation of mixed 0021-9797/$ – see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.10.061